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Path: utzoo!mnetor!seismo!rutgers!ames!cit-vax!oberon!bacall!papa
From: papa@bacall.UUCP (Marco Papa)
Newsgroups: comp.graphics
Subject: Re: Amiga World Ray-tracing article...
Message-ID: <2383@bacall.UUCP>
Date: Sun, 19-Apr-87 21:56:39 EST
Article-I.D.: bacall.2383
Posted: Sun Apr 19 21:56:39 1987
Date-Received: Wed, 22-Apr-87 00:56:57 EST
References: <239@rocky.STANFORD.EDU> <662@aurora.UUCP>
Distribution: comp
Organization: CS&CE Depts, U.S.C., Los Angeles, CA
Lines: 65

> in article <239@rocky.STANFORD.EDU>, ali@rocky.STANFORD.EDU (Ali Ozer) says:
> + 
> + In article <647@puff.WISC.EDU> beilke@puff.WISC.EDU (Matthew Beilke) writes:
> +>Make that 320X400 6-bit H.A.M.  The Amiga doesn't have the capability to
> +>display > 6 bit planes.
> + 
> + Despite that (just to set the record straight for comp.graphics 
> + readers not familiar with the Amiga), the Amiga can display 4096 
> + colors at once, using 6 bit planes.
>
Jeff Mulligan replys: 
> Could you elaborate a bit on how they achieve this remarkable feat?
> (No sarcasm intended.)

I am quoting from the Amiga Hardware Manual:

"Hold and Modify Mode.

In this mode you can display up to 4,096 colors on the screen at the same time.
... Hold and Modify allows very fine gradients of color or shading to be 
produced on the screen. ... In Hold and Modify mode, you use all 6 bitplanes.
Planes 5 and 6 are used to modify the way bits from planes 1 - 4 are treated, as
follows:

* if the 6-5 bit combination from planes 6 and 5 for any given pixel is 00,
normal color selecton procedure is followed. Thus, the bit combinations from
planes 4-1, in that order of significance, are used to choose one of 16-color
registers (registers 0 -15).

 if only 5 bit-planes are used, the data from the 6-th plane is automatically 
supplied with the value as 0.

* if the 6-5 combination is 01, the color of the pixel immediately to the left
of this pixel is duplicated and then modified. The bit combinations from planes
4 - 1 are used to replace the 4 "BLUE" bits in the corresponding color 
registers.

* if the 6-5 bit combination is 10, [same as the previous one except that ] 
we replace the 4 "RED" bits.

* if the 6-5 bit combination is 11, [same as the previous one except that]
we replace the 4 "GREEN' bits."

Note that this is not that complex to handle. There are now a variety of
programs for the AMIGA that use hold and modify.  Some great demos, like
the 3-D ray-traced real-time juggler (see it on the cover of AmigaWorld),
a "freeware" ray-tracing program by Dave Wecker, and a 4096-color paint 
program called Digi-View by NewTek, for example.  Dave's program comes with
source-code, so you can even see how it is done.

Rumour has it that Jay Miner, the designer of the Amiga graphics chips, almost
left out "Hold and Modify" since "nobody would use it".

-- Marco

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