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What are chunky and planar displays?

Computer images are arranged as a grid of pixels, each of which can be thought of as a number representing the color number of the pixel, sort of like a paint-by-numbers scheme. For example, here's a simplified example image, in four colors:

   00302132

The Amiga stores this image in a bitplane mode. That is, it is represented by several planes of bits (binary digits, 1s or 0s). This is a four-color image, so each color number could be represented by two bits. Therefore there are two bitplanes:

   00100110    Here's bitplane 0
   00101011    And here's bitplane 1
   --------    Now, let's add them up, binary style:
   00302132

Which is the final image. If the image was in two dimensions, it would truly be composed of bit planes. However, I'd need three dimensions to show multiple bitplanes overlayed, and therefore for simplicity we're working in one dimension (which is all we need).

Now, there's another way of storing this image. How about if we localize the bit data in little chunks?

   00 00 11 00 01 10 11 01 = 00302132

This is the principle of the chunky pixel mode.

Both methods of image storage are perfectly logical, and no one can say that one is better than the other. However, there are certain technical aspects which cause certain advantages and disadvantages.

First, if you've seen colored text scroll on your Amiga, you know there is a bit of "flicker" that arises. Specifically, what happens is that while the text is scrolling, its color temporarily changes to something completely different. What's happening is that the computer's moving several bitplanes of data while the raster (monitor electron gun) is sweeping across the screen. What that means is that, if the raster catches the data while it's being moved, you can end up with some bitplanes being moved and some not. What if we filled bitplane 1 in the example above with 0s? Instantly all the 3s become 1s, and the 2s become 0s! This is what causes "flicker" when certain colors are scrolled. By contrast, if a chunky pixel display is caught while scrolling, all we see is a partially-scrolled image; the colors are preserved (since their units are the small ones).

That's a disadvantage to planar pixels, but what about chunky pixels? Well, recall that a computer organizes information in terms of 8 bit bytes. These groups are static; you cannot decide to all of a sudden organize data in terms of three bytes or something! Therefore, when using chunky pixels, things get complicated if we decide to use a nonconvenient number of bits per pixel. In practice, the 8-bit (256-color) mode, and 24-bit (16 million color) modes are the most common candidates for chunky pixel displays.

Finally, certain effects can be accomplished with the different systems. Bitplanar mode is particularly useful for things like shadows (where an extra bitplane is set with 1s instead of 0s), and chunky mode is great for perspective and "mapping" (since the data for each pixel is localized in a single "chunk"). The latter advantage makes chunky pixel mode really great for games, and is what made Wolfenstein 3-D possible.

We all know that Amigas use the bitplane system for storing images. However, the Macintosh and PC(VGA) both use chunky pixel modes. While we can optimize our RAM usage with "bizarre" modes like 8- and 128-color, they gain the advantages of non-flicker scrolling, and the programming simplicity of just writing a byte where you want the pixel to go.

The difference between the two modes becomes problematic in things like emulation. EMPLANT has a "chunky to planar" routine which it uses to convert a Macintosh display into an Amiga one. "Chunky to planar" routines are also useful for getting chunky-inclined things to run on Amigas (see TMAPDemo, rotdemo). On a side note, there was some confusion as to what EMPLANT used the MMU for with regard to chunky to planar. The MMU itself is incapable of performing the algorithm for the conversion; rather, it is used to detect what portions of the display memory are updated from the Mac side, and therefore the processor is saved from having to perform the chunky to planar conversion for the entire display.

I sincerely hope that helped clear up most of the mystery concerning the terms "Chunky" and "Planar"!

(Joseph Luk, jluc@eis.calstate.edu)

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