GEMini Atari/GEMini_Atari_CD-ROM_Walnut_Creek_December_1993.iso/files/program/progem/gem6.asc

Text File | 1987-10-10 | 20.0 KB | 434 lines

                        *PROFESSIONAL GEM*
                           by Tim Oren
                  Column #6 - Raster operations

                        SEASONS GREETINGS!

     This is the Yuletide installment of ST PRO GEM, devoted to
explaining the raster, or "bit-blit" portion of the Atari ST's VDI
functions.

     Please note that this is NOT an attempt to show how to write
directly to the video memory, although you will be able to deduce a
great deal from the discussion.

     As usual, there is a download with this column.  You will find
it in ATARI16 (PCS-58) in DL3 under the name of GEMCL6.C.

                          DEFINING TERMS

     To understand VDI raster operations, you need  to understand the
jargon used to describe them.  (Many programmers will be tempted to
skip this section and go directly to the code.  Please don't do it
this time:  Learning the jargon is the larger half of understanding
the raster operations!)

     In VDI terms a raster area is simply a chunk of contiguous words
of memory, defining a bit image.  This chunk is called a "form".  A
form  may reside in the ST's video map area or it may be in the data
area of  your application.  Forms are roughly analogous to "blits" or
"sprites"  on other systems.  (Note, however, that there is no sprite
hardware on the ST.)

     Unlike other systems, there is NO predefined organization of the
raster form.  Instead, you determine the internal layout of the form
with an auxiliary data structure called the MFDB, or Memory Form
Definition Block.  Before going into the details of the MFDB, we need
to look at the various format options.  Their distinguishing features
are monochrome vs. color, standard vs. device-specific and even-word
vs. fringed.

                       MONOCHROME VS. COLOR

     Although these terms are standard, it might be better to say
"single-color vs. multi-color".  What we are actually defining is the
number of bits which correspond to each dot, or pixel, on the screen.
In the ST, there are three possible answers. The high-resolution mode
has one bit per pixel, because there is only one "color": white.

     In the medium resolution color mode, there are four possible
colors for each pixel.  Therefore, it takes two bits to represent
each dot on the screen.  (The actual colors which appear are
determined by  the settings of the ST's pallette registers.)

     In the low resolution color mode, sixteen colors are generated,
requiring four bits per pixel.  Notice that as the number of bits per
pixel has been doubled for each mode, so the number of pixels on the
screen has been halved: 640 by 400 for monochrome, 640 by 200 for
medium-res, and 320 by 200 by low-res.  In this way the ST always
uses  the same amount of video RAM: 32K.

     Now we have determined how many bits are needed for each pixel,
but not how they are laid out within the form.  To find this out, we
have  to see whether the form is device-dependent or not.

               STANDARD VS. DEVICE-SPECIFIC FORMAT

     The standard raster form format is a constant layout which is
the same for all GEM systems.  A device-specific form is one which is
stored in the internal format of a particular GEM system.  Just as
the ST has three different screen modes, so it has three different
device-specific form formats.  We will look at standard form first,
then the ST-specific forms.

     First, it's reasonable to ask why a standard format is used. Its
main function is to establish a portability method between various
GEM systems.  For instance, an icon created in standard format on an
IBM PC GEM setup can be moved to the ST, or a GEM Paint picture from
an AT&T 6300 could be loaded into the ST version of Paint.

     The standard format has some uses even if you only work with the
ST, because it gives a method of moving your application's icons and
images amongst the three different screen modes.  To be sure,  there
are limits to this.  Since there are different numbers of pixels  in
the different modes, an icon built in the high-resolution mode will
appear twice as large in low-res mode, and would appear oblong in
medium-res.  (You can see this effect in the ST Desktop's icons.)
Also,  colors defined in the lower resolutions will be useless in
monochrome.

     The standard monochrome format uses a one-bit to represent
black, and uses a zero for white.  It is assumed that the form begins
at the  upper left of the raster area, and is written a word at a
time left to right on each row, with the rows being output top to
bottom.  Within each word, the most significant bit is the left-most
on the screen.

     The standard color form uses a storage method called "color
planes". The high-order bits for all of the pixels are stored just as
for monochrome, followed by the next-lowest bit in another contiguous
block, and so on until all of the necessary color bits have been
stored.

     For example, on a 16-color system, there would be four different
planes.  The color of the upper-leftmost bit in the form would be
determined by concatenating the high-order bit in the first word of
each plane of the form.

     The system dependent form for the ST's monochrome mode is very
simple: it is identical to the standard form!  This occurs because
the  ST uses a "reverse-video" setup in monochrome mode, with the
background  set to white.

     The video organization of the ST's color modes is more
complicated. It uses an "interleaved plane" system to store the bits
which make up a pixel.  In the low-resolution mode, every four words
define the values of 16 pixels.  The high-order bits of the four
words are merged to form the left-most pixel, followed by the next
lower bit of each word, and so on.  This method is called
interleaving because the usually separate color planes described
above have been shuffled together in memory.

     The organization of the ST's medium-resolution mode is similar
to low-res, except the only two words are taken at a time.  These are
merged to create the two bits needed to address four colors.

     You should note that the actual color produced by a particular
pixel value is NOT fixed.  The ST uses a color remapping system
called a palette.  The pixel value in memory is used to address a
hardware  register in the palette which contains the actual RGB
levels to be sent to the display.  Programs may set the palette
registers with  BIOS calls, or the user may alter its settings with
the Control Panel desk accessory.  Generally, palette zero
(background) is left as white, and the highest numbered palette is
black.

                      EVEN-WORD VS. FRINGES

     A form always begins on a word boundary, and is always stored
with an integral number of words per row.  However,  it is possible
to use only a portion of the final word.  This partial word is called
a "fringe".  If, for instance, you had a form 40 pixels wide,  it
would be stored with four words per row: three whole words, and one
word with the eight pixel fringe in its upper byte.

                              MFDBs

     Now we can intelligently define the elements of the MFDB.  Its
exact C structure definition will be found in the download. The
fd_nplanes entry determines the color scheme: a value of one is
monochrome, more than one denotes a color form.  If fd_stand is zero,
then the form is device-specific, otherwise it is in standard format.

     The fd_w and fd_h fields contain the pixel width and height of
the form respectively.  Fd_wdwidth is the width of a row in words.
If fd_w is not exactly equal to sixteen times fd_wdwidth, then the
form has a fringe.

     Finally, fd_addr is the 32-bit memory address of the form
itself. Zero is a special value for fd_addr.  It denotes that this
MFDB is for the video memory itself.  In this case, the VDI
substitutes the actual address of the screen, and it ignores ALL of
the other parameters. They are replaced with the size of the whole
screen and number of planes in the current mode, and the form is (of
course) in device-specific format.

     This implies that any MFDB which points at the screen can only
address the entire screen.  This is not a problem, however, since the
the VDI raster calls allow you to select a rectangular region within
the form.  (A note to advanced programmers:  If this situation is
annoying, you can retrieve the address of the ST's video area from
low memory, add an appropriate offset, and substitute it into the
MFDB yourself to address a portion of the screen.)

                          LET'S OPERATE

     Now we can look at the VDI raster operations themselves.  There
are actually three: transform form, copy raster opaque, and copy
raster transparent.  Both copy raster functions can perform a variety
of logic operatoins during the copy.

                          TRANSFORM FORM

     The purpose of this operation is to change the format of a form:
from standard to device-specific, or vice-versa.  The calling
sequence is:

     vr_trnfm(vdi_handle, source, dest);

where source and dest are each pointers to MFDBs.  They ARE allowed
to be the same.  Transform form checks the fd_stand flag in the
source MFDB, toggles it and writes it into the destination MFDB after
rewriting the form itself.  Note that transform form CANNOT change
the number of color planes in a form: fd_nplanes must be identical in
the two MFDBs.

     If you are writing an application to run on the ST only, you
will probably be able to avoid transform form entirely.  Images and
icons are stored within resources as standard forms, but since they
are monochrome, they will work "as is" with the ST.

     If you may want to move your program or picture files to another
GEM system, then you will need transform form.  Screen images can be
transformed to standard format and stored to disk.  Another system
with the same number of color planes could the read the files, and
transform the image to ITS internal format with transform form.

     A GEM application which will be moved to other systems needs to
contain code to transform the images and icons within its resource,
since standard and device-specific formats will not always coincide.

     If you are in this situation, you will find several utilities in
the download which you can use to transform G_ICON and G_IMAGE
objects. There is also a routine which may be used with map_tree()
from the  last column in order to transform all of the images and
icons in a resource tree at once.

                        COPY RASTER OPAQUE

     This operation copies all or part of the source form into the
destination form.  Both the source and destination forms must be in
device-specific form.  Copy raster opaque is for moving information
between "like" forms, that is, it can copy from monochrome to
monochrome, or between color forms with the same number of planes.
The calling format is:

     vro_cpyfm(vdi_handle, mode, pxy, source, dest);

  As above, the source and dest parameters are pointers to MFDBs
(which in turn point to the actual forms).  The two MFDBs may point
to memory areas  which overlap.  In this case, the VDI will perform
the move in a  non-destructive order.  Mode determines how the pixel
values in the source and destination areas will be combined.  I will
discuss it separately later on.

     The pxy parameter is a pointer to an eight-word integer array.
This array defines the area within each form which will be affected.
Pxy[0] and pxy[1] contain, respectively, the X and Y coordinates of
the upper left corner of the source rectangle.  These are given as
positive  pixel displacements from the upper left of the form.
Pxy[2] and pxy[3]  contain the X and Y displacements for the lower
right of the source rectangle.

     Pxy[4] through pxy[7] contain the destination rectangle in the
same format.  Normally, the destination and source should be the same
size.  If not, the size given for the source rules, and the whole are
is transferred beginning at the upper left given for the destination.

     This all sounds complex, but is quite simple in many cases.
Consider an example where you want to move a 32 by 32 pixel area
from one part of the display to another.  You would need to allocate
only one MFDB, with a zero in the fd_addr field.  The VDI will take
care of counting color planes and so on.  The upper left raster
coordinates of the source and destination rectangles go into pxy[0],
pxy[1] and pxy[4], pxy[5] respectively.  You add 32 to each of these
values and insert the results in the corresponding lower right
entries, then make the copy call using the same MFDB for both source
and destination.  The VDI takes care of any overlaps.

                     COPY RASTER TRANSPARENT

     This operation is used for copying from a monochrome form to a
color form.  It is called transparent because it "writes through" to
all of the color planes.  Again, the forms need to be in
device-specific form.  The calling format is:

     vrt_cpyfm(vdi_handle, mode, pxy, source, dest, color);

  All of the parameters are the same as copy opaque, except that
color has been added.  Color is a pointer to a two word integer
array. Color[0] contains the color index which will be used when a
one appears in the source form, and color[1] contains the index for
use when a zero occurs.

     Incidentally, copy transparent is used by the AES to draw
G_ICONs and G_IMAGEs onto the screen.  This explains why you do not
need to convert them to color forms yourself.

     (A note for advanced VDI programmers: The pxy parameter in both
copy opaque and transparent may be given in normalized device
coordinates (NDC) if the workstation associated with vdi_handle was
opened for NDC work.)

                        THE MODE PARAMETER

     The mode variable used in both of the copy functions is an
integer with a value between zero and fifteen. It is used to select
how the copy function will merge the pixel values of the source and
destination forms.  The complete table of functions is given in the
download.  Since a number of these are of obscure or questionable
usefulness, I will only discuss the most commonly used modes.

                           REPLACE MODE

     A mode of 3 results in a straight-forward copy: every
destination pixel is replaced with the corresponding source form
value.

                            ERASE MODE

     A mode value of 4 will erase every destination pixel which
corresponds to a one in the source form.  (This mode corresponds to
the "eraser" in a Paint program.)  A mode value of 1 will erase every
destination pixel which DOES NOT correspond to a one in the source.

                             XOR MODE

     A mode value of 6 will cause the destination pixel to be toggled
if the corresponding source bit is a one. This operation is
invertable, that is, executing it again will reverse the effects.
For this reason it is often used for "software sprites" which must be
shown and then removed from the screens.  There are some problems
with this in color operations, though - see below.

                         TRANSPARENT MODE

     Don't confuse this term with the copy  transparent function
itself.  In this case it simply means that ONLY  those destination
pixels corresponding with ones in the source form will  be modified
by the operation.  If a copy transparent is being performed, the
value of color[0] is substituted for each one bit in the source form.
A mode value of 7 selects transparent mode.

                     REVERSE TRANSPARENT MODE

     This is like transparent mode except that only those destination
pixels corresponding to source ZEROS are modified.  In a copy
transparent, the value of color[1] is substituted for each zero bit.
Mode 13 selects reverse transparent.

                       THE PROBLEM OF COLOR

     I have discussed the various modes as if they deal with one and
zero pixel values only.  This is exactly true when both forms are
monochrome, but is more complex when one or both are color forms.

     When both forms are color, indicating that a copy opaque is
being performed, then the color planes are combined bit-by-bit using
the rule for that mode.  That is, for each corresponding source and
destination pixel, the VDI extracts the top order bits and processes
them, then operates on the next lower bit, and so on, stuffing each
bit back into the destination form as the copy progresses.  For
example, an XOR operation on pixels valued 7 and 10 would result in a
pixel value of 13.

     In the case of a copy transparent, the situation is more
complex. The source form consists of one plane, and the destination
form has two  or more.  In order to match these up, the color[] array
is used.  Whenever a one pixel is found, the value of color[0] is
extracted and used in the bit-by-bit merge process described in the
last paragraph.  When a zero is found, the value of color[1] is
merged into the destination form.

     As you can probably see, a raster copy using a mode which
combines the source and destination can be quite complex when color
planes are used!  The situation is compounded on the ST, since the
actual color values may be remapped by the palette at any time.   In
many cases, just using black and white in color[] may achieve the
effects you desire.  If need to use full color, experimentation is
the  best guide to what looks good on the screen and what is garish
or  illegible.

                   OPTIMIZING RASTER OPERATIONS

     Because the VDI raster functions are extremely generalized, they
are also slower than hand-coded screen drivers which you might write
for your own special cases.  If you want to speed up your
application's raster operations without writing assembl language
drivers, the following hints will help you increase the VDI's
performance.

                       AVOID MERGED COPIES

     These are copy modes, such as XOR,  which  require that words be
read from the destination form.  This extra memory access increases
the running time by up to fifty percent.

                   MOVE TO CORRESPONDING PIXELS

     The bit position within a word of the destination rectangle
should correspond with the bit position of the source rectangle's
left edge.  For instance, if the source's left edge is one pixel in,
then the destination's edge could be at one, seventeen, thirty-three,
and so.  Copies which do not obey this rule force the VDI to shift
each word of the form as it is moved.

                          AVOID FRINGES

     Put the left edge of the source and destination  rectangles on
an even word boundary, and make their widths even multiples  of
sixteen.  The VDI then does not have to load and modify partial words
within the destination forms.

                        USE ANOTHER METHOD

     Sometimes a raster operation is not the fastest way to
accomplish your task.  For instance, filling a rectangle with zeros
or ones may be accomplished by using raster copy modes zero and
fifteen, but it is faster to use the VDI v_bar function instead.
Likewise, inverting an area on the screen may be done more quickly
with v_bar by using BLACK in XOR mode.  Unfortunately, v_bar cannot
affect  memory which is not in the video map, so these alternatives
do not always work.

                         FEEDBACK RESULTS

     The results of the poll on keeping or  dropping the use of
portability macros are in.  By a slim margin, you have voted to keep
them.  The vote was close enough that in future columns I will try to
include ST-only versions of routines which make  heavy use of the
macros.  C purists and dedicated Atarians may then use the alternate
code.

                        THE NEXT QUESTION

     This time I'd like to ask you to drop by the Feedback Section
and tell me whether the technical level of the columns has been:

     A)  Too hard!  Who do you think we are, anyway?
     B)  Too easy!  Don't underestimate Atarians.
     C)  About right, on the average.

     If you have the time, it would also help to know a little about
your background, for instance, whether you are a professional
programmer, how long you have been computing, if you owned an 8-bit
Atari, and so on.

                          COMING UP SOON

     The next column will deal with GEM menus: How they are
constructed, how to decipher menu messages, and how to change menu
entries at run-time.  The following issue will contain more feedback
response, and a discussion on designing user interfaces for GEM
programs.