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Remove colour attributes from body and strip most of the font tags out

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James Gregory 2013-12-30 13:57:02 +11:00
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<!--CHAPTER=43//-->
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<P>I&rsquo;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&mdash;the Map Mask register, mode sets, word versus byte <B>OUT</B> instructions to the VGA&mdash;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 (<B>INT</B> 10H), function 10H, subfunction 2.</P>
<P>Bit-plane animation does have inherent limitations, which we&rsquo;ll get to in a second. One limitation that is <I>not</I> 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&rsquo;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.</P>
<P>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&rsquo;s a sight worth seeing, particularly for those who doubt the PC&rsquo;s worth when it comes to animation.</P>
<H3><A NAME="Heading6"></A><FONT COLOR="#000077">Limitations of Bit-Plane Animation</FONT></H3>
<H3><A NAME="Heading6"></A>Limitations of Bit-Plane Animation</H3>
<P>As I&rsquo;ve said, bit-plane animation is not perfect. For starters, bit-plane animation can only be used in the VGA&rsquo;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.
</P>
<P>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.</P>
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<font face="Verdana,sans-serif" size="1">Graphics Programming Black Book &copy; 2001 Michael Abrash</font>
Graphics Programming Black Book &copy; 2001 Michael Abrash
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