On Thu, Nov 15, 2001 at 04:12:08PM +0000, Edd Robbins wrote: > Hi. Can anyone tell me what packed pixels are and what is meant by their > support in X?
As I understand it, "packed pixel modes" are those in which pixel data is not aligned on (Intel 80386+) word boundaries, which are 32 bits. For instance, if you only have 24 bits of color depth there's no inherent reason you can't have a pixel every contiguous 3 bytes in the video card's framebuffer memory. The problem is that most PC hardware is accustomed to transferring 32 bits (4 bytes) at a time. Also, you have to a little more bit-banging in the video driver software because your pixels are aligned on word boundaries (with pixels that are 8, 16, or 32 bits wide, you know that a new pixel starts at every word fetched from, pushed to, the framebuffer). IIRC packed pixel modes were a little bit of a hack introduced by some video card manufacturers (like ATI) so that you could squeeze bigger 2D framebuffers into smaller amounts of RAM, since at depth 24 but 32 bits per pixel, 8 bits of information were unused. Nowadays, this is unnecessary because 1) RAM has gotten cheap and 2) most video cards have far more RAM than anyone would need for a 2D framebuffer, because 3D acceleration -- which demands more RAM -- is now the controlling factor in how much video memory goes on a video card. So, there's plenty of room for as big a 2D framebuffer as you want on most modern cards. The limiting factor for 2D these days is, in my limited experience, high-quality RAMDACs. Now, one of the guys who actually writes video drivers for XFree86 can come along and point out how everything I've just told you is incorrect. :) -- G. Branden Robinson | A committee is a life form with six Debian GNU/Linux | or more legs and no brain. [EMAIL PROTECTED] | -- Robert Heinlein http://people.debian.org/~branden/ |
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