Path: utzoo!utgpu!news-server.csri.toronto.edu!rpi!zaphod.mps.ohio-state.edu!ncar!gatech!hubcap!ncrcae!ncr-sd!se-sd!cns!dltaylor
From: dltaylor@cns.SanDiego.NCR.COM (Dan Taylor)
Newsgroups: comp.sys.amiga.graphics
Subject: Re: So, does *anyone* make 1024x768 x256 for the A3000?
Message-ID: <911@cns.SanDiego.NCR.COM>
Date: 25 Apr 91 21:18:39 GMT
References: <15230@helios.TAMU.EDU> <1991Apr24.162452.22106@hubcap.clemson.edu> <15250@helios.TAMU.EDU> <1991Apr24.211729.19270@bronze.ucs.indiana.edu>
Organization: NCR Corp. SE-San Diego
Lines: 43

In <1991Apr24.211729.19270@bronze.ucs.indiana.edu> tagreen@lothario.ucs.indiana.edu (Todd Green) writes:


>I think you mean 24 bits of information or a 24-bit plane, (i.e. 24
>bits of information for each pixel) and not 24 planes per se.

Todd, what's the difference?

A quick display tutorial:

	The Amiga display thinks in bit-planes.  The bytes in A bit-
	plane represent 1 degree of freedom for the display of 8 con-
	secutive PIXELS.  The degree of freedom could be part of a color
	lookup table index, one bit of a direct grey-scale or direct-
	mapped color, or a special function such as alpha or z.  The
	"normal" Amiga uses it as part of a color lookup table index.
	So, if monochrome is all I need in some portion, or all, of
	the display, I allocate the number of bytes which is the number
	of pixels, divided by 8 (plus boundary rounding).  A 640x400
	monochrome display is, therefore, 32000 bytes.  Similarly, a 16-
	color display takes 4 bitplanes, one for each degree of freedom
	required.  The bitplanes need not be contiguous, and different
	displays on the screen can be different depths, thereby conserving
	memory.  A 24-bit display, then would have 24 bitplanes.

	Other display families allocate display memory in nibbles, bytes,
	words, long-words, etc. such the each nibble... represents the
	entire value of 1 pixel.  A 4-color (two bits) display on a byte-
	oriented system "wastes" 6 bits per pixel.  Often, 24-bit display
	systems are direct-mapped.  For ease of use the pixel addresses
	are located on long-word boundaries, creating "holes" in the
	display address map.  Some systems fill the holes with alpha or
	z information.  A special purpose "true-color" display that
	is not going to be used for any other output can be built cheaper
	in this fashion, rather than having to keep track of the 24
	pointers to the bitplanes.  There are some systems that will
	remap the ENTIRE display memory from 2 to 4 to 8 or whatever,
	but they cost more.

So, there is NO difference in the color resolution between a "24-bit"
display and a display that supports 24 bitplanes.

Dan Taylor