Bit blit

From Academic Kids

Bit blit (bitblt, blitting etc.) is a computer graphics operation in which two bitmap patterns are combined into one using a RasterOp.



The name derives from the BitBLT machine instruction for the Xerox Alto computer, standing for "Bit Block Transfer". The invention of this instruction is credited to Dan Ingalls at Xerox PARC.

The development of fast methods for various bit blit operations was key in the evolution of computer displays from using character graphics, to using bitmap graphics for everything. Machines that rely heavily on the performance of 2D graphics (such as video game consoles) come with a special-purpose chip called a Blitter.

Bit Blit techniques and uses

A classic use for blitting is to render sprites with transparencies onto a background. It is usually not feasible go through every pixel and find out if it needs to be displayed for reasons of speed. For example, if we have the following background image:
Missing image

and wish to display various sprite on top of it representing for example objects within a game to produce this:
Missing image

Missing image
Sprites (Left) and Masks (Right)

What first is needed are the bitmaps for the sprites and the corresponding masks. The colours are very important as the sprite background and the mask foreground are black, stored in binary as 0's. The mask background is white, stored in binary as 1's.

The first blit uses the RasterOp of AND with the background and the mask of the sprites. Because any value ANDed with 0 equals 0, and any value ANDed with 1 is unchanged, we can create black areas where the actual sprites will appear, and leave the rest of the background alone.

Missing image

The second blit uses the RasterOp of OR with the altered background and the actual sprite. Because any value OR'd with 0 is unchanged, the background is unaffected and the black areas are filled with the actual sprite image.

Blitting vs. sprites

Blitting is similar to hardware-sprite drawing, in that both systems reproduce a pattern, typically a square area, at different locations on the screen. 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 every time the pattern is placed on the screen the display "under" it is overwritten, or "damaged". It is up to the software to clean this damage up by blitting twice, once to remove the damage, and then again to place the bit in its new location. However, there are several ways to optimise 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.

As one might imagine, this makes blitting significantly slower than sprite manipulation. However blitting has one very big advantage, there's no physical limit to the number of patterns you can blit, or to the size of the patterns. Thus you can use blitting to display anything on the screen, including simulating sprites (through the double-write pattern noted above), or even text.

See also

External link

ja:Bit Block Transfer


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