Bitplane
A bit plane in computer technology is a two-dimensional field of 1- bit information. If a bit plane is in the computer's main memory , a single memory word (e.g. a byte ) contains a corresponding number of consecutive bit positions. Bitplanes used to be used in computers with comparatively little memory in order to only use as much memory as was really necessary. However, this often resulted in a disadvantage in access when individual bits in a memory word in the computer architecture were not directly addressable, but could only be manipulated by masking with several assembler commands, which slowed down access by the CPU . With so-called blitters , however, hardware modules optimized for bitplanes were available that could perform fast two-dimensional block operations (e.g. delete, copy, move) on bitplanes.
Use in the graphics area
Bitplanes were mainly used in home computers for graphic display until the mid-1990s , as well as for managing floppy disks .
In the graphic area, the color information of the pixels is saved in one or more bitplanes. The color information of an individual pixel is obtained from a bit word into which a bit from each existing bit plane is entered. To z. For example, to display a graphic of 640 × 480 pixels in two colors, a bit plane with 38,400 bytes (640 × 480/8) is required. One bit corresponds to one pixel. With two bitplanes two bits per pixel are available, so four colors can be displayed, with three bitplanes eight colors etc. The bitplanes are "stacked on top of each other" and thus result in a two-dimensional field of bit words. This representation is called "planar", in contrast to "chunky", in which each pixel occupies an entire physical memory word (e.g. one byte for 256 colors).
Bitplan-based graphics have the following advantages:
- The bandwidth of the graphics DAC is easily scalable, i. This means how much data the graphics module has to fetch from the RAM per display period.
Disadvantages are:
- Access to individual bits is generally much slower than to entire memory words
- Therefore poorly suited for three-dimensional graphics, in which the color information for each pixel is calculated individually. 3D graphics on a planar graphics system require a "chunky to planar" implementation, since the color information of each individual pixel has to be distributed to the bitplanes used.
Previous computers tended to have limited available memory and bandwidth . So was z. For example, the CPU of the Amiga was clearly slowed down by the graphics DAC as the color depth increased, so that operations in the ChipRAM became slower and the computer reacted slowly.
When, in the mid-1990s, more and more fast RAM was integrated on graphics cards and 3D graphics became more dominant over 2D graphics, the planar display in the PC area was completely replaced by "chunky" and is no longer relevant today.
Typical examples of planar graphics are computer games for home computers such as the C64 or the Amiga . There was often vertical or horizontal scrolling by moving the bitplanes, as well as the use of bobs . A very good example for the use of bitplanes is the Amiga's dual playfield mode, in which the bitplanes were divided into two groups that were displayed independently of each other.