It might be obvious from the name, but there's also just one bit per pixel in a bitplane, and then you stack five bitplanes to get 32 colors.
You can have different offsets into each bitplane, which is why the grid could stay in the same place while the ball bounced.
The Amiga only supported bitplanes and not "chunky" modes like VGA where you have one byte per pixel. This made graphics with 256 colors (supported by the later AGA chipset) really slow as you had to do eight writes per pixel.
For a while there was a lot of competition between coders to write the best chunky to planar routine. This enabled you to write graphics effects with a chunky graphics buffer and then convert every frame on the fly to planar. One of the fastest was written by Kalms:
You can have different offsets into each bitplane, which is why the grid could stay in the same place while the ball bounced.
The Amiga only supported bitplanes and not "chunky" modes like VGA where you have one byte per pixel. This made graphics with 256 colors (supported by the later AGA chipset) really slow as you had to do eight writes per pixel.
For a while there was a lot of competition between coders to write the best chunky to planar routine. This enabled you to write graphics effects with a chunky graphics buffer and then convert every frame on the fly to planar. One of the fastest was written by Kalms:
http://www.lysator.liu.se/~mikaelk/doc/c2ptut/