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Pixel
Short for picture element. There are two common meanings: the pixels that textures, or bitmaps are made of, and the pixels that are rendered onto your computer screen by the engine.
You tell each pixel where to go and what color to be by giving it two sets of values. For position, it needs coordinates (coords), called X and Y, usually written as (X,Y). The X coord is horizontal, the Y coord is vertical, and the numbers usually start at (0,0) in the upper-left corner. For the pixel's color it needs the three RGB color values, written as (R,G,B). These go from 0 (no color) to 1 (full-on color). For instance, green is (0,1,0). Most paint programs use 0 to 255, but 0 to 1 is easier for RT3D programmers to use. See also texel.