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Bit blit (also writtenBITBLT,BIT BLT,BitBLT,Bit BLT,Bit Blt etc., which stands forbit block transfer) is a data operation commonly used incomputer graphics in which severalbitmaps are combined into one using aboolean function.[1]
The operation involves at least two bitmaps: a "source" (or "foreground") and a "destination" (or "background"), and possibly a third that is often called the "mask". The result may be written to a fourth bitmap, though often it replaces the destination. The pixels of each are combined using a program-selectableraster operation, a bit-wiseboolean formula. The most obvious raster operation overwrites the destination with the source. Others may involveAND,OR,XOR, andNOT operations.[1] The CommodoreAmiga's graphics chipset (and others) could combine three source bitmaps using any of the 256 possible 3-inputboolean functions.
Modern graphics software has almost completely replaced bitwise operations with more general mathematical operations used for effects such asalpha 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 still uses XOR to draw interactive highlight rectangles or region borders; when this is done to color images, the unusual resulting colors are easily seen.
The name derives from theBitBLT routine for theXerox Altocomputer, standing forbit-boundary block transfer.Dan Ingalls,Larry Tesler,Bob Sproull, andDiana Merry programmed this operation atXerox PARC in November 1975 for theSmalltalk-72 system.Dan Ingalls later implemented a redesigned version inmicrocode.
The development of fast methods for various bit blit operations gave impetus to the evolution of computer displays from using character graphics (text mode) to usingraster graphics (bitmap) for everything. Machines that rely heavily on the performance of2D graphics (such asvideo game consoles) often have special-purpose circuitry called ablitter.
A classic use for blitting is torender transparentsprites onto a background. In this example a background image, a sprite, and a 1-bit mask are used. As the mask is 1-bit, there is no possibility for partial transparency viaalpha blending.
A loop that examines each bit in the mask and copies thepixel from the sprite only if the mask is set will be much slower than hardware that can apply exactly the same operation to every pixel. Instead amasked blit can be implemented with two regular BitBlit operations using the AND and OR raster operations.
| Background image | Sprite (left) and mask (right) |
|---|---|
 |  |
The sprite is drawn in various positions over the image to produce this:
| Intended Result |
|---|
 |
When preparing the sprite, the colors are very important. Themask pixels are 0 (black) wherever the corresponding sprite pixel is to be displayed, and 1 (white) wherever the background needs to be preserved. Thesprite must be 0 (black) anywhere where it is supposed to be transparent, but note that black can be used in the non-transparent regions.
In the first blit, themask is blitted onto thebackground using the raster operatorAND. Because any value ANDed with 0 equals 0, and any value ANDed with 1 is unchanged, black areas are created where the actual sprites will appear, while leaving the rest of the background alone.
| Result of the first blit |
|---|
 |
In the second blit, thesprite is blitted onto the newly altered background using the raster operator ofOR. Because any value ORed with 0 is unchanged, the background is unaffected and the black areas are filled with the actual sprite image.
| Final result |
|---|
|
It is also possible to achieve the same effect using a sprite with a white background and awhite-on-black mask. In this case, the mask would be ORed first, and the sprite ANDed next.
Blitting is similar to hardware-sprite drawing, in that both systems reproduce a pattern, typically a square area, at different locations on the screen.[2] Hardware sprites have the advantage of being stored in separate memory, and therefore don't disturb the main display memory. This allows them to be moved about the display, covering the "background", with no effect on it.
Blitting moves the same types of patterns about the screen, but does so by writing into the same memory as the rest of the display. This means that every time a foreground pattern is placed on the screen, any background pixels underneath it are overwritten, or "damaged". It is up to the software to repair this damage by blitting twice, once to restore the pixels that were changed, and then again to place the foreground pattern in its new location. One way to do it is to store the required patterns in VRAM offscreen and to reserve another area offscreen as a sort of stack to temporarily store the affected display section. Assuming the graphics chip has dedicated VRAM this is useful to lessen the strain on system RAM but also the bandwidth limited ISA expansion slot on older PC systems.
However, there are several ways to optimize this. If large areas of the screen are taken over by the patterns, it may be more efficient to blit the background to the screen instead of erasing each pattern individually. A variation involves dividing the screen into segments and erasing only the segments where patterns have been drawn on. This technique is known as dirty rectangles.
A blit operation is a special form of copy operation; it copies a rectangular area of pixels from one framebuffer to another. This function also has some very specific properties with regard to multisampling.