Saturday, August 18, 2012

Moire Effect (1999)

In 1999, when starting university, I was still operating on somewhat older hardware (a pentium 133 and a 486/66, where most roommates had 400-500 MHz machines).  This being as it was, there were still a number of interesting demoscene effects I hadn't written up for myself, and I was still interested in learning about them.  On a rainy afternoon I tried my hand at writing a moiré effect.

The technique is pretty simple: a giant sprite is precomputed where the value of the color of a pixel is the radius from the center of the sprite, times a scalar constant to control radius, mod 32.  This yields a picture whose colour values consist of concentric rings with values increasing from 0 to 31, then 0 through 31 again, etc.  The palette is configured to consist of a gradient from black to white in colors 0 through 15, then white back to black in 16 to 31, so a single copy of the sprite rendered to the screen essentially looks like a series of concentric blurry circles.

At runtime, two copies of this sprite are offset in a simple moving pattern, and they are XORed into the framebuffer.  The XOR operation always a yields a value between 0 and 31, keeping the resulting colors in the frame buffer within the range of the preconfigured palette.  This yields the pattern which can be seen in the following movie.



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