24-bit vs 32-bit colour difference?

[Pawnless Endgame]

I know the maths: 8-bit colour is 2 to the power of 8 = 256 colours. 24-bit is 2-to-the-8 3 times, 1 for each primary colour.

So, silly question, probably answered back in the mid 90s when 24/32-bit was available: When you select 32-bit colour in display properties, you still get 256 levels in each primary colour, the same as what you get for 24-bit. What happens to the extra 8-bit? Is it reserved for art programs to allow them to understand transparency as an 8-bit alpha channel? The only thing I can think of. It wouldn't be for Photoshop's understanding of CMYK (4 primary colours, 256 levels each), as ultimately, the graphics card can still only output 24-bit to the screen.

 

[Sin_Chase]

Some choice quotes from Google results:

32-bit color is simply 24-bit color with an added 8-bit alpha channel. This alpha channel is for transparency, and also used for an 8-bit grayscale mask, which is for icon or cursor shadows, animations etc.

The main purpose of 32-bit colour is really to pad out the pixels so they take up one longword. This allows much faster indexing of a pixel, speeding up graphics at the expense of RAM used.

 

[Craiig]

i believe this is for grayscale..

 

[Pawnless Endgame]

Ahhhh thanks guys So I kinda guessed right with the allowing of alpha / transparency in Paintshop Pro / GIMP / Photoshop etc.

I vaguely remember from my AS level computing back in 1997 that a variable declared as 'long' was over a certain value. Can't remember what the value was though.