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This shows how to decide which pixels to fill when drawing a line on the screen. The thin black line is the "true" line, and the scenario provides a zoomed-in view of the pixels (white and orange squares) that are filled or left empty during the line-drawing.
Left-click to set the start point of the line, and right-click to set the end point.
A full explanation of the algorithm is here: http://sinepost.wordpress.com/2012/05/24/drawing-in-a-straight-line/