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sam-morgenfeld
02-21-2005, 03:25 AM
hello,

i hope you can help me. I have searched through quite a number of pages to get the right answer to this question:

what are the differences between 24/32 bit modes on graphiccards and which one is the best for OpenGL? well it seemed to me that every page has it own explanation for this.

I think the following point are right: the amount of colors are the same, 16,8 Million.

Then the informations about this 32bit-mode differs from page to page: is it true that it improves performance by aligning the data for each pixel on a 32-bit address boundary? Or are there more colors available?

then the 32 bit mode is told to have the same amount of colors like the 24 bit-mode but for each color AND alpha channel there is byte of data.

Are there any further differences between 24bit and 32 bit?

:confused:

02-21-2005, 08:31 AM
It is true that 24 BPP and 32 BPP modes both offer 2^24 colors.

The additional 8 bits in 32 BPP are usually used to store an per pixel transparancy value (alpha) which is very usefull for textures.

It is also true that it's less costly to access an individual pixel when each pixels consumes 32 bit (4 byte), this can be done by a simple bit shift, while you need an integer multiplication if each pixel takes 24 bit (3 byte). Of course the multiplication can be replaced by an bit shift and an addition (or simply three additions) but its still more costly compared to 32 BPP and requires more silicon (and given the current transistor counts for 3d gpu's, thats a valueable resource).

On the other hand it takes less memory and less bandwith to store or transfer an image in 24 BPP then it does in 32 BPP.

There is no other difference and both modes have their pro's and con's.

I personaly prefer 32 BPP because:

a.) Its a power of two value and "feels" better in a power of two world
b.) Most drivers will convert 24 BPP textures to 32 BPP anyway because the hardware is optimized for 32 BPP.

02-21-2005, 03:24 PM
If you are talking about your desktop, then you should set your BPP to 32 or 16, never 24.
24 will probably give you software mode for GL and D3D may not eve run.

I'm not sure if 24 is even offered these days. Very old cards had it.

sam-morgenfeld
02-22-2005, 02:11 AM
thanks for your answers. I think I see it clear, now. :cool: