Amiga Bitplanes & Planar vs Chunky - Technical Expert Required

[CodyJarrett]

APoV 4 includes an article about Doom clones on the Amiga and I need someone to check my technical explanation. Is there any technical expert who wants to volunteer to help with this?

The article mentions bitplanes, numbers of colours and planar versus chunky storage.

[Zetr0]

I can help with this Cody, what do you need?

[CodyJarrett]

I just need a verification of the text in case I'm writing a load of cobblers. Here's the text:

[CodyJarrett]

Chunky vs Planar

How are graphics stored in memory? One possible graphical storage system would be to store the value (colour) of each pixel in a long row. For example, in order to store an image with up to 256 colours the following values could be placed in each consecutive memory address:

00000000 (colour no. 0)
10010000 (colour no. 144)
00010110 (colour no. 22)
00010110 (colour no. 22)

In the above examples there are four pixels with colours 0, 144, 22 and 22. This is a Chunky graphics system and is the method used by the Doom-era PC.

The Amiga uses a different system; Planar graphics. The bits for each pixel are displayed in different locations (bitplanes) in memory and must be combined in order to find the colour of a pixel that is to be displayed. For example, an eight colour Planar system would use the following bytes:

Bitplane 1 0 0 1 1
Bitplane 2 1 1 0 0
Bitplane 3 0 1 1 1
Bitplane 4 0 0 0 0

Compare this to the Chunky system:

00000100 00000110 00001010 00001010

In the Chunky system each colour is displayed one after the other. In the Planar system each Bitplane row is stored at a different location in memory.

In order to find the first colour the Chunky system needs only to make one read operation: 0100. The Planar system needs to go to bitplane 1 then read 0, go to bitplane 2 then read 1, go to bitplane 3 then read 0 and finally to bitplane 4 and read 0.

A 256 colour display would need eight reads since eight bits are required to store up to 256 numbers e.g. this is one pixel:

Planar
Bitplane 1 0...
Bitplane 2 0...
Bitplane 3 1...
Bitplane 4 0...
Bitplane 5 0...
Bitplane 6 1...
Bitplane 7 0...
Bitplane 8 0...

The Chunky system is clearly faster than Planar since only one memory read is required per pixel rather than up to 8 reads. This is one reason why Doom clones on the Amiga are quite slow compared to the PC.

Planar, on the other hand is supposedly better for scrolling, parallax type games. Less memory is also used by the Planar system to display low numbers of colours. This is because it doesn't require a full memory address for a small number. For example, a sixteen colour Planar pixel needs 4 bits whereas Chunky would probably use the whole byte (8 bits).

[CodyJarrett]

Memory Storage and Colours

A computer display is made up of individual pixels each of which can have a specific colour. A two colour display needs only two numbers to record each possible colour in memory; 0 and 1, where 0 might represent black and 1 might represent white. In binary terms only 1 bit of memory is required to store each pixel's colour.

The more colours are used the more memory is required for each pixel. For example, 16 colours can be stored in 4 bits - decimal 0 or binary 0000 to decimal 15 or binary 1111) and 256 colours could be stored in 8 bits (1 byte) - decimal 0 or binary 00000000 to decimal 255 or 11111111.

The number of colours clearly affects how much memory is used and how many memory addresses must be accessed to update the display.

In order to work out how many colours can be displayed using the Amiga’s bitplanes calculate:

2 power y where y is the number of bitplanes

Bitplanes
1 = 2 colours
2 = 4 colours
3 = 8 colours
4 = 16 colours
5 = 32 colours
6 = 64 colours
7 = 128 colours
8 = 256 colours

The number of bitplanes most commonly used on the Amiga are 4 (16 colours), 5 (32 colours) and 8 (256 colours e.g. AGA).