GEMini Atari/GEMini_Atari_CD-ROM_Walnut_Creek_December_1993.iso/files/program/progem/gem9.asc

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                        *PROFESSIONAL GEM*
                           By Tim Oren
            Column #9 - VDI Graphics: Lines and Solids

     This  issue  of ST PRO GEM is the first in a  series  of  two
which  will explore the fundamentals of VDI graphics  output.   In
this installment, we will take a look at the commands necessary to
output simple graphics such as lines,  squares and circles as well
as  more complex figures such as polygons.   The following episode
will  take a first look at graphics text output,  with an emphasis
on  ways  to  optimize its drawing speed.   It will  also  include
another  installment  of ONLINE Feedback.   As usual,  there is  a
download  with  this column.   You should find it under  the  name
GEMCL9.C in DL3 of ATARI16 (PCS-58).

                        A  BIT  OF HISTORY

    One  of  the reasons that the VDI can  be  confusing  is  that
drawing anything at all,  even a simple line,  can involve setting
around four different VDI parameters before making the draw  call!
(Given  the state of the GEM documents,  just FINDING them can  be
fun!)  Looking backwards a bit sheds some light on why the VDI  is
structured this way,  and also gives us a framework for organizing
a discussion of graphics output.

     The GEM VDI closely follows the so-called GKS standard, which
defines  capabilities  and calling sequences  for  a  standardized
graphic  input/output system.   GKS is itself an evolution from an
early system called "Core".   Both of these standards were born in
the  days  when  pen plotters,  vectored  graphics  displays,  and
minicomputers  were  the  latest items.   So,  if you  wonder  why
setting  the drawing pen color is a separate command,  just  think
back  a  few  years when it actually meant  what  it  says!   (The
cynical   may   choose   instead  to  ponder   the   benefits   of
standardization.)

     When  doing  VDI  output,  it helps if you pretend  that  the
display screen really is a plotter or some other separate  device,
which  has  its own internal parameters which you can set  up  and
read  back.   The class of VDI commands called Attribute Functions
let  you set the parameters.   Output Functions cause the "device"
to  actually  draw  someone once it is  configured.   The  Inquire
Functions let you read back the parameters if necessary.

     There  are two parameters which are relevant no  matter  what
type of object you are trying to draw.   They are the writing mode
and  the clipping rectangle.   The writing mode is similar to that
discussed in the column on raster operations.   It determines what
effect the figure you are drawing will have on data already on the
screen.  The writing mode is set with the call:

          vswr_mode(vdi_handle, mode);

   (Vdi_handle,  here  and  below,  is the  handle  obtained  from
graf_handle at the beginning of the program.  Mode is a word which
may be one of:

          1 - Replace Mode
          2 - Transparent Mode
          3 - XOR mode
          4 - Reverse Transparent Mode

     In  replace mode,  whatever is on the screen is  overwritten.
If  you are writing characters,  this means the background of each
character cell will be erased.

     In  transparent  mode,  only  the pixels directly  under  the
"positive"  part  of the image,  that is,  where 1-bits are  being
written, will be changed.  When writing characters, the background
of the cell will be left intact.

     In XOR mode,  an exclusive or is performed between the screen
contents and what is being written.   The effect is to reverse the
image under areas where a 1-bit occurs.

     Reverse transparent is like transparent,  but with a "reverse
color  scheme".   That  is,  only  places where a 0-bit is  to  be
put   are  changed  to  the  current  writing  color.    When  you
write  characters in reverse transparent (over white),  the effect
is reverse video.

     The  other  common parameter is the clipping  rectangle.   It
defines the area on the screen where the VDI is permitted to draw.
Any output which would fall outside of this area is ignored; it is
effectively a null operation.   The clip rectangle is set with the
call:

          vs_clip(vdi_handle, flag, pxy);

   Pxy is a four-word array.   Pxy[0] and pxy[1] are the X  and  Y
screen coordinates,  respectively,  of one corner of your clipping
rectangle.    Pxy[2]  and  pxy[3]  are  the  coordinates  of   the
diagonally opposite corner of the rectangle.   (When working  with
the  AES,  use  of  a  GRECT to define  the  clip  is  often  more
convenient.  The routine set_clip() in the download does this.)

     Flag is set to TRUE if clipping is to be used.  If you set it
to  FALSE,  the  entire  screen is assumed to be fair  game.

     Normally,  you should walk the rectangle list for the current
window to obtain your clipping rectangles.  (See ST PRO GEM #2 for
more details.)  However, turning off the clip speeds up all output
operations,  particularly text.  You may do this ONLY when you are
absolutely certain that the figure you are drawing will not extend
out of the top-most window, or out of a dialog.

                    THE LINE FORMS ON THE LEFT

     The  VDI  line  drawing  operations  include  polyline,  arc,
elliptical  arc,  and rounded rectangle.   I'll first look at  the
Attribute Functions for line drawing,  then go through the drawing
primitives themselves.

     The  most  common used line attributes are color  and  width.
The color is set with:

          vsl_color(vdi_handle, color);

   where color is one of the standard VDI color  indices,  ranging
from  zero to 15.   (As discussed in column #6,  the  color  which
actually appears will depend on the pallette setting of your ST.)

     The  line width may only be set to ODD positive  values,  for
reasons  of  symmetry.   If you try to set an even value,  the VDI
will take the next lower odd value.  The call is:

          vsl_width(vdi_handle, width);

     The  two  less  used line parameters are the  end  style  and
pattern.   With  the  end style you can cause the output  line  to
have rounded ends or arrowhead ends.  The call is:

          vsl_ends(vdi_handle, begin_style, end_style);

   Begin_style  and end_style are each words which  may  have  the
values zero for square ends (the default),  one for arrowed  ends,
or  two  for  rounded ends.   They determine the  styles  for  the
starting and finishing ends of the line, respectively.

     The line pattern attribute can select dotted or dashed  lines
as  well  as more complicated patterns.   Before  continuing,  you
should  note one warning:  VDI line output DOES NOT compensate for
pixel aspect ratio.  That is, the dashes on a line will look twice
as long drawn vertically on a medium-res ST screen as they do when
drawn horizontally.  The command for setting the pattern is:

          vsl_type(vdi_handle, style);

   Style  is  a word with a value between 1  and  7.   The  styles
selected are:

          1 - Solid (the default)
          2 - Long Dash
          3 - Dot
          4 - Dash, Dot
          5 - Dash
          6 - Dash, Dot, Dot
          7 - (User defined style)

   The  user  defined  style is determined  by  a  16-bit  pattern
supplied  by the application.   A one bit in the pattern  turns  a
pixel on, a zero bit leaves it off.  The pattern is cycled through
repeatedly,  using the high bit first.  To use a custom style, you
must make the call:

          vsl_udsty(vdi_handle, pattern);

  before doing vsl_type().

     As  I  mentioned  above,   the  line  type  Output  Functions
available are polyline,  circular and ellliptical arc, and rounded
rectangle.   Each  has  its own calling sequence.   The call for a
polyline is:

          v_pline(vdi_handle, points, pxy);

  Points tells how many vertices will appear on the polyline.  For
instance,  a  straight  line has two vertices:  the  end  and  the
beginning.   A closed square would have five,  with the first  and
last  identical.    (There  is  no  requirement  that  the  figure
described be closed.)

     The pxy array contains the X and Y raster coordinates for the
vertices,  with a total of 2 * points entries.   Pxy[0] and pxy[1]
are the first X-Y pair, and so on.

     If you happen to be using the XOR drawing mode, remember that
drawing  twice  at  a point is equivalent to no  drawing  at  all.
Therefore,  for  a figure to appear closed in XOR mode,  the final
stroke  should actually stop one pixel short of the origin of  the
figure.

     You  may  notice  that  in the GEM  VDI  manual  the  rounded
rectangle  and arc commands are referred to as  GDPs  (Generalized
Drawing Primitives).  This denotation is historical in nature, and
has  no effect unless you are writing your own VDI bindings.

     The  rounded rectangle is nice to use for customized  buttons
in  windows and dialogs.   It gives a "softer" look to the  screen
than the standard square objects.  The drawing command is:

          v_rbox(vdi_handle, pxy);

   Pxy  is  a  four word array  giving  opposite  corners  of  the
rectangle,  just as for the vs_clip() call.   The corner  rounding
occurs  within  the  confines of  this  rectangle.   Nothing  will
protrude  unless  you specify a line thickness greater  than  one.
The  corner rounding is approximately circular;  there is no  user
control over the degree or shape of rounding.

     Both the arc and elliptical arc commands use a curious method
of  specifying  angles.   The units are tenths of degrees,  so  an
entire  circle is 3600 units.   The count starts at ninety degrees
right of vertical, and proceeds counterclockwise.  This means that
"3 o'clock" is 0 units,  "noon" is 900 units,  "9 o'clock" is 1800
units, and 2700 units is at "half-past".  3600 units take you back
to "3 o'clock".

     The command for drawing a circular arc is:

          v_arc(vdi_handle, x, y, radius, begin, end);

   X  and y specify the raster coordinates of the  center  of  the
circle.   Radius specifies the distance from center to all  points
on the arc.   Begin and end are angles given in units as described
above,  both with values between 0 and 3600.   The drawing of  the
arc  ALWAYS  proceeds  counterclockwise,   in  the  direction   of
increasing arc number.   So values of 0 and 900 for begin and  end
would  draw  a  quarter circle from  "three  o'clock"  to  "noon".
Reversing  the values would draw the other three quarters  of  the
circle.

     A  v_arc()  command  which specifies a  "full  turn"  is  the
fastest  way to draw a complete circle on the screen.   Be warned,
however,  that  the circle drawing algorithm used in the VDI seems
to  have  some  serious  shortcomings at  small  radii!   You  can
experiment  with  the  CIRCLE primitive in  ST  Logo,  which  uses
v_arc(), to see what I mean.

     Notice  that if you want an arc to strike one or more  given
points on the screen,  then you are in for some trigonometry.   If
your  math  is  a  bit  rusty,  I highly  recommend  the  book  "A
Programmer's  Geometry",  by  Bowyer  and Woodwark,  published  by
Butterworths (London, Boston, Toronto).

     Finally, the elliptical arc is generated with:

          v_ellarc(vdi_handle, x, y, xrad, yrad, begin, end);

 X,  y,  begin, and end are just as before.  Xrad and yrad give the
horizontal and vertical radii of the defining ellipse.  This means
that  the distance of the arc from center will be yrad  pixels  at
"noon"  and  "half-past",  and it will be xrad pixels at "3 and  9
o'clock".  Again, the arc is always drawn counterclockwise.

     There  are  a  number  of approaches  to  keeping  the  VDI's
attributes "in sync" with the actual output operations.   Probably
the  LEAST efficient is to use the Inquire Functions to  determine
the  current  attributes.   For  this  reason,  I have  omitted  a
discussion of these calls from this column.

     Another  idea  is  to keep a local copy  of  all  significant
attributes, use a test-before-set method to minimize the number of
Attribute  Functions which need to be called.   This puts a burden
on  the programmer to be sure that the local  attribute  variables
are correctly maintained.   Failure to do so may result in obscure
drawing  bugs.   If  your  application employs  user  defined  AES
objects, you must be very careful because GEM might call your draw
code  in the middle of a VDI operation (particularly if  the  user
defined objects are in the menu).

     Always  setting  the attributes is a simplistic  method,  but
often   proves  most  effective.    The  routines  pl_perim()  and
rr_perim()  in the download exhibit this  approach.   Modification
for  other  primitives  is straightforward.   This style  is  most
useful  when  drawing  operations  are  scattered  throughout  the
program,  so that keeping track of the current attribute status is
difficult.  Although inherently inefficient, the difference is not
very  noticable if the drawing operation requested is itself  time
consuming.

     In  many  applications,  such  as data graphing  programs  or
"Draw"  packages,  the output operations are centralized,  forming
the primary functionality of the code.   In this case,  it is both
easy  and  efficient  to keep track of  attribute  status  between
successive drawing operations.

                              SOLIDS

    There  are  a wider variety of VDI  calls  for  drawing  solid
figures.   They  include rectangle or  bar,  disk,  pie,  ellipse,
elliptical  pie,  filled rounded rectangle,  and filled  polygonal
area.   Of course,  filled figure calls also have their own set of
attributes which you will need to set.

     The  fill color index determines what pen color will be  used
to draw the solid.  It is set with:

          vsf_color(vdi_handle, color);

     Color is just the same as for line drawing.   A solid may  or
may not have a visible border.  This is determined with the call:

          vsf_perimeter(vdi_handle, vis);

   Vis is a Boolean.   If it is true,  the figure will be given  a
solid one pixel outline in the current fill color index.   This is
often  useful  to improve the appearance of solids  drawn  with  a
dithered fill pattern.  If vis is false, then no outline is drawn.

     There are two parameters which together determine the pattern
used  to  fill your figure.   They are called interior  style  and
interior  index.   The style determines the general type of  fill,
and the index is used to select a particular pattern if necessary.
The style is set with the command:

          vsf_interior(vdi_handle, style);

  where style is a value from zero through four.   Zero selects  a
hollow  style:  the  fill is performed in  color  zero,  which  is
usually  white.   Style one selects a solid fill with the  current
fill  color.   A style of two is called "pattern" and a  three  is
called "hatch", which are terms somewhat suggestive of the options
which can then be selected using the interior index.   Style  four
selects the user defined pattern, which is described below.

     The  interior  index is only significant for styles  two  and
three. To set it, use:

          vsf_style(vdi_handle, index);

   (Be careful here: it is very easy to confuse this call with the
one  above  due  to the unfortunate choice of  name.)   The  index
selects the actual drawing pattern.  The GEM VDI manual shows fill
patterns corresponding to index values from 1 to 24 under style 2,
and  from  1 to 12 under style 3.   However,  some  of  these  are
implemented  differently on the ST.   Rather than try to  describe
them all here, I would suggest that you experiment.  You can do so
easily  in  ST Logo by opening the Graphics  Settings  dialog  and
playing with the style and index values there.

     The user defined style gives you some interesting options for
multi-color fills.  It is set with:

          vsf_udpat(vdi_handle, pattern, planes);

    Planes  determines the number of color planes in  the  pattern
which  you  supply.   It  is  set to one  if  you  are  setting  a
monochrome  pattern.   (Remember,  monochrome is  not  necessarily
black).   It may be set to higher values on color systems: two for
ST medium-res mode, or four for low-res mode.  If you use a number
lower than four under low-res, the other planes are zero filled.

     The  pattern  parameter  is  an array of  words  which  is  a
multiple  of  16 words long.  The pattern determined is 16  by  16
pixels,  with each word forming one row of the pattern.   The rows
are  arranged top to bottom,  with the most significant bit to the
left.   If  you  have selected a multi-plane pattern,  the  entire
first plane is stored, then the second, and so on.

       Note that to use a multi-plane pattern, you set the writing
mode  to replace using vswr_mode().   Since the each plane can  be
different,  you can produce multi-colored patterns.   If you use a
writing   color  other  than  black,   some  of  the  planes   may
"disappear".

     Most  of  the  solids Output Functions  have  analogous  line
drawing commands.   The polyline command corresponds to the filled
area primitive.  The filled area routine is:

          v_fillarea(vdi_handle, count, pxy);

    Count  and pxy are just the same as  for  v_pline().   If  the
polygon  defined  by pxy is not closed,  then the VDI  will  force
closure  with  a straight line from the last to the  first  point.
The  polygon may  be concave or self-intersecting.   If  perimeter
show is on, the area will be outlined.

     One  note of caution is necessary for both  v_fillarea()  and
v_pline().   There is a limit on the number of points which may be
stored in pxy[].   This limit occurs because the contents of pxy[]
are copied to the intin[] binding array before the VDI is  called.
You  can  determine  the maximum number of  vertices  by  checking
intout[14] after using the extended inquire function vq_extnd().

     For  reasons unknown to this writer,  there are TWO different
filled rectangle commands in the VDI.  The first is

          vr_recfl(vdi_handle, pxy);

    Pxy is a four word array defining two opposite corners of  the
rectangle,  just  as  in  vs_clip().   Vr_recfl()  uses  the  fill
attribute settings, except that it NEVER draws a perimeter.

     The other rectangle routine is v_bar(), with exactly the same
arguments  as  vr_recfl().    The  only  difference  is  that  the
perimeter  setting  IS  respected.   These two  routines  are  the
fastest way to produce a solid rectangle using the VDI.   They may
be  used in XOR mode with a BLACK fill color to quickly invert  an
area  of  the screen.   You can improve the speed even further  by
turning off the clip (if possible), and byte aligning the left and
right edges of the rectangle.

     Separate commands are provided for solid circle and  ellipse.
The circle call is:

          v_circle(vdi_handle, x, y, radius);

   and the ellipse command is:

          v_ellipse(vdi_handle, x, y, xrad, yrad);

    All of the parameters are identical to those given  above  for
v_arc()  and v_ellarc().   The solid analogue of an arc is a  "pie
slice".  The VDI pie commands are:

          v_pieslice(vdi_handle, x, y, radius, begin, end);

  for a slice from a circular pie, and

          v_ellpie(vdi_handle, x, y, xrad, yrad, begin, end);

   for a slice from a "squashed" pie.   Again,  the parameters are
identical  to  those in v_arc() and  v_ellarc().   The  units  and
drawing  order  of angles are also the  same.   The  final  solids
Output Function is:

          v_rfbox(vdi_handle, pxy);

  which draws a filled rounded rectangle.   The pxy array  defines
two  two  opposite  corners of the  bounding  box,  as  shown  for
vs_clip().

     The  issues involved in correctly setting the VDI  attributes
for a fill operation are identical to those in drawing lines.  For
those  who  want to employ the "always set" method,  I have  again
included  two  skeleton  routines in the download,  which  can  be
modified as desired.

                         TO BE CONTINUED

   This concludes the first part of our expedition  through  basic
VDI  operations.   The  next  issue will tackle  the  problems  of
drawing  bit mapped text at a reasonable speed.   This first  pass
will  not attempt to tackle alternate or  proportional  fonts,  or
alternate font sizes.   Instead,  I will concentrate on techniques
for  squeezing greater performance out of the standard  monospaced
system fonts.