[GEM Development] Video mode - bitplane vs packed pixels

[Michael Bernstein]

Hi Cyprian,

> I'm working on a small application which operates on offscreen graphics 
> data, that I transfer via vro_cpyfm to the screen.

There is one step missing in your handling.

> Now I wonder how to distinguish video mode: bitplane vs packed pixels. 
> Do you have any tips?

Do you speak about the graphic format of the Video RAM itself used by 
the hardware? Then the answer is: you should take it as unknown. But 
there is no need for applications to have this information.

The VDI distinguish between the hardware-dependent format which is used 
by hardware and the VDI graphics driver. And the standard format which 
is well defined and a bitplane mode. You can query the number of planes 
with the VDI function vq_extnd()

So the proper handling of offscreen bitmap is:

- Make your graphic handling with a buffer in the standard format. The 
field fd_stand of the MFDB structure for this buffer is 1.
- Then call vr_trnfm() to transform the standard format graphic buffer 
to a device-dependant buffer (fd_stand = 0 for destination MFDB). This 
transformation may be done in place. But this took a longer time. So 
better use a second buffer for the destination.
- Then use vro_cpyfm() to copy the graphic buffer in hardware dependant 
format to the screen buffer. The field fd_addr of the destination MFDB 
is set to NULL (0) and all other field can stay uninitialized.

Atari ST/TT/Falcon computers use with GEM the same graphic system. So 
you can use the GEM part of https://toshyp.atari.org/ also as a 
documentation. This documentation is available in german and english. 
Another source of information is the Atari Profibuch, which is now 
available for download in pdf format. But this is only in german. The 
Profibuch contains an example function how to calcuate the RAM size of a 
graphic buffer for the actual graphic resolution.

Best regards
Michael