Editing Amiga Original Chip Set (section)

===Blitter===
The ''[[blitter]]'' is a sub-component of Agnus. "Blit" is shorthand for "block image transfer" or [[bit blit]]. The blitter is a highly parallel memory transfer and logic operation unit. It has three modes of operation: copying blocks of memory, filling blocks (e.g. polygon filling) and line drawing.

The blitter allows rapid copying of video memory, meaning that the CPU can be freed for other tasks. The blitter was primarily used for drawing and redrawing graphics images on the screen, called "bobs", short for "blitter objects".

The blitter's block copying mode takes zero to three data sources in memory, called A, B and C, performs a programmable [[Boolean function]] on the data sources and writes the result to a destination area, D. Any of these four areas can overlap. The blitter runs either from the start of the block to the end, known as "ascending" mode, or in reverse, "descending" mode.

Blocks are "rectangular"; they have a "width" in multiples of 16 bits, a height measured in "lines", and a "stride" distance to move from the end of one line to the next. This allows the blitter to operate on any video resolution up to 1,024×1,024 pixels.<ref>The ''BLTSIZE'' register holds six bits for horizontal words and ten bits for vertical lines.</ref> The copy automatically performs a per-pixel logical operation. These operations are described generically using [[minterms]]. This is most commonly used to do direct copies (D = A), or apply a pixel mask around blitted objects (D = (C AND B) OR A). The copy can also barrel shift each line by 0 to 15 pixels. This allows the blitter to draw at pixel offsets that are not exactly multiples of 16.

These functions allow the Amiga to move GUI windows around the screen rapidly as each is represented in graphical memory space as a rectangular block of memory which may be shifted to any required screen memory location at will.

The blitter's line mode draws single-pixel thick lines using [[Bresenham's line algorithm]]. It can also apply a [[16-bit computing|16-bit]] repeating pattern to the line. The line mode can also be used to draw rotated bobs: each line of bob data is used as line pattern while the line mode draws the tilted bob line by line.

The blitter's filling mode is used to fill per-line horizontal spans. On each span, it reads each pixel in turn from right to left. Whenever it reads a set pixel, it toggles filling mode on or off. When filling mode is on, it sets every pixel until filling mode is turned off or the line ends. Together, these modes allow the blitter to draw individual flat-shaded polygons. Later Amigas tended to use a combination of a faster CPU and blitter for many operations.