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+#### Stacking the Palette Registers {#Heading4}
+
+Suppose that instead of viewing the four bits per pixel coming out of
+display memory as selecting one of sixteen colors,we view those bits as
+selecting one of *four* colors. If the bit from plane 0 is 1, that would
+select color 0 (say, red). The bit from plane 1 would select color 1
+(say, green), the bit from plane 2 would select color 2 (say, blue), and
+the bit from plane 3 would select color 3 (say, white). Whenever more
+than 1 bit is 1, the 1 bit from the lowest-numbered plane would
+determine the color, and 1 bits from all other planes would be ignored.
+Finally, the absence of any 1 bits at all would select the background
+color (say, black).
+
+That would give us four colors and the background color. It would also
+give us nifty image precedence, with images in plane 0 appearing to be
+in front of images from the other planes, images in plane 1 appearing to
+be in front of images from planes 2 and 3, and so on. It would even give
+us transparency, where rearward images would show through holes within
+and around the edges of images in forward planes. Finally, and most
+importantly, it would meet all the criteria needed to allow us to store
+each image in a single plane, letting us manipulate the images very
+quickly and with no reprogramming of the VGA’s hardware other than the
+few **OUT** instructions required to select the plane we want to write
+to.
+
+Which leaves only one question: How do we get this magical
+pixel-precedence scheme to work? As it turns out, all we need to do is
+reprogram the palette registers so that the 1 bit from the plane with
+the highest precedence determines the color. The palette RAM settings
+for the colors described above are summarized in Table 43.1.
+
+Remember that the 4-bit values coming from display memory select which
+palette register provides the actual pixel color. Given that, it’s easy
+to see that the rightmost 1-bit of the four bits coming from display
+memory in Table 43.1 selects the pixel color. If the bit from plane 0 is
+1, then the color is red, no matter what the other bits are, as shown in
+Figure 43.4. If the bit from plane 0 is 0, then if the bit from plane 1
+is 1 the color is green, and so on for planes 2 and 3. In other words,
+with the palette register settings we instantly have exactly what we
+want, which is an approach that keeps images in one plane from
+interfering with images in other planes while providing precedence and
+transparency.
+
+* * * * *
+
+Bit Value For Plane\
+ 3 2 1 0
+
+Palette Register
+
+Register setting
+
+* * * * *
+
+0 0 0 0
+
+0
+
+00H (black)
+
+0 0 0 1
+
+1
+
+3CH (red)
+
+0 0 1 0
+
+2
+
+3AH (green)
+
+0 0 1 1
+
+3
+
+3CH (red)
+
+0 1 0 0
+
+4
+
+39H (blue)
+
+0 1 0 1
+
+5
+
+3CH (red)
+
+0 1 1 0
+
+6
+
+3AH (green)
+
+0 1 1 1
+
+7
+
+3CH (red)
+
+1 0 0 0
+
+8
+
+3FH (white)
+
+1 0 0 1
+
+9
+
+3CH (red)
+
+1 0 1 0
+
+10
+
+3AH (green)
+
+1 0 1 1
+
+11
+
+3CH (red)
+
+1 1 0 0
+
+12
+
+39H (blue)
+
+1 1 0 1
+
+13
+
+3CH (red)
+
+1 1 1 0
+
+14
+
+3AH (green)
+
+1 1 1 1
+
+15
+
+3CH (red)
+
+* * * * *
+
+Table 43.1 Palette RAM settings for bit-plane animation.
+
+* * * * *
+
+![](images/43-04.jpg)\
+ **Figure 43.4**  *How pixel precedence works.*
+
+Seems almost too easy, doesn’t it? Nonetheless, it works beautifully, as
+we’ll see very shortly. First, though, I’d like to point out that
+there’s nothing sacred about plane 0 having precedence. We could
+rearrange the palette register settings so that any plane had the
+highest precedence, followed by the other planes in any order. I’ve
+chosen to make plane 0 the highest precedence only because it seems
+simplest to think of plane 0 as appearing in front of plane 1, which is
+in front of plane 2, which is in front of plane 3.
+
+### Bit-Plane Animation in Action {#Heading5}
+
+Without further ado, Listing 43.1 shows bit-plane animation in action.
+Listing 43.1 animates 13 rather large images (each 32 pixels on a side)
+over a complex background at a good clip *even on a primordial
+8088-based PC*. Five of the images move very quickly, while the other 8
+bounce back and forth at a steady pace.
+
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+
+* * * * *
+
+Graphics Programming Black Book © 2001 Michael Abrash