Bitplane

A bitplane in computer technology is a two-dimensional array of 1- bit information. If a bitplane in the computer, so a single memory word contains (eg one byte) corresponding number of consecutive bit positions. Bitplanes were formerly used in computers with relatively little memory to consume only as much memory as was really necessary. In this case, however, often resulted in a disadvantage in access when individual bits were in the computer architecture in a memory word is not directly addressable, but could only be manipulated by a mask with multiple assembler instructions, which slowed down the access by the CPU. With so-called Blittern but stood on bitplanes optimized hardware blocks are available, the rapid two-dimensional block operations ( such as deleting, copying, moving ) were able to carry on bitplanes.

Use in the graphics area

Bitplanes were until about the mid-1990s used in home computer mainly for graphic representation, moreover for managing disks.

In the graphic, the color information of the pixel is stored in one or more bitplanes. This results in the color information of a single pixel of a bit word in which one bit is entered from any existing bitplane. To display an image of 640x480 pixels in two colors, for example, a bit-plane, with 38400 bytes ( 640x480 / 8 ) is required. One bit corresponds to one pixel. With two bitplanes are two bits per pixel available, so it can be shown four colors, with three bitplanes eight colors, etc. The bitplanes of its " stacked " and thus found to be two dimensional array of bit words. This representation is referred to as " planar", as opposed to " chunky ", wherein each pixel an entire physical memory word (e.g., one byte for 256 colors) in use.

Bitplanebasierte Graphics has the following advantages:

Disadvantages are:

In earlier computers both the available memory as well as its bandwidth were usually limited. For example, the CPU of the Amiga was significantly slowed down with increasing color depth of graphics - DAC, so operations were slower Chip RAM and the computer responded tough.

In the mid- 1990s, an increasing number of fast RAM was integrated graphics card and 3D compared with 2D graphic was dominant, the planar representation in the PC sector has been completely replaced by " chunky " and now no longer plays a role.

Typical examples of planar graphics are computer games for home computers like the Commodore 64 or the Amiga. This often was vertical or horizontal scrolling by shifting the bitplanes, and the use of Bob's. A very good example of the application of bitplanes is the dual playfield mode of the Amiga where the bitplanes were divided into two groups that were represented independently.