C++ Games Programming/CPPGAMES.ISO/fgl/fglight/manuals.arj/USER12.DOC

Text File | 1995-02-06 | 26KB | 654 lines

Chapter 12





Animation Techniques
274   Fastgraph User's Guide


Overview

     Unlike other microcomputers, the IBM PC and PS/2 family of systems do not
have any special graphics hardware or firmware to help in performing
animation. This means any animation done on these systems must be implemented
entirely through software. This chapter will show how to do this using
Fastgraph's video page management, image display, and block transfer routines.
The methods described in this chapter are not intended to be all inclusive,
for that would itself fill a separate volume at least as large as this manual.
However, the animation techniques presented here should provide a basis that
you can readily extend to develop more sophisticated uses of animation. The
examples in this chapter are restricted to graphics video modes.


Simple Animation

     The first type of animation we'll examine is called simple animation. In
simple animation, we display an object, erase it, and then display it in a new
position. When we perform this "erase and redisplay" sequence repetitively,
the object moves. This method, however, has two drawbacks. First, unless the
object is rather small, it will flicker because the erasing and display of the
object does not coincide with the refresh rate of the video display. Second,
and perhaps more importantly, anything underneath the object is not saved as
the object moves across it. Despite these limitations, simple animation is
sometimes useful, and it is a good place to begin our discussion of animation
techniques.

     Example 12-1 moves a small bright green rectangle (magenta in CGA) from
left to right across the screen in any 320x200 color graphics mode. The
program moves the rectangle, 20 pixels wide and 10 pixels high, using a for
loop. This loop first uses fg_clprect to display the rectangle, then uses
fg_waitfor to leave the object on the screen momentarily, and finally uses
fg_clprect again to erase the rectangle by redisplaying it in the original
background color (fg_waitfor is described in Chapter 16). We use fg_clprect
rather than fg_rect because the first few and last few loop iterations result
in at least part of the rectangle being off the screen. Each successive loop
iteration displays the rectangle five pixels to the right of its previous
position.

                                 Example 12-1.

          #include <fastgraf.h>
          #include <stdio.h>
          #include <stdlib.h>
          void main(void);

          void main()
          {
             int new_mode, old_mode;
             int x;

             /* initialize the video environment */

             fg_initpm();
             new_mode = fg_bestmode(320,200,1);
                                       Chapter 12:  Animation Techniques   275

             if (new_mode < 0 || new_mode == 12) {
                printf("This program requires a 320 ");
                printf("x 200 color graphics mode.\n");
                exit(1);
                }
             old_mode = fg_getmode();
             fg_setmode(new_mode);

             /* move the object across the screen */

             for (x = -20; x < 320; x+=5) {
                fg_setcolor(10);
                fg_clprect(x,x+19,95,104);
                fg_waitfor(1);
                fg_setcolor(0);
                fg_clprect(x,x+19,95,104);
                }

             /* restore the original video mode and return to DOS */

             fg_setmode(old_mode);
             fg_reset();
          }


     Example 12-2 is the same as example 12-1, but it shows what happens when
we move the rectangle across an existing background (in this case, the
background is solid white). If you run this program, you'll see that the
rectangle leaves a trail of color 0 pixels behind it. While this might be
occasionally useful, it demonstrates that simple animation is destructive
because it does not preserve the background. In this example, if we changed
the second call to fg_setcolor within the for loop to revert to color 15
instead of color 0, the background would be restored. In general, though, it
may not be this easy to replace the background, so we must rely on some other
method for preserving it.

                                 Example 12-2.

          #include <fastgraf.h>
          #include <stdio.h>
          #include <stdlib.h>
          void main(void);

          void main()
          {
             int new_mode, old_mode;
             int x;

             /* initialize the video environment */

             fg_initpm();
             new_mode = fg_bestmode(320,200,1);
             if (new_mode < 0 || new_mode == 12) {
                printf("This program requires a 320 ");
                printf("x 200 color graphics mode.\n");
                exit(1);
276   Fastgraph User's Guide

                }
             old_mode = fg_getmode();
             fg_setmode(new_mode);

             /* draw some type of background */

             fg_setcolor(15);
             fg_rect(0,319,0,199);

             /* move the object across the screen */

             for (x = -20; x < 320; x+=5) {
                fg_setcolor(10);
                fg_clprect(x,x+19,95,104);
                fg_waitfor(1);
                fg_setcolor(0);
                fg_clprect(x,x+19,95,104);
                }

             /* restore the original video mode and return to DOS */

             fg_setmode(old_mode);
             fg_reset();
          }


     To summarize, we've seen that simple animation is easy to implement, but
it is destructive and typically causes the animated object to flicker. For
these reasons, it is not used too frequently.


XOR Animation

     "Exclusive or" animation, or XOR animation for short, is an interesting
extension of simple animation and is most useful when animating a single-color
object against a single-color background. Like simple animation, it uses the
"erase and redisplay" technique to move an object, but it does this
differently. Instead of erasing the object by displaying it in the background
color, XOR animation does so by displaying it in the same color using an
exclusive or, or XOR, operation. This method relies on a specific property of
the exclusive or operator:

                (object XOR background) XOR object = background

In other words, if you XOR something twice in the same position, the result is
the same as the original image in that position.

     Example 12-3 demonstrates XOR animation. This program is similar to
example 12-2, but it only runs in the 320x200 EGA graphics mode (mode 13).
After establishing the video mode, it uses the Fastgraph routine fg_setfunc to
select XOR mode. This causes any subsequent graphics output to be XORed with
the contents of video memory instead of just replacing it. The fg_setfunc
routine is described further in Chapter 17.

     The other differences between examples 12-3 and 12-2 are that the call to
fg_setcolor has been moved outside the for loop, and that fg_setcolor takes a
                                       Chapter 12:  Animation Techniques   277

different value. Since the existing background is bright white (color 15), we
can't just use color 10 if we want to display a bright green object. The
desired value is that which when XORed with color 15 produces color 10; the
easiest way to obtain this value is to XOR these two numbers. The call to
fg_setcolor can be moved outside the loop because we display the object using
the same color index throughout.

                                 Example 12-3.

          #include <fastgraf.h>
          #include <stdio.h>
          #include <stdlib.h>
          void main(void);

          void main()
          {
             int old_mode;
             int x;

             /* initialize the video environment */

             fg_initpm();
             if (fg_testmode(13,1) == 0) {
                printf("This program requires EGA.\n");
                exit(1);
                }
             old_mode = fg_getmode();
             fg_setmode(13);
             fg_setfunc(3);

             /* draw some type of background */

             fg_setcolor(15);
             fg_rect(0,319,0,199);

             /* move the object across the screen */

             fg_setcolor(10^15);
             for (x = -20; x < 320; x+=5) {
                fg_clprect(x,x+19,95,104);
                fg_waitfor(1);
                fg_clprect(x,x+19,95,104);
                }

             /* restore the original video mode and return to DOS */

             fg_setmode(old_mode);
             fg_reset();
          }


     Fastgraph only supports the XOR pixel operation in the native EGA and VGA
graphics video modes (modes 13 through 18) and 16-color SVGA modes (28 and
29). Thus, you cannot use XOR animation in CGA, Tandy/PCjr, Hercules, or 256-
color graphics modes.
278   Fastgraph User's Guide

     While XOR animation is non-destructive (that is, it restores the original
background), it still suffers from the flickering encountered in simple
animation. In spite of this, it may be useful when animating a single-color
object against a single-color background.


Static Frame Animation

     Static frame animation uses a different strategy than simple animation or
XOR animation. The general scheme of this method is to create the entire
animation sequence off-screen and then successively display each item, or
frame, in this sequence on one position of the visual video page. This results
in a visually appealing animation that is non-destructive and does not include
the flickering associated with simple animation and XOR animation. Static
frame animation requires the visual video page and one or more additional
pages (or virtual buffers) to implement. The number of pages needed depends on
the number of frames and the size of each frame.

     Example 12-4 runs in any 320x200 color graphics video mode and
illustrates a simple use of static frame animation. The program displays an
animation sequence containing 12 frames; it displays this sequence three
times. The animation sequence consists of a bright green rectangle (magenta in
CGA) moving from left to right across the center of the frame. Each frame is
96 pixels wide and 50 pixels high. The 12 frames are set up on an off-screen
video page as shown here:

                         0       95 96     191 192    287

                      0
                          frame 1    frame 2    frame 3
                     49

                     50
                          frame 4    frame 5    frame 6
                     99

                    100
                          frame 7    frame 8    frame 9
                    149

                    150
                          frame 10   frame 11   frame 12
                    199


     Example 12-4 first establishes the video mode and allocates the
additional video page (needed if using a video mode in which page 1 is a
virtual video page). The program then generates the background for frame 1;
the background is a blue rectangle (cyan in CGA) with a white ellipse centered
on it. After the call to fg_ellipse, the first frame is ready.

     The next step is to create the remaining 11 frames. In frame 2, the right
half of the 20-pixel wide rectangle will enter the left edge of the frame. In
frame 3, the rectangle will be ten pixels farther right, or aligned against
the left edge of the frame. In frames 4 through 12, the rectangle will be ten
pixels farther right in each frame, so by frame 12 only the left half of the
                                       Chapter 12:  Animation Techniques   279

rectangle appears on the right edge of the frame. The first for loop in the
program builds frames 2 through 12 by copying the background from frame 1 and
then displaying the rectangle (that is, the animated object) in the proper
position for that frame.

     The second for loop performs the animation sequence. To display the 12-
frame sequence three times, it must perform 36 iterations. The loop simply
copies each frame from the proper position on video page 1 to the middle of
the visual video page. Note how fg_waitfor is used to pause momentarily
between frames.

                                 Example 12-4.

        #include <fastgraf.h>
        #include <stdio.h>
        #include <stdlib.h>
        void main(void);

        #define VISUAL 0
        #define HIDDEN 1

        int xmin[] = {  0, 96,192,  0, 96,192,  0, 96,192,  0, 96,192};
        int ymax[] = { 49, 49, 49, 99, 99, 99,149,149,149,199,199,199};

        void main()
        {
           int new_mode, old_mode;
           int frame, offset;
           int i, x, y;

           /* initialize the video environment */

           fg_initpm();
           new_mode = fg_bestmode(320,200,2);
           if (new_mode < 0 || new_mode == 12) {
              printf("This program requires a 320 ");
              printf("x 200 color graphics mode.\n");
              exit(1);
              }
           old_mode = fg_getmode();
           fg_setmode(new_mode);
           fg_allocate(HIDDEN);

           /* draw the background in the upper left corner */

           fg_setpage(HIDDEN);
           fg_setcolor(1);
           fg_rect(0,95,0,49);
           fg_setcolor(15);
           fg_move(48,25);
           fg_ellipse(20,20);

           /* display the animated object against each background */

           fg_setcolor(10);
           offset = -10;
280   Fastgraph User's Guide

           for (i = 1; i < 12; i++) {
              x = xmin[i];
              y = ymax[i];
              fg_transfer(0,95,0,49,x,y,HIDDEN,HIDDEN);
              fg_setclip(x,x+95,0,199);
              fg_clprect(x+offset,x+offset+19,y-29,y-20);
              offset += 10;
              }

           /* slide the object across the background three times */

           for (i = 0; i < 36; i++) {
              frame = i % 12;
              x = xmin[frame];
              y = ymax[frame];
              fg_transfer(x,x+95,y-49,y,112,124,HIDDEN,VISUAL);
              fg_waitfor(2);
              }

           /* restore the original video mode and return to DOS */

           fg_freepage(HIDDEN);
           fg_setmode(old_mode);
           fg_reset();
        }




Dynamic Frame Animation

     Dynamic frame animation is similar to static frame animation, but all the
animation frames are built as needed during the animation sequence instead of
in advance. When using this method, you must first store a copy of the
background on an off-screen video page. Then, to build a frame, create another
copy (called the workspace) of the background elsewhere on the off-screen page
(or even to a different off-screen page) and display the object on that copy.
Finally, transfer the workspace to the visual page. Like static frame
animation, this method produces a non-destructive, flicker-free animation
sequence.

     Example 12-5 is functionally identical to example 12-4, but it uses
dynamic rather than static frame animation. As before, the program builds the
background in the upper left corner of video page 1, but it then uses
fg_transfer to copy it to the center of the visual video page. The for loop
builds each frame as it is needed and also copies it to the center of the
visual page. Again, fg_waitfor creates the necessary pause between frames.

                                 Example 12-5.

          #include <fastgraf.h>
          #include <stdio.h>
          #include <stdlib.h>
          void main(void);

          #define VISUAL 0
                                       Chapter 12:  Animation Techniques   281

          #define HIDDEN 1

          void main()
          {
             int new_mode, old_mode;
             int frame, offset;
             int i;

             /* initialize the video environment */

             fg_initpm();
             new_mode = fg_bestmode(320,200,2);
             if (new_mode < 0 || new_mode == 12) {
                printf("This program requires a 320 ");
                printf("x 200 color graphics mode.\n");
                exit(1);
                }
             old_mode = fg_getmode();
             fg_setmode(new_mode);
             fg_allocate(HIDDEN);

             /* draw the background in the upper left corner */

             fg_setpage(HIDDEN);
             fg_setcolor(1);
             fg_rect(0,95,0,49);
             fg_setcolor(15);
             fg_move(48,25);
             fg_ellipse(20,20);

             /* copy it to the center of the visual page */

             fg_transfer(0,95,0,49,112,124,HIDDEN,VISUAL);

             /* slide the object across the background three times */

             fg_setcolor(10);
             for (i = 0; i < 36; i++) {
                frame  = i % 12;
                offset = 10 * frame - 10;
                fg_transfer(0,95,20,29,112,105,HIDDEN,HIDDEN);
                fg_rect(112+offset,131+offset,96,105);
                fg_transfer(112,207,96,105,112,105,HIDDEN,VISUAL);
                fg_waitfor(2);
                }

             /* restore the original video mode and return to DOS */

             fg_freepage(HIDDEN);
             fg_setmode(old_mode);
             fg_reset();
          }


     Two items in example 12-5 merit further discussion. First, we have chosen
our workspace on page 1 so it uses the same screen space coordinates as the
282   Fastgraph User's Guide

image area on the visual page. This is not necessary unless you are using
fg_restore instead of fg_transfer. Second, the program can use the faster
fg_rect routine in place of fg_clprect. It can do this because even though the
object will extend beyond the workspace limits, we only transfer the workspace
itself. However, for this to function properly, the workspace's horizontal
limits must fall on byte boundaries.

     Note too that we do not need to transfer the entire frame during the
animation sequence. In example 12-5, we know the vertical extremes of the
moving image are y=96 and y=105, so we only transfer 10 rows instead of the
entire frame. We could similarly compute the x extremes for each frame and
only transfer the necessary portion. Recall, however, that fg_transfer extends
the horizontal coordinates to byte boundaries, so we may copy a few extra
pixels as well. This may or may not affect the animation sequence. Again, the
problem is eliminated if you align your workspace on byte boundaries.

     When we use dynamic frame animation, it is easy to change the number of
frames in the animation sequence. Suppose we wish to produce a smoother
animation by increasing the number of frames from 12 to 24. This means the
object will move in increments of five pixels instead of ten. The only changes
needed are to double the number of loop iterations, modify the calculations
for the frame number and offset values as shown here, and reduce the
fg_waitfor pause from 2 to 1.


                           frame  = i % 24;
                           offset = 5 * frame - 10;


Compare this to all the changes that would be necessary if we were using
static frame animation.


Page Flipping

     Page flipping is a variation of frame animation in which you construct
images on off-screen video pages and then repetitively make those pages the
visual page. We can further divide the page flipping technique into static and
dynamic variants, as we did with frame animation.

     In static page flipping, we construct the entire animation sequence in
advance, with one frame per video page. Once this is done, we can display each
frame by using fg_setvpage to switch instantly from one video page to another.
Although this produces a smooth, flicker-free animation, we cannot carry the
sequence very far before running out of video pages (and hence animation
frames).

     In dynamic page flipping, we construct each animation frame when it is
needed. As in static page flipping, we construct each frame on a separate
video page. However, as example 12-6 demonstrates, we only need three video
pages to produce the animation sequence, regardless of the number of frames in
the sequence. Two of the three video pages will alternate as the visual page,
while the remaining video page keeps a copy of the background.

     Example 12-6, which performs an animation sequence similar to examples
12-4 and 12-5, illustrates dynamic frame animation in the 320x200 EGA graphics
                                       Chapter 12:  Animation Techniques   283

video mode (mode 13). The program begins by displaying the background on video
page 2. Video pages 0 and 1 will alternate as the visual page; the page that
is not the visual page is called the hidden page. We start with page 0 as the
visual page, and hence page 1 as the hidden page. To build each frame, the
program uses fg_transfer to copy the background from page 2 to the hidden page
and then uses fg_clprect to display the animated object at the correct
position on the hidden page. After this, it displays the next frame by using
fg_setvpage to make the hidden page the visual page. Before beginning the next
iteration, the program toggles the hidden page number in preparation for the
next frame.

                                 Example 12-6.

          #include <fastgraf.h>
          #include <stdio.h>
          #include <stdlib.h>
          void main(void);

          void main()
          {
             int old_mode;
             int hidden;
             int x;

             /* initialize the video environment */

             fg_initpm();
             if (testmode(fg_13,3) == 0) {
                printf("This program requires EGA.\n");
                exit(1);
                }
             old_mode = fg_getmode();
             fg_setmode(13);

             /* draw the background on page two */

             fg_setpage(2);
             fg_setcolor(1);
             fg_rect(0,319,0,199);
             fg_setcolor(15);
             fg_move(160,100);
             fg_ellipse(20,20);

             /* slide the object across the screen */

             hidden = 1;
             setcolor(10);
             for (x = -10; x < 320; x+=4) {
                fg_setpage(hidden);
                fg_transfer(0,319,0,199,0,199,2,hidden);
                fg_clprect(x,x+19,96,105);
                fg_setvpage(hidden);
                hidden = 1 - hidden;
                fg_waitfor(1);
                }
284   Fastgraph User's Guide

             /* restore the original video mode and return to DOS */

             fg_setmode(old_mode);
             fg_reset();
          }


     A problem with either page flipping technique arises if we use virtual
video pages. Page flipping relies on the fact that changing the visual page
number occurs instantly, which is exactly what happens when we use physical
video pages. However, such is not the case with virtual or logical pages
because Fastgraph must copy the entire page contents into video memory. While
this occurs quite rapidly, it is not instantaneous, and its effects are
immediately apparent on the animation.


An Animation Example: The Fastgraph Fish Tank

     If you installed the example programs when you installed Fastgraph, the
EXAMPLES subdirectory will include a fully-commented program called the fish
tank that illustrates dynamic frame animation. The fish tank is an excellent
example of multi-object non-destructive animation in which several types of
tropical fish swim back and forth against a coral reef background. As a
picture is worth 1,024 words, we suggest studying the fish tank source code
for some useful techniques in developing a complete animation program. The
source code for the fish tank program is in FISHTANK.C, FISHTANK.BAS,
FISHTANK.FOR, or FISHTANK.PAS, depending on what language support you've
installed with Fastgraph.


Summary of Animation Techniques

     This chapter has presented five animation techniques: simple animation,
XOR animation, static frame animation, dynamic frame animation, and page
flipping. The following table summarizes their behavior.

               technique     destructive?        flicker-free?

               simple             yes                 no
               XOR                no                  no
               static frame       no                  yes
               dynamic frame      no                  yes
               page flipping      no                  yes

Simple animation and XOR animation are elementary techniques that are seldom
used once you master frame animation and page flipping.

     As stated at the beginning of this chapter, the simple examples presented
here serve as the basis for understanding the mechanics of the animation
techniques we have discussed. In "real world" programs, you'll typically want
to display an image using the fg_drwimage or fg_drawmap family of routines
instead using rudimentary images such as the rectangles in our examples. A
helpful rule is to use PCX, GIF, or pixel run files for both backgrounds and
moving objects, and then use fg_getimage or fg_getmap to retrieve the moving
objects as bitmapped images for later display. Of course, it's desirable to do
this "behind the scenes" work on video pages other than the visual page. This
                                       Chapter 12:  Animation Techniques   285

is precisely the technique used in the Fastgraph fish tank.
286   Fastgraph User's Guide