Path: utzoo!utgpu!news-server.csri.toronto.edu!rpi!zaphod.mps.ohio-state.edu!think.com!spool.mu.edu!uunet!cbmvax!amix!undrground!accangel From: undrground!accangel@amix.commodore.com (Mark Gardner) Newsgroups: comp.sys.amiga.advocacy Subject: Re: Amiga Color Capabilities Message-ID: Date: 2 May 91 04:12:31 GMT Article-I.D.: undrgrou.kJXa21w164w References: <91120.110153MBS110@psuvm.psu.edu> Lines: 66 Okay, this is as definitive a description of the Amiga graphics modes as I can give. An Amiga without version 2.0 or later of the operating system can display screens at 320x200, 640x200, 320x400, and 640x400 pixel resolution. The latter two's vertical resolution is achieved by having the screen's electronic beam draw every other line, then moving back up to the top of the screen, down a half-line, and then drawing the other 200, in between the first set. This is called interlacing, and is why high-contrast screens of this nature appear to "flicker" on the Amiga - the odd lines are beginning to fade as the even ones are being drawn, and vice versa. Mow, as far as color resolution is concerned, an Amiga without any add-on graphics boards has what are known as 32 color registers. Think of them as electronic inkwells - they each store three numbers from 0 to 15, defining a color by the amounts of red, green, and blue mixed into it. By doing a little arithmetic, it is shown that the Amiga has 4096 colors to choose from to put in these color registers (16 to the power of 3). This is where it gets tricky. For each pixel on the screen, the Amiga assigns a number referring to one of the color registers. However, there are certain limits imposed - for instance, both 640x200 and 640x400 can only choose from 16 color registers. Amiga techies say these screens are limited to four bit-planes; that is, they are limited to 2 to the power of 4 number of colors. 320x200 and 320x400 can do up to six bit-planes - 2 to the power of 6, or 64 different values that each pixel can hold. But hold on a minute. Didn't I say earlier the Amiga only has 32 color registers (capacity to fill five bit-planes)? What is the other bit-plane used for? Well, in the case of a screen that is set to "Extra-Halfbrite" mode, it is used to tell the Amiga display to either show the regular color found in whatever color register it's set to, or to tell the display to show a half-brightness version of that color. Confused enough yet? There's another way a six bit-plane display can be used to display more than 32 colors onscreen. This mode is called HAM, short for Hold-And-Modify. This is REALLY hard to explain, so I'll try to simplify. Basically, each pixel can tell the display one of four things - A) show one of the first sixteen color registers; B) take the color of the pixel immediately to the left of this one and change its red value to something else; C) do the same as (B), only to the green value; or D) do the same as (B), only to the blue value. This is why it's called Hold-And-Modify - each pixel can HOLD some of the color found in its left neighbor and MODIFY a part of it to obtain a different color. Using that method, one can create a display that shows all 4096 of the Amiga's color palette, indirectly, using only 6 bit-planes. This saves memory, always a precious commodity on a multitasking computer like the Amiga. Mmmm. Seems I've used a LOT of lines to explain all this. I'll leave the explanation of the new graphics modes available using the Enhanced Chip Set (included in all Amigas currently being produced) and version 2.0 of the operating system (only available for the Amiga 3000 and 3000T at this time) to someone who HAS such a beastie. Me, I'm stuck with an Amiga 500, version 1.3 of the OS, and no ECS. Ah, woe is me.... -MG "Can you say LONG-WINDED, boys and girls...? Hope this helps someone out there, anyway...." ---------- Mark Gardner UUCP: uunet!cbmvax!amix!undrground!accangel Internet: undrground!accangel@amix.commodore.com