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For those of you who haven’t experienced the frustrations of animation programming on a PC, there’s a whole lot of animation going on in Listing 43.1. What’s more, the animation is virtually flicker-free, partly thanks to bit-plane animation and partly because images are never really erased but rather are simply overwritten. (The principle behind the animation is that of redrawing each image with a blank fringe around it when it moves, so that the blank fringe erases the part of the old image that the new image doesn’t overwrite. For details on this sort of animation, see the above-mentioned PC Tech Journal July 1986 article.) Better yet, the red images take precedence over the green images, which take precedence over the blue images, which take precedence over the white backdrop, and all obscured images show through holes in and around the edges of images in front of them.

In short, Listing 43.1 accomplishes everything we wished for earlier in an animation technique.

If you possibly can, run Listing 43.1. The animation may be a revelation to those of you who are used to weak, slow animation on PCs with EGA or VGA adapters. Bit-plane animation makes the PC look an awful lot like—dare I say it?—a games machine.

Listing 43.1 was designed to run at the absolute fastest speed, and as I mentioned it puts in a pretty amazing performance on the slowest PCs of all. Assuming you’ll be running Listing 43.1 on an faster computer, you’ll have to crank up the DELAY equate at the start of Listing 43.1 to slow things down to a reasonable pace. (It’s not a very good game where all the pieces are a continual blur!) Even on something as modest as a 286-based AT, Listing 43.1 runs much too fast without a substantial delay (although it does look rather interesting at warp speed). We should all have such problems, eh? In fact, we could easily increase the number of animated images past 20 on that old AT, and well into the hundreds on a cutting-edge local-bus 486 or Pentium.

I’m not going to discuss Listing 43.1 in detail; the code is very thoroughly commented and should speak for itself, and most of the individual components of Listing 43.1—the Map Mask register, mode sets, word versus byte OUT instructions to the VGA—have been covered in earlier chapters. Do notice, however, that Listing 43.1 sets the palette exactly as I described earlier. This is accomplished by passing a pointer to a 17-byte array (1 byte for each of the 16 palette registers, and 1 byte for the border color) to the BIOS video interrupt (INT 10H), function 10H, subfunction 2.

Bit-plane animation does have inherent limitations, which we’ll get to in a second. One limitation that is not inherent to bit-plane animation but simply a shortcoming of Listing 43.1 is somewhat choppy horizontal motion. In the interests of both clarity and keeping Listing 43.1 to a reasonable length, I decided to byte-align all images horizontally. This saved the many tables needed to define the 7 non-byte-aligned rotations of the images, as well as the code needed to support rotation. Unfortunately, it also meant that the smallest possible horizontal movement was 8 pixels (1 byte of display memory), which is far enough to be noticeable at certain speeds. The situation is, however, easily correctable with the additional rotations and code. We’ll see an implementation of fully rotated images (in this case for Mode X, but the principles generalize nicely) in Chapter 49. Vertically, where there is no byte-alignment issue, the images move 4 or 6 pixels at a times, resulting in considerably smoother animation.

The addition of code to support rotated images would also open the door to support for internal animation, where the appearance of a given image changes over time to suggest that the image is an active entity. For example, propellers could whirl, jaws could snap, and jets could flare. Bit-plane animation with bit-aligned images and internal animation can look truly spectacular. It’s a sight worth seeing, particularly for those who doubt the PC’s worth when it comes to animation.

Limitations of Bit-Plane Animation

As I’ve said, bit-plane animation is not perfect. For starters, bit-plane animation can only be used in the VGA’s planar modes, modes 0DH, 0EH, 10H, and 12H. Also, the reprogramming of the palette registers that provides image precedence also reduces the available color set from the normal 16 colors to just 5 (one color per plane plus the background color). Worse still, each image must consist entirely of only one of the four colors. Mixing colors within an image is not allowed, since the bits for each image are limited to a single plane and can therefore select only one color. Finally, all images of the same precedence must be the same color.

It is possible to work around the color limitations to some extent by using only one or two planes for bit-plane animation, while reserving the other planes for multi-color drawing. For example, you could use plane 3 for bit-plane animation while using planes 0-2 for normal 8-color drawing. The images in plane 3 would then appear to be in front of the 8-color images. If we wanted the plane 3 images to be yellow, we could set up the palette registers as shown in Table 43.2.

As you can see, the color yellow is displayed whenever a pixel’s bit from plane 3 is 1. This gives the images from plane 3 precedence, while leaving us with the 8 normal low-intensity colors for images drawn across the other 3 planes, as shown in Figure 43.5. Of course, this approach provides only 1 rather than 3 high-precedence planes, but that might be a good tradeoff for being able to draw multi-colored images as a backdrop to the high-precedence images. For the right application, high-speed flicker-free plane 3 images moving in front of an 8-color backdrop could be a potent combination indeed.


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Graphics Programming Black Book © 2001 Michael Abrash