AMOS:Working with bitplanes

(this is a work-in-progress tutorial and has not been tested yet!!!!!)

Amiga 500 display

Amiga uses bitplanes, so instead of using one or more bytes per pixel, the screen is made of several layers (bitplanes).

Each pixel has only one bit by bitplane. So using only one bitplane in a screen, there are only two colors available for a pixel: the color of index 0 (bit=0) and the color of index 1 in the palette (bit=1). Using 2 bitplanes, there are 4 colors available for a pixel : index 0 (bit 0 and bit 0), index 1 (bit 1 and bit 0), index 2 (bit 0 and bit 1) and index 3 (bit 1 and bit 1 of both bitplanes). And so on.

The Amiga 500 has 6 bitplanes. So the color depth for a screen goes from 2 until 2 power 6=64 colors.

For every bitplane added, the maximum amount of colors doubles:

• 2^1 = 2 = 2 colors
• 2^2 = 2*2 = 4 colors
• 2^3 = 2*2*2 = 8 colors
• 2^4 = 2*2*2*2 = 16 colors
• 2^5 = 2*2*2*2*2 = 32 colors
• 2^6 = 2*2*2*2*2*2 = 64 colors

Example (taken from amcaf extension guide) :

We have an 16 colours screen. So it has got 4 bitplanes. Normally, the bitplanes are counted from 0 to n-1...

  Binary 0 1 0 1 = Decimal 5
         | | | |    .---v---v---v---v--------------------
         | | | `----+ 1 |   |   |   | Bitplane 0
         | | |      |--v^--v^--v^--v^--v-------------------
         | | `------+--+ 0 |   |   |   | Bitplane 1
         | |        |  |--v^--v^--v^--v^--v------------------
         | `--------+--+--+ 1 |   |   |   | Bitplane 2
         |          |  |  |--v^--v^--v^--v^--v-----------------
         `----------+--+--+--+ 0 |   |   |   | Bitplane 3
                    |  |  |  |---^---^---^---'
                    |  |  |  |
                    |  |  |
                    |  |
                    |

Note 1 : There are in fact 2 special modes : "Extra Half Bright" (64 colors so 6 bitplanes) and Hold And Modify (4096 colors)

Note 2 : if we consider chip memory, one byte in a bitplane is associated to 8 pixels.

If you want to know how many bytes a line takes in a bitplane use this formula:

size of one line within a bitplane	ScreenWidth / 8
size of one bitplane	ScreenWidth / 8 * ScreenHeight
Size of full picture	ScreenWidth / 8 * ScreenHeight * AmountOfBitplanes

Reading and writing to a bitplane

AMOS brought to us the concept of screen : a number of bitplanes (between 1 and 6) with a dimension and a resolution (low, mid or high). Up to 8 different screens can be used at the same time and even displayed with different color depth and dimension at the same time. This powerfull function is actually possible because AMOS programs for us a lot of chipset registers using an important amiga coprocessor called the Copper.

Moreover, for each screen, using the double buffer instruction, AMOS has two screen buffers, the physical and logical. The physical screen buffer is the one that's visible and in case you're using double buffered screens the logical contains the screen you're drawing on. You can get the address of these with =Phybase(bitplane) and =Logbase(bitplane). Bitplanes are counted from 0 to 6.

So say you want to copy a bitplane from an AMOS screen to an AMOS bank:

BitplaneSize = Screen Width / 8 * Screen Height

Reserve As Work 10,BitplaneSize Copy Phybase(0),Phybase(0)+BitplaneSize To Start(10) Wsave "RAM:Bitplane",10

Or do it the other way around and copy an bitplane from an AMOS bank to an AMOS screen:

BitplaneSize = Screen Width / 8 * Screen Height

Reserve As Work 10,BitplaneSize Wload "RAM:Bitplane",10 Copy Start(10),Start(10)+BitplaneSize To Phybase(0)