Path: utzoo!attcan!uunet!munnari.oz.au!uokmax!apple!sun-barr!cs.utexas.edu!wuarchive!texbell!merch!spudge!johnm
From: johnm@spudge.UUCP (John Munsch)
Newsgroups: comp.sys.amiga.tech
Subject: Re: GIF viewer
Keywords: GIF
Message-ID: <10412@spudge.UUCP>
Date: 7 Feb 90 18:38:59 GMT
References: <2758@umbc3.UMBC.EDU>
Reply-To: johnm@spudge.UUCP (John Munsch)
Distribution: comp.sys.amiga.tech
Organization: Friends of Guru Bob
Lines: 72

In article <2758@umbc3.UMBC.EDU> jhon@menkar.gsfc.nasa.gov (Jhon Honce) writes:
>I have a number of GIF files that look real nice on my Sun at work, 256
>color and all.  But I've only found one PD program that will display
>these files on the amiga "hamgif"  This program doesn't seem to display
>the same number of colors as on the Sun.
				[stuff deleted]
>1) What is the limit to the number of colors displayed by the amiga
>   for given modes? (I think HAM and SHAM should do the trick but if
>   I know for sure I wouldn't be asking.)

Under HAM mode you can theoretically show 4096 colors simultaneously (the whole
spectrum the Amiga is capable of producing).  In practice it doesn't usually
work out that way because HAM is kind of kludgy.  You have 16 base colors you 
can use anywhere on the screen but if you want to show a color that isn't in
that basic palette you can do so by specifying that only ONE of its three
components (Red, Green, and Blue) will change with respect to the pixel to its
left.  An example will help.

	Lets say you have a line of black pixels with one red pixel midway
	into it.  If black is in your basic palette but red isn't,  you
	can put black pixels across until you get to the one that you want
	to be red and then just change the red component with respect to
	the pixel to its left.  Yippee!  Everything looks just like you wanted.

	Unfortunately we hit a big stumbling block sometimes.  What happens
	when you have another pixel just to the right of the red pixel that
	is a little bit redder and also a little bit bluer (and that color
	isn't in your basic palette).  There's no way to make that pixel
	come out looking correct.  You can change the red component or the
	blue component to match what you wanted but not both.

	Since this sort of thing usually occurs at the boundaries of objects
	in pictures it frequently ends up looking like there is a strangely
	colored fringe along the right hand side of objects.

SHAM mode is an attempt to fix that problem by having a different basic
palette for each line.  When the basic palette can be optimized on a line
by line basis it is a whole lot easier to pick colors that will eliminate
the need for color fringe.  It still displays the same 4096 colors but the
accuracy with which the colors can match what you want on a pixel by pixel
basis is a lot better.

>2) Has anyone had any luck with the GIFtoTMP/TMPtoIFF programs?

I've used them without problem on everything except very large pictures
(640 x 400 or larger).  On very large pictures the 1.0 versions of the
GIFtoIFF stuff had real problems.  Fortunately version 2.0 just came out
and should be appearing soon on a BBS near you.

>3) Is there a better GIF viewer for the Amiga, and if so, do you know
>   where I could ftp it from?

Ftp...some people just don't know how good they have it :->.

I sometimes use a program called VirtGif.  It really isn't much better about
showing ham pics than Hamgif is but it can show really big ones on a
superbitmap screen so you can scroll around, which is kind of neat.

The best you'll get for displaying GIF pictures is to use the publicly
available GIF to SHAM converters and then show them in SHAM mode.  The
results should be much closer to what you're used to on the SUN.

>4) Where can I read about the different display modes for the Amiga?
>   (Then I will be quiet and just read.)

One way is to purchase the new Amiga Rom Kernal Manual: Libraries and Devices.
It will give you a good view of what the amiga's video can do and its a
must have if you ever intend to program yours.

Hope that answered some questions.

John Munsch