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+Chapter 44\
+ Split Screens Save the Page Flipped Day {#Heading1}
+----------------------------------------
+
+### 640x480 Page Flipped Animation in 64K...Almost {#Heading2}
+
+Almost doesn’t count, they say—at least in horseshoes and maybe a few
+other things. This is especially true in digital circles, where if you
+need 12 MB of hard disk to install something and you only have 10 MB
+left (a situation that seems to be some sort of eternal law) you’re
+stuck.
+
+And that’s only infuriating until you dredge up the gumption to go in
+there and free up some space. How would you feel if you were up against
+an “almost-but-not-quite” kind of a wall that couldn’t be breached by
+freeing up something elsewhere? Suppose you were within a few KB of
+implementing a wonderful VGA animation scheme that provided lots of
+screen space, square pixels, smooth motion and more than adequate
+speed—but all the memory you have is all there is? What would you do?
+
+Scream a little. Or throw something that won’t break easily. Then you
+sit down and let your right brain do what it was designed to do. Sure
+enough, there’s a way, and in this chapter I’ll explain how a little VGA
+secret called *page splitting* can save the day for page flipped
+animation in 640x480 mode. But to do that, I have to lay a little
+groundwork first. Or maybe a lot of groundwork.
+
+No horseshoes here.
+
+#### A Plethora of Challenges {#Heading3}
+
+In its simplest terms, computer animation consists of rapidly redrawing
+similar images at slightly differing locations, so that the eye
+interprets the successive images as a single object in motion over time.
+The fact that the world is an analog realm and the images displayed on a
+computer screen consist of discrete pixels updated at a maximum rate of
+about 70 Hz is irrelevant; your eye can interpret both real-world images
+and pixel patterns on the screen as objects in motion, and that’s that.
+
+One of the key problems of computer animation is that it takes time to
+redraw a screen, time during which the bitmap controlling the screen is
+in an intermediate state, with, quite possibly, many objects erased and
+others half-drawn. Even when only briefly displayed, a partially-updated
+screen can cause flicker at best, and at worst can destroy the illusion
+of motion entirely.
+
+Another problem of animation is that the screen must update often enough
+so that motion appears continuous. A moving object that moves just once
+every second, shifting by hundreds of pixels each time it does move,
+will appear to jump, not to move smoothly. Therefore, there are two
+overriding requirements for smooth animation: 1) the bitmap must be
+updated quickly (once per frame—60 to 70 Hz—is ideal, although 30 Hz
+will do fine), and, 2) the process of redrawing the screen must be
+invisible to the user; only the end result should ever be seen. Both of
+these requirements are met by the program presented in Listings 44.1 and
+44.2.
+
+#### A Page Flipping Animation Demonstration {#Heading4}
+
+The listings taken together form a sample animation program, in which a
+single object bounces endlessly off other objects, with instructions and
+a count of bounces displayed at the bottom of the screen. I’ll discuss
+various aspects of Listings 44.1 and 44.2 during the balance of this
+article. The listings are too complex and involve too much VGA and
+animation knowledge for for me to discuss it all in exhaustive detail
+(and I’ve covered a lot of this stuff earlier in the book); instead,
+I’ll cover the major elements, leaving it to you to explore the finer
+points—and, hope, to experiment with and expand on the code I’ll
+provide.
+
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+
+* * * * *
+
+Graphics Programming Black Book © 2001 Michael Abrash