On Tuesday 02 Aug 2005 00:01, Arnt Karlsen wrote: > On Mon, 01 Aug 2005 18:14:16 -0400, Josh wrote in message > > <[EMAIL PROTECTED]>: > > Arnt Karlsen wrote: > > > ..pass, what I learned from my own research on gpu's > > > before buying an ATI 9250 clone, is ATI are "native 24bpp" > > > and "24bpp only", where Nvidia is "1x32bpp or 2x16bpp", > > > suggesting "ATI would suck at 16bpp doing less than > > > 3x8bpp" and "at 32bpp not being able to see or make any > > > use of the top 8 bits." > > > My understanding of Nvidea is "their cards should work > > > better at 32bpp and 16bpp than at 24bpp, because 24bpp > > > wastes half a 16bpp engine." > > > > > > > > > > > >From what I understand, 24bpp is the same amount of data as > > > 32bpp. It > > > > just signifies that there is a separate alpha channel. Since > > this is not strictly 'color' the last 8 alpha bits are not > > counted in the color depth. > > ..yes, but does this impact 32bpp performance relative to > 24bpp and "not" 24bpp relative to 16bpp like it "should" on > ATI's and "should not" on Nvidea and vice versa? > > > Still, each pixel takes up 32 bits of memory. > > ..my understanding is ATI cannot do 32bpp math at all, their > gpus are "24bpp only", while Nvidea gpus does both 16bpp and > 32bpp "but not 24bpp." > Strategic gpu HW design choises made a decade or so back. > > > ATI cards do 16bpp just the same as all the other cards, 16 > > bits of color and nothing else. (red and blue get 5bpp, > > green I think is the one that gets 6bpp) > > ..true, at the same speed as they will do 24bpp, 15bpp and > possibly also 8bpp, I doubt ATI gpu's has a 3x8bpp mode, > Nvidea however talks about a 2x16bpp and an 1x32bpp mode.
As Josh said, in a 32 bpp mode 8 bits are used for an alpha channel so there isn't really any 32 bpp maths to worry about. It probably makes more sense to think in terms of 3x8 bpp for 24 bit modes or 4x8 bpp for 32 bit display modes, with each 8 bit channel giving 256 levels of intensity (brightness). 8 bit and lower colour modes work differently and use an indexed palette and a look-up table of absolute rgb values. The actual colours in the palette can have any value but you're limited to a total of 256 of 'em. An 8 bit greyscale mode is essentially the same as one of the 24 bit colour modes channels except you're only dealing with absolute brightness - all colour info is ignored. 24 bpp data can be displayed on a 15 or 16 bit mode simply by discarding the least significant bits of each channel. This can produce some colour banding and other undesired artifacts but it's economical as the data requires no conversion. After 24 bpp the next commonly used colour mode is 36 bpp - 3x12 bpp. This is mainly used for print stuff. LeeE _______________________________________________ Flightgear-devel mailing list [email protected] http://mail.flightgear.org/mailman/listinfo/flightgear-devel 2f585eeea02e2c79d7b1d8c4963bae2d
