po-xor

A paint operation that merges the colors of new content that is being drawn with the current colors of the destination stream's pixels. It does this by doing an exclusive or on the bits of the pixel values. See paint-operation.

An exclusive or is particularly useful for drawing something temporarily and then erasing it in such a way that the original drawing remains undisturbed. This can be done by simply drawing the temporary content twice using po-xor, where the second draw does the non-destructive erasing. The rubber-banding functions such as get-line and get-shape-box use po-xor internally to erase the rubber-banding line.

The color that results with exclusive or drawing is rather arbitrary, but that is the trade-off for non-destructive erasure.

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Created 2010.1.21.