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9:MATHS FUNCTIONS 111
-------------------------
AMOS Basic includes a wide variety of the more commonly needed
mathematical functions. To conserve memory, AMOS uses the standard
Amiga library routines. The appropriate libraries will be loaded
automatically from your workbench disc the first time you call one of
these functions in a particular session. You should therefore ensure
that the current disc contains the file MATHTRANS.LIBRARY in the LIBS
folder.
Trigonometric functions
=======================
The trigonometric functions provide you with a useful array of
mathematical tools. These can be used for a variety of purposes, from
education to the creation of complex musical wabeforms.
DEGREE (use degrees)
DEGREE
Generally all angles are specified in radians. Since radians are rather
difficult to work with, it's possible to instruct AMOS to accept angles
in degrees. Once you've activated this feature any subsequent calls to
the trig functions will expect you to use degrees.
RADIAN (use radian measure)
RADIAN
THe RADIAN directive informs AMOS that all future angles are to be
entered using radians - this is the default.
=PI# (a constant PI)
a#=PI#
This function returns the number called PI which represents the result
of the division of the diameter of a circle by the circumference. PI is
used by most of the trigonometric functions to calculate angels. Note
that a # character is part of the token name! This is to avoid clashes
with your own variable names.
=SIN (sine)
s#=SIN(a)
s#=SIN(a#)
The SIN functions calculates the sine of the angle in n. Note that the
function always returns a floating point number.
=COS (cosine) 112
c#=COS(a[#])
The cosine function computes the cosine of an angle.
=TAN (tangent)
t#=TAN(a[#])
TAN generates the tangent of an angle.
=ACOS (arc cos)
c#=ACOS(n#)
The ACOS function takes a number between -1 and +1 and calculates the
angle which would be needed to generate this value with COS.
Note, we haven't provided you with ASIN, because it's not really
needed. It can be readily computed using the formula:
ASIN(X)=90-ACOS(X) : Rem Measured in degrees.
ASIN(X)=1.5708-ACOS(X) : Rem using radians
=ATAN (arc tangent) 113
t#=ATAN(n#)
ATAN returns the arctan of a number.
=HSIN (hyperbolic sine)
s#=HSIN(a[#])
HSIN computes the hyperbolic sine of angle a.
=HCOS (hyperbolic cosine)
c#=HCOS(a[#])
HCOS calculates the hyperbolic cosine of angle a.
=HTAN (hyperbolic tangent)
t#=HTAN(a[#])
HTAN returns the hyperbolic tangent of the angle a.
Standard mathematical functions 114
===============================
=LOG (logarithm)
r#=LOG(v[#])
LOG returns the logarithm in base 10 (log 10) of the expression in v#.
=EXP (exponential function)
r#=EXP(e#)
Calculates the exponential of e#. Example:
Print Exp(1)
( result : 2.71828 )
=LN (natural logarithm)
r#=LN(l#)
LN computes the natural of naperian logarithm of l#.
=SQR (square root)
s#=SQR(v[#])
SQR calculates the square root of a number.
=ABS (absolute value) 115
r=ABS(v[#])
ABS returns the absolute value of v, taking no account of its sign.
=INT (convert floating point number to an integer)
i=INT(v#)
INT rounds a floating point number in v down to the nearest whole
integer.
=SGN (find the sign of a number)
s=SGN(v[#])
SGN returns a value of representing the sign of a number. There are
three possibilities.
-1, if v is negative
0, if v is zero
1, if v is positive
Creating random sequences
=========================
=RND (random number generation)
RND generates a random integer between 0 and n inclusive. But if n is
less than zero, RND will return the last value it produced. This can be
very useful when debugging one of your programs.
RANDOMIZE (set the seed of a random number) 116
RANDOMIZE seed
In practice, the numbers produced by the RND function are not really
random. They're computed internally using a complex mathematical
formula. The starting point for this calculation is taken from a number
known as the "seed". This seed is set to a standard value whenever you
load AMOS Basic into the computer. So the sequence of numbers generated
by RND will be exactly the same every time you run your game!
The RANDOMIZE command allows you to set the seed value directly, so
that the numbers would really look like random every time.
"seed" can be any value you wish. In order to generate a true random
numbers, you need some way of varying the seed from game to game. This
can be achieved using the TIMER instruction:
Randomize Timer
TIMER is a Basic function which returns the amount of time which has
elapsed since your Amiga was switched on in the current session. All
timings are measured in units of a 50th of a second.
Manipulating numbers
====================
=MAX (get the maximum of two values)
r=MAX(x,y)
r#=MAX(x#,y#)
r$=MAX(x$,y$) MAX compares two expressions and returns the largest.
These expressions can be composed of numbers or
strings of characters, providing you don't try to mix different types
of expressions in one instruction.
Print Max(10,4)
( result : 10 )
=MIN (return the minimum of two values) 117
r=MIN(x,y)
r#=MIN(x#,y#)
r$=MIN(x$,y$) This works the same way the =MAX does, except returns
the minimum value of compared numbers/strings.
SWAP (swap the contents of two variables)
SWAP x,y
SWAP x#,y#
SWAP x$,y$ Swaps the data between any two variables of the same
type.
FIX (set precision of floating point output)
FIX(n)
Changes the way your floating point numbers will be displayed on the
screen or printer. There are four possibilities.
If 0<n<16 then n denotes the number of figures to be output after
the decimal point.
If n> 16 the printout will be proportional and any trailing zeros will
be removed.
If n<0 Then all floating point numbers will be displayed in
exponential format, and the absolute value of n will
determine the number of digits after the decimal point.
If n=16 then the format will be returned to normal
Fix(-4) : Print PI#
DEF FN (create a user-defined function) 118
DEF FN name [(list)]=expression
The DEF FN command lets you create your own user-defined functions
within an AMOS Basic program. These can be used to compute commonly
needed values quickly and easily.
"nane" is the name of the function you wish to define. "list" is a
set of variables separated by commas. Only the type of these variables
is significant. When you call your function, any variables you enter
with, will be automatically subsituted in the appropriate positions.
"expression" can include any of the standard AMOS functions you wish.
Like all Basic expressions, it's limited just to a single line of prog.
FN (call a user-defined function)
FN name [(variable list)]
FN executes a function defined using DEF FN. Example:
Def Fn Asin(X)=90-Acos(X)
Degree
Print Fn Asin(0.5)
10:SCREENS 119
-------------------------
The default screen
==================
Whenever you run an AMOS Basic program a default screen is created as
screen zero. This forms a standard display which will be used for all
your normal drawing operations.
The system defaults to a 16-colour screen with dimensions 320x200,
which can easily be altered from within your program. In addition, you
can also define up to seven further screen with power SCREEN OPEN
command.
Defining a screen
=================
SCREEN OPEN (open a screen)
SCREEN OPEN n, w, h, nc, mode
SCREEN OPEN opens a screen, and reserves some memory it. The new screen
will now be used as the destination of all subsequent text and
graphical operations in your program.
n is the identification number of the screen which is to be created
by this instruction. Possible values range from 0-7. If this screen
already exists, it will be totally replaced by your new definition.
w holds the width of the screen in pixels. This is not limited to the
physical size of your display. It's perfectly lefal to define extra
large screens which may be manupulated using SCREEN OFFSET.
h sets the height of your screen using the same system. Providing
you've enough memory, you can easily create screens which are much
larger than the visible screen area. These screens can be used in
conjunction with all the normal screen operations. So you can construct
your images off-screen, and scroll them into view with the SCREEN
OFFSET command.
nc requests the number of colours required for the new screen. The
range of available colours varies from 2 to 64 (EHB). You can also
access the Amiga's special HAM mode with a value of 4096.
"mode" allows you to choose the width of the individual points on the
screen. The Amiga supports screen widths of either 320 or 640 pixels.
You can select the required width by setting "mode" either LOWRES (0)
or HIRES ($8000).
Here's a list of the possible screen options along with an indication
of the amount of memory they consume.
Colours Resolution Memory Notes 120
------- ----------- ------ ------------------------------
2 320 x 200 8 k Paper=0 Pen=1 Crsr=1, no flash
640 x 200 16 k " " " "
4 320 x 200 16 k Paper=1 Pen=2 Crsr=3, flash=3
640 x 200 32 k " " " "
8 320 x 200 24 k " " " "
640 x 200 48 k " " " "
16 320 x 200 32 k This is a default screen 0
640 x 200 64 k
32 320 x 200 40 k
64 320 x 200 48 k Extra Half-Bright mode (EHB)
4096 320 x 200 48 k Hold and Modify mode (HAM)
Note that the memory sizes in the table only apply to a standard
screen. If you create taller of wider screens, the amount of memory is
consumed will obviously be considerable greater. Screen zero is
equivalent to:
SCREEN OPEN 0,320,200,16,Lowres
SCREEN CLOSE (erase a screen) 121
SCREEN CLOSE n
SCREEN CLOSE deletes screen number n, and frees the memory for use.
AUTO VIEW ON/OFF (control viewing mode)
AUTO VIEW OFF
WHen you open a screen using SCREEN OPEN the new screen is usualyy
displayed immediately. This can be very incovenient during the
initialisation stages of your programs.
The AUTO VIEW OFF command provides you with full control over the
updating process. It turns off the automatic display system copletely.
You can then update the screen display at a convenient point in your
program using the VIEW instruction.
AUTO VIEW ON activates automatic screen updating.
DEFAULT (reset screen to its default)
DEFAULT
Closes all current open screens and restores the display back to its
original default setting. Example:
Load Iff "AMOS_DATA:IFF/Amospic.IFF",0
Wait Key
Defaul
VIEW (display the current screen settings)
VIEW
Displays any changes to the current screen settings at the next
vertical blank period. You only have to use this command when AUTOVIEW
is OFF.
Special screen modes
====================
The colour of every point on the screen is determined by a value held
in one of the Amiga's 32 colour registers. Each register can be loaded
from a selection of 4096 different colours.
Although 32 colours may seem rather a lot, particularly by ST
standards, it wasn't enough for the Amiga's designers. The easiest
solution would have been to increase the number of colour registers,
but this was quickly ruled out from reasons of cost.
Instead, they invented two special graphics modes which cleveroly 122
exploited the existing registers to increase the maximum number of
colours on the screen.
You've propably encountered these modes already. They're the infamous
Extra Half Bright and HAM modes. AMOS Basic provides full support for
both HAM and Half Bright modes. Here's a brief explanation.
Extra Half Bright mode (EHB)
----------------------------
Doubles the maximum colours on the screen to a grand total of 64. It
works by generating two colours for each of the 32 possible colour
registers.
The first 32 colours load the colour value directly from one of the
registers. Each register contains a value between 0 and 4095 which sets
the precise shade of the final colour.
The second group of colours, with numbers from 32 to 63, take one of
the previous registers and divide its contents by two. This produces 32
extra colours which are exactly half as bright as the normal colour
registers.
In order to exploit EHB mode to the full, it's necessary to load the
32 registers with the brightest shades in your palette. This will
automatically generate a list of intermediate tones in colours 32-63.
Aside from t
Hold and Modigy mode (HAM)
--------------------------
The Amiga's hardware currently limits you to a maximum of six bit
planes per screen. This allows you to display up to 64 different
colours on the screen at once. If you wanted to display a photograph 123
though, you'd require hunderds or even thousands of colours on the
screen.
This was the problem faced by Jay Miner when he was designing the
Amiga's display system. His solution was to exploit a trick which has
been known by artists for centuries. If a professional aritst had to
take every conceivable colour on an assignment, he would be faced with
an impossible task. It's therefore common parctice to mix the exact
shade on the spot, out of a small set of basic colours. This provides
millions of potential shades, without the need to carry several large
lorry loads worth of paint. The same technique can also be applied to a
computer screen. Instead of specifying each colour individually, you
can take an existing colour and modify it slightly. This increases the
number of available colours tremendously, and forms the basis of the
Amiga's powerfl Hold And Modify mode.
Each colour value on the Amiga is created from a mixture of the three
separate components. These determine the relative strength of the
primary colours Red, Green and Blue in the final colour. Possible
intenses range from 0 to 15.
Ham mode splits the Amiga's colour values into four separate groups:
* Colour registers 0-15: The first 16 colour take a value directly
from a colour register. These colours are
treadted just like those on a standard 16 colour screen.
* Red components 16-31: However, if a point is set to a colour
number in the range 16 to 31, the colour
value is loaded from the pixel to its immediate left.
The Red component of this colour is now replaced with a
value from 0 to 15 which is calculated from the formula:
Intensity=Colour index - 16
* Green components 32-47: Similarly, a colour number from 32 to 47
takes the current shade, and changes the
green component. The intensity of this component is set
to a value of colour - 32.
* Blue components 48-63: These colour numbers grab the colour value
from the point on the left of the current
pixel, and load a new blue component from your colour
number like so:
Intensity = Colour Index - 48
The colour of a particular point therefore depends on the colours of
all the points to the left of it. This allows you to create smooth
gradiations of colour which are ideal for flesh tones. However, you
can't choose the colour of each point on the screen independently. In
practice, it takes a maximum of three pixles to shift from one colour
to another.
When the Amiga was first released, Ham initially was regarded as
little more than curiosity. Nowadays, the situation is very different,
with the advent of excellent Ham graphics packages such as Photon
Paint.
AMOS allows you to perform the full range text and graphics
operations directly on to a Ham screen. EXAMPLE 10.1 provides you with
a simple example of how you can generate an entire screen in just a few
lines of Basic code.
Another point to consider, is that Ham screens are manipulated using
the normal SCREEN DISPLAY and SCREEN OFFSET commands. Here are some
simple guidelines to their use:
* The first point in each horizontal line should be set to a colour
number from 0 to 15. This will serve as the starting colour for all
the shades on the current line.
* Don't attempt to subject your Ham screens to horizontal scrolling.
If you try to scroll one of these screens, you'll get colour fringes
at the sides of your picture. These are generated by the changes in
the starting colours for each line. There are no such restrictions
to vertical scrolling.
* Fringing effects can also be produced by SCREEN COPY. The solution
is to ensure that the border of your zone is drawn using a colour
from 0 to 15. This will ensure that your Ham screens will be redrawn 124
at their new position with their original colours.
Loading a screen
================
LOAD IFF (load an IFF screen from the disc)
LOAD IFF "filename"[,screen]
Loads an IFF format picture from the disc. "Screen" indicates the
number of the screen which is to be loaded with your picture. This
screen will be opened automatically for your use, if it didn't exist.
Anything already inside your screen will be totally erased.
To load the picture into the present screen, omit the "screen"
parameter altogether.
Example:
Load Iff "AMOS_DATA:IFF/AMOSPIC.IFF",1
Saving a screen
===============
SAVE IFF (save an IFF scree)
SAVE IFF "filename"[,compression]
Saves the current screen as an IFF picture file on the disc.
"compression" is a flag which allows you to choose whether your file
will be compacted before it's saved. A value of one specifies that the
standard file compressiong system is to be employed and zero saves the
picture as it stands. As a default all AMOS screens are compressed.
SAVE IFF automatically appends a small IFF "chunck" to your picture
file. This stores the present screen settings including SCREEN DISPLAY,
SCREEN OFFSET and SCREEN HIDE/SHOW. When you load this file back into
AMOS Basic it will be returned to exactly its original condition. This
extra IFF data will be completely ignored by external graphics packages
such as DPaint 3.
Note that it's possible to save double buffered or dual playfield
screens with this command.
Moving a screen 125
===============
SCREEN DISPLAY (position a screen)
SCREEN DISPLAY n [, x, y, w, h]
Once you have defined your screen with SCREEN OPEN, you'll need to
position it on your screen. Unlike most other computers, the Amiga is
capable of displaying a picture anywhere you like on the TV screen.
This can be easily exploited to produce amazing "bouncing" screen
effects. With AMOS Basic, it's even possible to perform these
animations using interrupts (see AMAL).
Another aplication is to overlay several screens alongside each
other. This allows you to create your display out of a combination of
different screen modes.
"n" indicates the number of the screen to be positioned. "x" and "y"
specify the location of the screen in hardware coordinates.
The x coordinates of a screen can range from 0 to 448 and are
automatically rounded down to the nearest 16-pixel boundary. Only the
positions between 112 and 448 actually visible on your TV though, and
you are strongly advised to avoid using an x coordinate below 112.
The y coordinates of your screen can range between 0 and 312. The
visible range will largely depend on your TV or monitor, but you'll
propably find that coordinates between 30 and 300 are satisfactory for
the majority of systems.
At the time of writing, there appears to be a minor bug in the
Amiga's HAM mode. These pictures cannot be displayed with a Y
coordinate of exactly 256. So set your coordinates to intermediate
values such as 255 or 257 instead. We're not sure if it's a hardware or
software fault yet but it won't restrict you by any means.
"w" holds the width of your screen in pixels. If this is different
from the original setting, only a part of your image will be shown,
starting from the top left corner of the display. Like the x
coordinates, the screen width will be rounded to the nearest 16 pixel
boundary.
Similarly, "h" sets the apparent height of the screen. Changing this
value will reduce the depth of your image.
Generally SCREEN OPEN will automatically select the display position
for you using a standard setting in the AMOS configuration file. If a
screen is larger than the display then AMOS sets the screen into
overscan.
SCREEN DISPLAY provides you with a simple way of changing these
values from the default. Any of the parameters x,y,h and w may be
omitted as appropriate. The unused values will be automatically
assigned to the default settings, and should be separated by commas.
Screen Display 0,112,45,, : Rem position the screen at 112,45.
When you are positioning your screens, try to ensure that the screen
starts at the left of the display and ends towards the right. This is
essential if the Amiga's hardware is to interpret your screen
correctly. In practice, you may need to experiment a little to get the
precise effect you want. Fortunately, the worst that can happen is that
you'll get a silly looking display. The Amiga won't crash if you make a
mistake. here are some guidelines to help you along:
* Only a single screen can be displayed on each horizontal line. 126
However, you can safely place several screens on top of each other.
All will be well, providing only one of the screens visible.
* There will always be a one pixel thick "dead zone" between each pair
of screens. This is generated by the copper list and is completely
unavoidable. The dead zone will be noticeable whenever you move a
sprite between the screens. As an example, try moving the mouse
pointer from the editor window to the menu line. You should see a
small black line through your mouse pointer at the border between
the two screens.
SCREEN OFFSET (hardware scrolling)
SCREEN OFFSET n,x,y
The Amiga's display is not just limited to the visible dimensios of
your TV screen. There's absolutely nothing stopping you from generating
images which are much larger than the actual screen. It's obviously not
possible to display such pictures in their entirety, but you can easily
view a section of your image using the SCREEN OFFSET command.
"n" is the number of the screen to be displayed. x,y measure the
offset from the top left hand corner of the screen to the starting
point for your display. x and y are specified in units of a single
pixel, so there's nothing stopping you from generating some
delightfully smooth scrolls.
You can also use negative offsets with this instruction, allowing you
to display any part of the Amiga's memory on the screen. See
EXAMPLE 10.2 for a full demonstration of this command.
Screen control commands 127
=======================
SCREEN CLONE (clone a screen)
SCREEN CLONE n
The SCREEN CLONE command assigns a second version of the current screen
to screen number n. This clone uses exactly the same memory area as the
original screen.
Normally, the cloned screen is displayed at the same place as its
parent. However it can be manupulated separately using any of the
normal screen operations such as SCREEN DISPLAY and SCREEN OFFSET.
Since there's only a *single* copy of the original screen data in
memory, you can't access a clone with the SCREEN command. You'll get an
"illegal screen parameter" error if you rty. Another point to consider
is that any colour flash sequences you've set up on the original screen
will NOT be copied during the cloning operation. See EXAMPLE 10.3.
Notice the use of the WAIT VBL command. This ensures that the clone is
repositioned off-screen and keeps the movements running smoothly.
If you experiment with SCREEN CLONE, you'll quickly find that there's
a real limit to the amount of movement you can perform without spoiling
the effect completely. Even something as trivial as an extra
calculation to your movement routine can often introduce an
unacceptable delay into your animations.
The screen display can also be adjusted directrly from the AMAL
animation language. This is capable of animating large numbers of
screens smoothly and easily. See EXAMPLE 10.4 for a demonstration.
DUAL PLAYFIELD (combine two screens
into dual playfield)
DUAL PLAYFIELD screen1, screen2
The Amiga's dual playfield mode allows you to display two complete
screens simultaneously at the same x and y coordinates. It's almost as
if you'd drawn eaxh screen on cellophane and overlayed them on top of
each other. Each screen can be manipulated totally independently. You
can exploit this to produce a smooth parallax effect which is ideal for
screen scrolling games such as Silkworm.
The two components of a dual playfield are treated just like any
other AMOS screen and can be written to in the normal way. They can
even be animated within AMAL or double buffered.
"screen1" and "screen2" refer to screens which have been previously
defined with the SCREEN OPEN command. Only certain screen combinations
are acceptable. Both screens MUST use the same resolution, as it's
illegal to use hires(meaning actually MedRes) and lowres in the same
playfield.
Here is a list of the possibilities 128
Screen 1 Screen 2 Notes
=============================
#of colours #of colours
----------- -----------
2 2
4 2
4 4
8 4 LowRes only
8 8 LowRes only
Although the colour ranges are predefined, the sizes of the two screens
can be completely different. By creating a background screen which is
larger than the foreground you can create a delightfully realistic
parallax effect.
The colours of these screens are all taken from the palette of
screen1 with colour zero being treated as transparent.
Screen Colour indexes (from screen 1)
------ ------------------------------
1 0 - 7
2 8 - 15
When you are drawing to the second screen, AMOS Basic will
automatically convert your colour index to the appropriate number
before using it. So INK 2 will use colour nine from the first palette.
This conversion process does not apply to the assignment statements
such as COLOUR or PALETTE. It's important to remember this when you are
changing the colour settings, otherwise your new colours will not be
reflected on the actual screen. Always make "screen1" the current
screen before changing your colour assignments.
There are a couple of important opints which you must be aware of
before setting up a dual playfield screen:
* The screen offsets for both screens must never be set to zero.
* If you set a dual playfield screen up and then want to position
it with SCREEN OFFSET be sure to specify dual screen 1 not the
second.
DUAL PLAYFIELD is an extremely powerful instruction. A full
demostration can be found in EXAMPLE 10.5.
DUAL PRIORITY (choose order of dual playfield screens)
DUAL PRIORITY screen1,screen2
The first screen of a dual playfield is normally displayed directrly
over the second. The DUAL PRIORITY command allows you to change this
order around so that screen2 appears in front if screen1
WARNING! This instruction only changes the order of the display. It
has *NO* effect on the screen organization. The first screen in the
dual playfield list should therefore still be used for all colour
assignments and with SCREEN DISPLAY.
SCREEN (set current screen) 129
SCREEN n
The SCREEN command allows you to direct all graphical and text
operations to screen number n.
=SCREEN (get the current screen #)
s=SCREEN
Returns the number of the currently active screen.
SCREEN TO FRONT (moves screen to front of display)
SCREEN TO FRONT [s]
This instruction moves screen "s" to the front of the TV display. If
the parameter is omitted, then the current screen will be used instead.
Note: if the AUTOVIEW system has been turned off, you'll need to call
the VIEW command before the effect will be visible on the screen.
SCREEN TO BACK (move screen to back of display)
SCREEN TO BACK [n]
SCREEN TO BACK moves a screen to the background of your display. If
there is another screen at the same coordinate this will now be
displayed in front of the selected screen.
SCREEN HIDE (temporarily hide a screen)
SCREEN HIDE [n]
Removes a selected screen from view copletely. This screen can be
redisplayed using a call to SCREEN SHOW. If n is omitted, this
instruction will hide the current screen.
SCREEN SHOW (restore a screen) 130
SCREEN SHOW [n]
Screen SHOW returns a screen onto the display after it has been hidden
with the SCREEN HIDE command.
=SCREEN HEIGHT (return height of screen)
h=SCREEN HEIGHT [n]
Returns the height of an AMOS screen. If you don't include the
parameter n, the height will be returned for the current screen.
=SCREEN WIDTH (return the width of screen)
w=SCREEN WIDTH [n]
SCREEN WIDTH retrieves the width of either the current screen or screen
number n. Example:
Print Screen Width
=SCREEN COLOUR (return the number of colours)
c=SCREEN COLOUR
Returns the maximum numbers of colours in the currently active screen.
=SCIN (returns screen number at a selected position)
s=SCIN(x,y)
Returns the number of screen which is underneath the *hardware*
coordinates x,y. If this screen does not exist, then s will be loaded
with a negative value (null).
SCIN is normally used in conjuction with the X MOUSE and Y MOUSE
functions to check whether the mouse cursor has entered a particular
screen. Example:
Print Scin(X Mouse, Y Mouse)
Defining the screen colours 131
===========================
DEFAULT PALETTE (load screen with standard palette)
DEFAULT PALETTE c1,c2,c3,,,c6,,-> up to 32 colours
This command simplifies the process of opening many screens with the
same palette. It defines a list of colours which will be used for all
subsequent screens which you create with the SCREEN OPEN instruction.
As usual, the allowable colour values range from $000 to $FFF.
GET PALETTE (set the palette from a screen)
GET PALETTE n [,mask]
The GET PALETTE instruction copies the colours from screen n and loads
them into the current screen. This can be very useful when you're
moving information from one screen to another with SCREEN COPY, as it's
usually vital that both the source and destination screens share the
same colour settings.
The optional "mask" parameter allows you to load just a selection of
the colours. See GET SPRITE PALETTE for full details of mask.
Clearing the screen
===================
CLS (clear the screen)
CLS erases all or part of the current screen. There are three possible
formats of this command:
CLS
Clears the current screen by filling it with colour zero and clears any
windows which may have been set up.
CLS col
Fills your screen with colour col.
CLS col,x1,y1 to x2,y2
Replaces the rectangular region at coordinates x1,y1,x2,y2 with a block
of colour col. Col can take any value from 0 to the max. number of
available colours. x1,y1,x2,y2 hold the coordinates for top left and
bottom right corners of the area to be cleared by this command.
Example:
Cls : Circle 100,09,09 : Cls 1,50,50 To 150,150
Manipulating the contents of a screen 132
=====================================
SCREEN COPY (copy sections of the screen)
SCREEN COPY scr1 TO scr2
SCREEN COPY scr1,x1,y1,x2,y2 TO scr2,x3,y3 [,mode]
SCREEN COPY makes it possible to copy large sections of a screen from
one place to another at amazing speed.
"scr1" holds the screen used as the source of your image. This can be
either a standard screen number or the number of a logical or physical
screen generated using the LOGIC and PHYSIC commands.
"scr2" selects an optional destination screen into which this data
will be copied. If it's omitted, the area will be copied into the
current screen.
x1,y1 and x2,y2 hold the dimensios of a rectangular source area, and
x3,y3 contain the coordinates of the destination. There are no
limitions to these coordinates whatsoever. Any parts of your image
which lie outside the current screen area will be automatically clipped
as appropriate.
The optional "mode" parameter chooses which of the 255 possible
blitter modes will be used for your copying operation. These modes
determine how your source and destination areas will be combined
together on the screen. The mode is set using a bit-pattern in the
following format:
Mode Bit Source Bit Destination Bit
-------- ---------- ---------------
4 0 0
5 0 1
6 1 0
7 1 1
Note that the bottom four bits in the pattern are not used by this
instruction and should always be set to zero.
Each bit in "mode" represents a single combination of bits in the
source and destination areas. If a mode bit is set to one, then the
associated bit on the screen will also be loaded with a one, otherwise
the result will be zero.
In order to select the correct drawing mode for you application, you
simply decide which combinations should result in a one and set the
appropriate bits in the "mode" parameter accordingly.
Supposing you only wanted to set a bit on the screen if both the
source and destination bits were the same. You would look the table for
the points where your requirement was satisfied. This would produce the 133
following vaue for "mode":
%10010000
If you're not familiar with binary notation, you may find this command
a little opaque. Rather than boring you silly with an explanation of
binay we'll now provide you with a detailed list of the more common
requirements along with the associated bit-maps.
Mode Effect Bit-pattern
------- --------------------------------------- -----------
REPLACE Replaces the destination with a direct %11000000
copy of the source image (default).
INVERT Replaces the destination image by a %00110000
reversed copy of the source image.
AND Combines the source and destination %10000000
with a logical AND operation.
OR OR's the source with the destination %11100000
image.
XOR Combines the source and destination %01100000
area with an Exclusive OR.
Technically-minded users should note that SCREEN COPY combines the
source and destination using blitter areas B and C and that blitter
area A is not used by the system at all.
Scrolling the screen
====================
DEF SCROLL (define a scroll zone)
DEF SCROLL n,x1,y1 to x2,y2,dx,dy
Allows you to define up to 16 different scrolling zones. Each of these
zones can be associated with a specific scrolling operation which is
determined by the variables dx and dy.
n holds the number of the zone and can range from 1 to 16. x1,y1
refer to the coordinates of the top left-hand corner of the area to be
scrolled and x2,y2 to the point diagonally opposite.
dx signifies the number of pixels the zone will be shifted to the
right in each operation. Negative numbers indicate that the scrolling
will be from right to left, and positive numbers from left to right.
Similarly, dy holds the number of pixels the zone will be advanced up
or down during the scroll. In this case negative values of dy are used
to indicate an upward movement and positive values a downward motion.
SCROLL (scroll the screen) 134
SCROLL n
The SCROLL command scrolls the screen using the settings you have
specified with the DEF SCROLL instruction. n refers to the number of
the zone you wish to scroll.
Load Iff "AMOS_DATA:IFF/Frog_Leap.IFF",2
Def Scroll 1,0,0, to 320,200,1,0
Do
Scroll 1
Loop
Larger examples can be found in EXAMPLE 10.7 and EXAMPLE 10.8. The
variable s holds the number of points the picture will be moved during
each SCROLL. Note the use of screen switching to improve the quality of
the motion.
Screen switching
================
In order to produce the smooth movement effects found in a computer
game, it's necessery to complete all the drawing operations within a
time span of no more than a 15th of a second. This represents a real
challenge for the fastest computer, and it's often impossible to
achieve even on the Amiga. If the animation is complex, your graphics
will therefore tend to flicker annoyingly as they are being drawn.
Fortunately, there's a solution at hand which has been succesfully
exploited in the vast majority of modern arcade games. This *screen
switching* technique can easily generate flicker-free screen animation
using just a fraction of Amiga's computing power.
The basic idea is extremely simple. Instead of constructing your
images on the actual screen, you perform all your drawing operations on
a separate logical screen, which is copletely invisible to the user.
This is distinct from the *physical screen* which is currently being
displayed on your TV. Once the graphics have been completed, you can
then swap the logical and physical screen to produce a smooth
transition between the two screen images. The old physical screen now
becomes the new logical screen, and is used to construct the next
picture in your sequence.
At fist glance, this process looks pretty complicated, but it's all
performed automatically by the AMOS Basic DOUBLE BUFFER command. This
forces all drawing operations to be performed directly on the logical
screen without affecting the current display. All you need to do within
your program is to synchronise your drawing operations with the screen
switches. This can be achieved with the help of SCREEN SWAP
instruction.
SCREEN SWAP (swap the logical and physical screens)
SCREEN SWAP [n]
SCREEN SWAP swaps the physical and logical screens. This enables you to
instananeously switch the physical display between the two screens. 135
If you're using DOUBLE BUFFER, these screens will have been created
for you already. However, you will need to switch off the automatic
screen switching system with BOB UPDATE OFF, as otherwise the screens
will be swapped 50 times a second, and will interfere with your own
drawing operations. It's also necessary to kill the autoback feature
with AUTOBACK OFF. This normally copies your graphical operatoins onto
both physical and logical screens. It's useful when you wish to combine
simple graphics with moving bobs, but it destroys the effect of your
screen switching operations totally.
As an illustration of the power of this command, have a look at the
programs EXAMPLE 10.9 and EXAMPLE 10.10.
=LOGBASE (return the address of part of
part of the logical screen)
address=LOGBASE(plane)
The LOGBASE function is aimed at expert programmers who wish to access
the Amiga's screen memory directly. "plane" referes one of the six
possible bit-planes which make up the current screen. After LOGBASE has
been called, "address" will contain either the address of the required
bit-plane, or zero if it doesn't exist.
=PHYSBASE (return the address of
the current screen)
address=PHYBASE
PHYBASE returns the address in memory of bit-plane number "plane" for
the current screen. If this plane does not exist, then a value of zero
will be returned by this function. Example:
Loke Phybase(0),0 : Rem pokes a thin line directly onto the
screen.
=PHYSIC (return identifier of
the physical screen)
=PHYSIC
=PHYSIC(s)
The PHYSIC function returns an identification number for the current
physical screen. This number allows you to directly access the physical
image which is being displayed by the double buffering system.
The result of this function can be substituted for the screen number
in the ZOOM, APPEAR and SCREEN COPY commands.
"s" is the number of an AMOS screen. If it's omitted, then the
present screen will be used instead. Don NOT confuse with the LOGBASE
function.
=LOGIC (return identifier of 136
the logical screen)
=LOGIC
=LOGIC(s)
Returns an identification number of a logical screen. This can be used
in conjunction with the SCREEN COPY, APPEAR and ZOOM commands to change
your image off-screen, without affecting the current display.
Screen synchronisation
======================
Like most home computers the AMIGA uses a memory-mapped display. This
is a technical term for a concept you are almost certainly already
familiar with. Put simply, a memory-mapped display is one which uses
special hardware to convert en image stored in memory into a signal
which can be displayed to your TV screen. Whenever AMOS Basic accesses
the scren it does so through the medium of this screen memory.
The screen display is updated by the hardware every 50th of a second.
Once a screen has been drawn, the electron beam turns off and returns
to the top left of the screen. This process is called the vertical
blank period VBL. At the same time, AMOS Basic performs a number of
important tasks, such as moving the sprites and switching the physical
screen address if it has changed. The actions of instructions such as
ANIM or SCREEN SWAP will therefore only be fully completed when the
screen is redrawn.
Since a 50th of a second is a quite long time for AMOS Basic, this
can lead to a serious lack of coordination between your program and the
screen, which is especially noticeable in tight program loops. The best
way of avoiding this is difficulty, is to wait until the screen has
been updated before you execute the next Basic command.
WAIT VBL (wait for a vertical blank)
The WAIT WBL instruction halts the AMIGA until ne next vertical blank
period. It is commonly used after either a PUT BOB insturction or a
SCREEN SWAP
Special effects
===============
APPEAR (fade between two pictures)
APPEAR source TO destination, effect [,pixels]
The APPEAR command enables you to produce fancy fades between the
"source" and "destination" screens. Source and destination are simply
the numbers of screens you've previously opened using SCREEN OPEN. You
can also substitute the LOGIC and PHYSIC functions in these positions
if required.
"effect" determines the type of fade which will be produced by this
insturction. The size of this parameter can vary from 1 to the number
of pixels in you current screen.
"pixels" specifies the number of points which are to affected.
Normally this value is set to the TOTAL screen area, but you can reduce
it to fade only a part of the screen. All screens are drawn in strict
order from the top of the screen to the bottom.
The appearance of your fades will naturally vary depending on the
screen mode you are using. A program is provided in EXAMPLE 10.11 to
allow you to experiment with the various possibilities.
FADE (blend one or more colours 137
to new colour values)
FADE speed [,colour list]
FADE speed TO screen [,mask]
The FADE command allows you to smoothly change the entire palette from
one set of colours to another. This can be used to generate
professional-looking fade effects for your loading screens.
The standard version of the instruction takes the current palette,
and slowly dissolves the screen colours to zero. Each colour value is
successively reduced by one until they reach zero. Example:
Fade 15 : Wait 225
"speed" is the number of vertical blank periods that must occur
before the next colour change is performed.
Since the fadig effects are executed using interrupts, it's best to
wait until the operation has completely finished before proceeding to
the nexy Basic instruction. The time taken for the fade WAIT can be
calculated by the formula:
wait value = fade speed * 15
Fade can be extended to generate a new palette directly from a list of
colour values.
Fade 15,$100,$200,$200,$300
Any number of colours can be specified in this instruction, up to the
maximum allowed in the current graphics mode. Like most AMOS commands,
it's possible to omit selected parameters completely. These colours
will be totally unaffected fy the FADE command.
Fade 15,,$100,$800,$F00
The most powerful form of FADE smoothly transforms the colours from the
current screen into a palette taken from an existing screen.
Fade speed TO s [,mask]
The present colours are slowly converted into the palette of screen s.
It's also possible to load the palette from the sprite bank using the
same technique. Simply use a negative value for the screen number s.
"mask" is a bit-pattern which specifies which colours should be
loaded. Each colour is associated with a single bit in this pattern
numbered from 0 to 15. If a bit is set to 1, then the relevant colour
will be changed. See EXAMPLE 10.12.
FLASH (set flashing colour sequence) 138
This command gives you the ability to periodically change the colour
assigned to any colour index. It does this with an interrupt similar to
that used by the sprite and the music instructions. The format of the
flash instruction is:
FLASH index,"(colour,delay)(colour,delay)(colour,delay)..."
"index" is the number of the colour which is to be animated. Delay is
set in units of a 50th of a second.
Colour is stored in the standard RGB format (See COLOUR) for mode
details. The action of FLASH is to take each new colour from the list
in turn, and then load it into the index for a length of time
specified by the delay. When the end of this list is reached, the
entire sequence of colours is repeated from the start. Note that you
are only allows to use a max. of 16 colour changes in any one FLASH
instruction. Here is a small example:
Flash 1,"(007,10)(000,10)"
This alternates colour number 1 between blue and black every 10/50th of
a second.
FLASH OFF
Turns off the flashing. Note that on start-up, colour number 3 is
automatically assigned a flash sequence for use by the cursor. It's a
good idea to turn this off before loading any pictures from the disc.
SHIFT UP (colour rotation)
SHIFT UP delay,first,last,flag
The SHIFT UP command rotates the values held in the colour registers
from the "first" to "last". The "first" colour in the list is copied
into the second, and the second into the third, and so on, until the
"last" colour in the series is reached.
Each AMOS screen can have its own unique set of colour animations.
Colour shifts can be used to create amazing hyperspace sequences
similar to those found in Captain Blood and Elite. Since these
animations are performed using interrupts, they can be executed while
your program is running, without affecting it in the slightest.
"delay" is the time interval between each stage of the rotation,
measured in 50ths of a second.
"flag" controls the type of rotation. If it's ste to one, the last
colour index in the list will be copied into the first, and the first
to the last. So the colours will rotate continuously on the screen.
When "flag" is set to zero, the contents of the first and last indexes
will be discarded, and the region between first and last will be
replaced by a copy of the first colour in the list. For example:
SHIFT UP 100,1,15,1
SHIFT UP 10,1,15,0
SHIFT DOWN (colour rotation) 139
This is similar to the SHIFT UP, except it rotates the colours in the
opposite direction.
SHIFT OFF (stops col.rotation for the current screen)
SHIFT OFF
Immediately terminates all colour rotations produced by the SHIFT UP or
SHIFT DOWN instructions
SET RAINBOW (define a rainbow effect)
Defines an attractive rainbow effect which can be subsequently
displayed using the RAINBOW command. It works by changing the shade of
a colour according to a series of simple rules.
"n" is the number of your rainbow. Possible values range from 0 to 3.
"colour" is a colour index which will be changed by the instruction.
This colour can be assigned a different value for each horizonal sreen
line (or scan line). Note that only colours 0-15 can be manipulated
using this system.
"length" sets the size of table to store your colours. There's one
entry in this table for each colour value on the screen. The size of
this table can range from 16 to 54400. If "length" is less than the
physical height of your rainbow, then the colour pattern will be
repeated several times on the screen.
The r$,g$,b$ command strings, progressibely change the intensities of 140
the red, green and blue components of your final colour. These values
are loaded into a special colour table. Each colour in the table
determines the appearance of a single horizontal scan line on the
screen.
At the start of the rainbow, all the components in your colour are
initially loaded with a value of zero. This will be changed according
to the information held in the colour table.
Any command string may be omitted if required, but you'll still have
to include the quotes and the commas in their expected positions.
Each string can contain a whole list of commands. These will be
cycled continually to produce the final rainbow pattern. The format is:
(n,step,count)
"n" sets the number of lintes to be assigned to a specific colour value
in the rainbow. Increasing this number will change the height of each
individual rainbow line.
"step" holds a number to be added to the component. This number will
be used to generate the colour of the succeeding line on the screen. A
positive step will increase the intensity of colour component, and a
negative value will reduce it.
Whenever a particular component exceeds the maximum of 15, a new
value will be calculated from the formula:
new component = old component Mod 15
"count" is the number of times the current operation is to be repeated.
The best way to demonstrate this command is with an example:
Set Rainbow 0,1,64,"(8,2,8)","",""
Rainbow 0,56,1,255
Wait Key
This creates a new rainbow with number zero using colour index one. As
you can see, SET RAINBOW only defines your rainbow. In order to display
it on the screen you need to make use of the RAINBOW command.
The rainbow effects first loads your colour with a value of zero.
Every four scan-lines, the red component will be automatically
incremented by two. So the contents of colour zero will progressively
change from $000 to $E00. WHen the component exceeds the maximum of 15,
its remainder will be calculated, and the colour will be returned to
its starting point (zero). The pattern will now be repeated down the
screen.
By defining a separate pattern for eaxh of the red, green and blue
components of your colour, you can easily generate some starling
patterns on the screen. Since each rainbow only uses a single colour
index, there's nothing stopping you from creating the same effects
using just two colour screens. These are ideal from the backgrounds of
an arcade game, as they consume very little memory. Example:
Screen Open 0,320,256,2,Lowres
Set Rainbow 0,1,128,"8,1,8)","(8,1,8)",""
Rainbow 0,1,30,128
Colour 1,0 : Curs Off : Cls 1 : Flash Off
Locate 0,2 : Centre "Amos Basic" : Wait Key
For further demonstration of the superb effects that can be achieved
with this instruction load up EXAMPLE 10.13.
Rainbows can also be animated using a powerful interrupt system. See
the section on AMAL for more details.
RAINBOW (create a rainbow effect) 141
RAINBOW n,base,y,h
Displays rainbow number n on the screen. If AUTOVIEW is set to OFF, the
rainbow will only appear when you next call the VIEW command.
"base" is an offset in the first colour in the table you created with
SET RAINBOW. Changing this value will cycle the rainbow on the screen.
y holds the vertical position of the rainbow in hardware coordinates.
The minimum calue for this coordinate is 40. If you attempt to use a
coordinate below this point, the rainbow will be displayed from line 40
onwards.
h sets the height of your rainbow scan lines.
Rainbows are totally compatible with the AMOS system including bobs
and sprits. However, don't attempt to rainbow a colour which is
currently being changed using the FLASH or SHIFT instructions, as this
will lead to unpredictable screen effects.
Note that only a single rainbow effect can be displayed on a
particular scan line, even if they use different colours on the screen.
Normally the rainbow with the highest screen position will be
displayed first. But if several rainbows start from the same scan line,
then the rainbow with the lowest identification number will be drawn in
front of the others.
=RAIN (change the colour of an
individual rainbow line)
RAIN(n,line)=c
c=RAIN(n,line)
This is the most powerful of all the rainbow creation commands, as it
allows to change the colour of an individual rainbow line to any value
you like.
n is the number of the rainbow you wish to access. "line" is the
individual scan line to be changed. Example:
Curs Off : Centre "Securitate Stinks!"
Set Rainbow 1,1,4097,"","",""
For Y=0 To 4095
Rain(1,Y)=Y
Next Y
For C=0 to 4095-255
Rainbow 1,C,40,255
Next C
Wait Key
ZOOM (magnify a section of the screen) 142
ZOOM source,x1,y1,x2,y2 TO dest,x3,y3,x4,y4
ZOOM is a simple instruction which allows you to change the size of any
rectangular region of the screen.
"source" is the number of a screen from which your picture will be
taken. You can also use the LOGIC function to grab your image from the
appropriate logical screen. The rectangular area to be affected by this
instruction is entered using the coordinates x1,y1,x2,y2. "dest" holds
the destination screen for your image. Like the source, it can be
either a screen number, or a logical screen specified using LOGIC.
The dimensios of this screen are taken from the cordinates x3,y3 and
x4,y4. These hold the dimensios of the rectangle into which the screen
segment will be compressed.
The effect of this instruction depends on the relative sizes of the
source and destination rectanges. The source image is automatically
resized to fit exactly into the destination rectangle. So the same
instruction can be used to reduce or enlarge your images as required.
See EXAMPLE 10.14 for a further demonstration.
Changing the copper list
========================
The Amiga's co-processor (copper) provides total control over the
appearance of every line on your screen. This copper is a separate
processor with its own internal memory and unique set of instructions.
By programming the copper it's possible to freely generate a massive
variety of different screen effects. Normally the copper is managed
automatically by the AMOS system. Each of the available copper effects
can be performed directly from within AMOS Basic without the need to
indulge in complicated machine-level programming. In practive these
intructions will be more than sufficient for the vast majority of
applications.
Obviously, no one can think of everything though. Expert programmers
may wish to access the copper directly to create their own special
screen modes.
Be warned: The copper list is notoriously difficult to program, and
if you don't know precisely what you are doing, you'll almost certainly
crash your Amiga. Before embarking on your copper experiments for the
first time, you are therefore adviced to read one of the many reference
books on the subject. A good explanation can be found the "Amiga System
Programmers Guite" from Abacus.
COPPER OFF (turn of the standard copper list) 143
COPPER OFF
Freezes the current AMOS copper list and turns off the screen display
copletely. You can now create your own display using a series of COP
MOVE and COP WAIT instructions.
As a default, all user-defined copper lists are limited to a maximum
of 12k. On average, each copper instruction takes up two bytes. So
there's a space for around 6000 instructions. This may be increased if
required, using a special option from the CONFIG utility.
Note that all copper instructions are written to a separate logical
list which is not displayed on the screen. This stops your program
corrupting the display while the copper list is being created. To
activate your new screen, you'll need to swap the physical and logical
lists around with the COP SWAP command.
It's also important to generate your copper lists in strict order,
starting from the top left of your screen and progressing downward to
the bottom right. See EXAMPLE 10.15.
COPPER ON (restart the copper list)
COPPER ON
Restarts the AMOS copper list calculations and displays the current
AMOS screens.
COP MOVE (write a MOVE instruction into
the logical copper list)
COP MOVE addr,value
Generates a MOVE instruction in the logical copper list.
"addr" is an address of a 16 bit register to be changed. This must
lie within the normal copper DATA ZONE ($7F-$1BE). "value" is a
word-sized integer to be loaded into the requested register.
COP MOVEL (write a long MOVE instruction
into copper list)
COP MOVEL addr,value
This is identical to the COP MOVE, except that "addr" now refers to a
32-bit copper register. "value" contains a long word intereger.
COP WAIT (copper WAIT instruction)
COP WAIT x,y [,x mask, y mask]
COP WAIT writes a WAIT instruction into your copper list. The copper
waits until the hardware coordinates x,y have been reached and returns
control to the main processor. Note that line 255 is automatically
managed by AMOS. So you don't have to worry about it at all.
x mask and y mask are bit maps which allow you to wait until just a
certain combination of bits in the screen coordinates have been set. As
a default both masks are automatically assignet to $1FF.
COP RESET (reset copper list pointers) 144
COP RESET
Restores the address used by the next copper instruction to the start
of the copper list.
=COP LOGIC (address of copper list)
addr=COP LOGIC
This function returns the absolute address in memory of the logical
copper list. This allows you to poke your COPPER instructions directly
into the buffer, possibly using assembly language.
Hints and tips
==============
* Before creating a screen with a user defined copper list, you'll
first need to allocate some memory for the appropriate bit-maps.
Although you can use RESERVE for this purpose, it's much easier to
define a dummy screen with the SCREEN OPEN command instead. The copper
registers can be loaded with the addresses of the required bit-maps
using the LOGBASE function.
You'll now be able to access your screen using all the standard AMOS
drawing features. In order to reserve the correct amount of memory, set
the number of colours to the MAXIMUM used in the new screen. This may
be a little wasteful, but simplifies things enormously.
* It's perfectly acceptable to combine user-defined screens with AMOS
bobs. If you're using double buffering though, you'll have to define
a separate copper list for both the logical and physical screens. This
may be achieved using the following procedure;
1 Define your copper list for the first screen
2 Swap the logical and physical copper lists with COP SWAP
3 Swap the physical and logical screens with SCREEN SWAP
4 Define your copper list for the second screen
This will ensure that your bobs will updated correctly on your new
screens. All the normal AMOS commands can be used including AMAL.
