Bit blit

Bit blit (also written BITBLT, BIT BLT, BitBLT, Bit BLT, Bit Blt etc., which stands for bit block transfer) is a data operation commonly used in computer graphics in which several bitmaps are combined into one using a boolean function.

The operation involves at least two bitmaps, one source and destination, possibly a third that is often called the "mask" and sometimes a fourth used to create a stencil. The pixels of each are combined bitwise according to the specified raster operation (ROP) and the result is then written to the destination. The ROP is essentially a boolean formula. The most obvious ROP overwrites the destination with the source. Other ROPs may involve AND, OR, XOR, and NOT operations. The Commodore Amiga's graphics chipset, for example, could combine three source bitmaps according to any of 256 boolean functions of three variables.

Modern graphics software has almost completely replaced bitwise operations with more general mathematical operations used for effects such as alpha compositing. This is because bitwise operations on color displays do not usually produce results that resemble the physical combination of lights or inks. Some software, like Windows 10, still uses XOR to draw interactive highlight rectangles; when this is done to color images, the unusual resulting colors are easily seen.

The name derives from the BitBLT routine for the Xerox Alto computer, standing for bit-boundary block transfer. Dan Ingalls, Larry Tesler, Bob Sproull, and Diana Merry programmed this operation at Xerox PARC in November 1975 for the Smalltalk-72 system. Dan Ingalls later implemented a redesigned version in microcode.

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