GIF viewers are inconsistent

GIF viewers are inconsistent

Post by p.. » Sat, 02 Jun 1990 19:37:00



I created a GIF output file using the program FRACTINT.  I then displayed
that file using different GIF viewers, VGIF, VUGIF, SHWGIF, and FASTGIF.
All showed DIFFERENT colors.  I thought the color pallette was a part of
the GIF format.  Can someone explain why different programs show the same
GIF file in different colors?  Is something defective in the file?


 
 
 

GIF viewers are inconsistent

Post by venu.p.gop » Sat, 02 Jun 1990 21:27:00



>I created a GIF output file using the program FRACTINT.  I then displayed
>that file using different GIF viewers, VGIF, VUGIF, SHWGIF, and FASTGIF.
>All showed DIFFERENT colors.  I thought the color pallette was a part of
>the GIF format.  Can someone explain why different programs show the same
>GIF file in different colors?  Is something defective in the file?

True color representation depends on the graphics display card you are
using.  GIF allows 256 colors, each one of which can be one of 16.7 million
possible colors.  If you are using a CGA, you are limited to 4 colors that
are pre-fixed.  How can you possibly do even a reasonable job on a CGA ?

If you are using an EGA, you have 16 colors that can be selected from
a possible set of 64 colors.  Better, but far from what is needed for
reasonable color representation.

VGA and SuperVGA 256 colors modes are much better.  Though you do not
get the full color resolution that GIF is capable of, you can still
select 256 colors out of 262 thousand colors.  You will find that when
you use these modes, the differences between the display programs
disappear.  The differences mainly exist because the GIF colors
specifications have to be approximated by what the display card can
show and different programs have different methods.

Venu P. Gopal
UUCP:   att!ihuxy!vgopal


Silence those silent letters and save the world 500 million keystrokes a day.

 
 
 

GIF viewers are inconsistent

Post by Peter da Sil » Sat, 02 Jun 1990 12:33:00


GIF is restricted to 256 unique colors? How is one to convert Amiga HAM
images (with up to 4096 unique colors) to GIF? Some will work, but others
won't.
--
Peter da Silva, Xenix Support, Ferranti International Controls Corporation.



 
 
 

GIF viewers are inconsistent

Post by Paul A. Thiess » Sat, 02 Jun 1990 17:15:00



>disappear.  The differences mainly exist because the GIF colors
>specifications have to be approximated by what the display card can
>show and different programs have different methods.

>Venu P. Gopal
>UUCP:       att!ihuxy!vgopal


>Silence those silent letters and save the world 500 million keystrokes a day.

More specifially, GIF can handle 8 bits of color resolution, meaning
every color can have red, green, and blue values from 0 to 255. But VGA
is only 6 bits, meaning RGB values from 0..63. So to display on VGA,
the GIF program has to scale the 8 bits down to 6 bits which can cause
color distortion and differences in colors depending on exactly how the
program "crunches" 256 values into 64.

           - Paul

--
------------------------------------------------------------


------------------------------------------------------------

 
 
 

GIF viewers are inconsistent

Post by Jim Alves-Fo » Sat, 02 Jun 1990 21:40:00




>>...  Can someone explain why different programs show the same
>>GIF file in different colors?  Is something defective in the file?
> ...
>select 256 colors out of 262 thousand colors.  You will find that when
>you use these modes, the differences between the display programs

                          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

Quote:>disappear.  The differences mainly exist because the GIF colors
 ^^^^^^^^^
>specifications have to be approximated by what the display card can
>show and different programs have different methods.

Different display programs also have different default modes. FASTGIF
displays GIF files in EGA modes but does a good approximation of better modes
using a dithering technique, VGIF uses the VGA modes and I'm unsure
about the others. Check the documentation that comes with these programs to
see what modes they support and then use the one best suited for YOUR
particular hardware configuration (ie FASTGIF for EGA, VGIF for VGA, ...)


 
 
 

GIF viewers are inconsistent

Post by p.. » Sat, 02 Jun 1990 17:24:00


Quote:> True color representation depends on the graphics display card you are
> using.  GIF allows 256 colors, each one of which can be one of 16.7 million
> possible colors.  If you are using a CGA, you are limited to 4 colors that
> are pre-fixed.  How can you possibly do even a reasonable job on a CGA ?

I would not be able to, but at that point I'd expect to see that same set
of 4 colors (or less) from all viewers that work, although not necessarily
assigned to the same original color.

Quote:> If you are using an EGA, you have 16 colors that can be selected from
> a possible set of 64 colors.  Better, but far from what is needed for
> reasonable color representation.
> VGA and SuperVGA 256 colors modes are much better.  Though you do not
> get the full color resolution that GIF is capable of, you can still
> select 256 colors out of 262 thousand colors.  You will find that when
> you use these modes, the differences between the display programs
> disappear.  The differences mainly exist because the GIF colors
> specifications have to be approximated by what the display card can
> show and different programs have different methods.

I have standard IBM VGA in a PS/2-30/286.

Quote:> More specifially, GIF can handle 8 bits of color resolution, meaning
> every color can have red, green, and blue values from 0 to 255. But VGA
> is only 6 bits, meaning RGB values from 0..63. So to display on VGA,
> the GIF program has to scale the 8 bits down to 6 bits which can cause
> color distortion and differences in colors depending on exactly how the
> program "crunches" 256 values into 64.

So, I'd expect the colors to be truncated this way, and therefore still have
the same colors displayed on my 256-of-262144 display from a file having
256-of-16777216 colors.  The colors are instead DRAMATICALLY different.
One viewer shows one area as several over saturated hues of the rainbow
and another viewer shows the same thing as a SINGLE ugly green hue.

Quote:> Different display programs also have different default modes. FASTGIF
> displays GIF files in EGA modes but does a good approximation of better modes
> using a dithering technique, VGIF uses the VGA modes and I'm unsure
> about the others. Check the documentation that comes with these programs to
> see what modes they support and then use the one best suited for YOUR
> particular hardware configuration (ie FASTGIF for EGA, VGIF for VGA, ...)

This indeed seems like what must be happening.  Although ALL viewers are
squishing the images (360x480) into the left side, some were truncating it
on the bottom, indicating the likelyhood of being in the EGA (640x350) mode.

The FRACTINT program has a tweaked VGA mode of 360x480x256 with non-square
pixels.  It works by loading the registers directly instead of calling BIOS
to set a graphics mode (limited choices apparently).  Documentation says this
works only on IBM VGA *REGISTER COMPATIBLES*.  I'd like to see a viewer that
can use this mode.  FRACTINT source is available, so why not.  So far, I have
NO SOURCE to any viewers, so I can't get in and FIX THEM.


 
 
 

GIF viewers are inconsistent

Post by Urs Zurbuch » Sat, 02 Jun 1990 20:50:00



>I created a GIF output file using the program FRACTINT.  I then displayed
>that file using different GIF viewers, VGIF, VUGIF, SHWGIF, and FASTGIF.
>All showed DIFFERENT colors.

I experienced a similar problem here with a .gif file showing a girl's
head as a photograph in the upper left corner and a logo 'grafx' at the
lower right. Using Vgif the picture displayed correctly, but with
Fastgif it was hardly recognizable.
None of the 10 other .gif files I have on my harddisk showed a similar
amount of deviation.

Any ideas?

                ...urs

UUCP (dumb): {backbone}!mcvax!cernvax!ethz!zu

Fido: 2:302/801.36

 
 
 

GIF viewers are inconsistent

Post by Jef Poskanz » Sat, 02 Jun 1990 07:58:00



Quote:}GIF is restricted to 256 unique colors? How is one to convert Amiga HAM
}images (with up to 4096 unique colors) to GIF? Some will work, but others
}won't.

To say nothing of converting professional-quality 24 bit deep images to
a 256-element colormap.  This is known as the color quantization problem.
If you read comp.graphics for a while, you will be sure to see solutions
posted.
---
Jef


              "What we've got here is a failure to communicate."

 
 
 

GIF viewers are inconsistent

Post by p.. » Sat, 02 Jun 1990 16:55:00


Quote:> GIF is restricted to 256 unique colors? How is one to convert Amiga HAM
> images (with up to 4096 unique colors) to GIF? Some will work, but others
> won't.

TIFF allows true RGB colors, and that result in 16777216 colors or more.
Conversion to GIF obviously must distort the colors some.  Better programs
will be able to better estimate a good GIF pallette to use from the frequency
of usage of the whole color range.

This is also one of the drawbacks of GIF.  TIFF does not allow pallettes in
the current version I have docs for, but this is less of a drawback since a
GIF file can be converted to a TIFF by just mapping the pallette into each
pixel.


 
 
 

GIF viewers are inconsistent

Post by venu.p.gop » Sat, 02 Jun 1990 18:44:00



Quote:>GIF is restricted to 256 unique colors? How is one to convert Amiga HAM
>images (with up to 4096 unique colors) to GIF? Some will work, but others
>won't.

Yes, the current GIF standard is restricted to 256 colors only;  however,
each of the 256 colors can be one of 16.7 million colors.  If you are
converting from a 4096 color image, obviously you will lose something.
The 4096 colors, depending on their red, blue and green content, will have
to be mapped to 256 colors that should be selected from the 16.7 million.
There was some discussion on color mapping algorithms in this newsgroup
sometime earlier.

The 256 color restriction is obviously based on the 8-bit byte so that a
color can be represented by a byte.  You can actually do a lot with
256 colors, I have seen a lot of very impressive images.

Venu P. Gopal
UUCP:   att!ihuxy!vgopal


Silence those silent letters and save the world 500 million keystrokes a day.

 
 
 

GIF viewers are inconsistent

Post by Klaas Hemst » Sat, 02 Jun 1990 08:02:00


I would like to get to the bottom of this, because I also have had some
strange colours.

I think the problem of the GIF viewers is when the size of the image is larger
than 320 x 200 AND there are more than 16 colours.
Some GIF viewers automatically choose EGA or VGA modes with 16 colours to
display these images. This will result in scaling down the number of colours
from x (mostly 256) to 16, which cannot be done very well. Most likely this
feature causes the inconsistent display of images.
Some of the GIF viewers give you the possibility to manually select the
displaymode, with that You might be able to check whether it is correct for
your pictures and monitors.

Anybody who thinks this is not correct ?
Please mail me with more information about your particular system.
When there are other reasons I will post them.

                                                        Klaas


uucp: ..{uunet!}hp4nl!mh.nl!hst              |   /--/  `-,    /  ___  |_/ |__|
Multihouse N.V., Gouda, the Netherlands      |  /  / ___/    /   ---  | \ |  |
"Most of us mindreaders are atheist, you know" A song for Lya: George Martin

 
 
 

GIF viewers are inconsistent

Post by Peter da Sil » Sat, 02 Jun 1990 15:24:00




> }GIF is restricted to 256 unique colors? How is one to convert Amiga HAM
> }images (with up to 4096 unique colors) to GIF? Some will work, but others
> }won't.
> To say nothing of converting professional-quality 24 bit deep images to
> a 256-element colormap.  This is known as the color quantization problem.
> If you read comp.graphics for a while, you will be sure to see solutions
> posted.

So, basically, GIF is an inadequate format. Too bad it's so popular.

So, what does that leave? IFF/ILBM should do a better job, but it's not
popular outside the Amiga.

What about this 'TIFF' format?
--
Peter da Silva, Xenix Support, Ferranti International Controls Corporation.



 
 
 

GIF viewers are inconsistent

Post by Paul A. Thiess » Sat, 02 Jun 1990 16:31:00



Quote:>GIF is restricted to 256 unique colors? How is one to convert Amiga HAM
>images (with up to 4096 unique colors) to GIF? Some will work, but others
>won't.

Just a note: GIF can only have 256 colors *on screen*, from a choice of
about 16 million (256**3). I don't know if that's what you meant, but
it is true that a choice of only 256 colors would be lame...

          - Paul
--
------------------------------------------------------------


------------------------------------------------------------

 
 
 

GIF viewers are inconsistent

Post by venu.p.gop » Sat, 02 Jun 1990 21:06:00


..
..So, I'd expect the colors to be truncated this way, and therefore still have
..the same colors displayed on my 256-of-262144 display from a file having
..256-of-16777216 colors.  The colors are instead DRAMATICALLY different.
..One viewer shows one area as several over saturated hues of the rainbow
..and another viewer shows the same thing as a SINGLE ugly green hue.

..> Different display programs also have different default modes. FASTGIF
..> displays GIF files in EGA modes but does a good approximation of better modes
..> using a dithering technique, VGIF uses the VGA modes and I'm unsure
..> about the others. Check the documentation that comes with these programs to
..> see what modes they support and then use the one best suited for YOUR
..> particular hardware configuration (ie FASTGIF for EGA, VGIF for VGA, ...)
..
..This indeed seems like what must be happening.  Although ALL viewers are
..squishing the images (360x480) into the left side, some were truncating it
..on the bottom, indicating the likelyhood of being in the EGA (640x350) mode.
..
..The FRACTINT program has a tweaked VGA mode of 360x480x256 with non-square
..pixels.  It works by loading the registers directly instead of calling BIOS
..to set a graphics mode (limited choices apparently).  Documentation says this
..works only on IBM VGA *REGISTER COMPATIBLES*.  I'd like to see a viewer that
..can use this mode.  FRACTINT source is available, so why not.  So far, I have
..NO SOURCE to any viewers, so I can't get in and FIX THEM.
..

You may want to try VUGIF, posted to c.b.i.p. a short while ago, and
probably available from clarkson, simtel etc. by ftp.

You can select the video mode yourself from the ump* choices on
the menu and see for yourself what is happening.

It will also scale the picture (if you want) to fit the screen so that
your image size and color quantization effects can be independently seen.

Venu P. Gopal
UUCP:   att!ihuxy!vgopal


Silence those silent letters and save the world 500 million keystrokes a day.

 
 
 

GIF viewers are inconsistent

Post by p.. » Sun, 03 Jun 1990 00:06:00


Quote:> To say nothing of converting professional-quality 24 bit deep images to
> a 256-element colormap.  This is known as the color quantization problem.
> If you read comp.graphics for a while, you will be sure to see solutions
> posted.

I hope some of those solutions show up as public domain source code to
display TIFF files on 256 color displays (PC VGA, Mac II, or even Amiga)
or TIFF to GIF conversion.


 
 
 

1. Tiff 5.0 (Re: GIF viewers are inconsistent)

Tiff 5.0 includes the following color Tags:
320 Color Map --  Defines the RGB color map for palatte color images.
262 PhotometricInterpretation
              --  New modes to handle Palette color and Greyscale

To get a Tiff Developer's ToolKit send $30 (includes postage) to
Aldus Corporation
411 First Avenue South
Suite 220
Seattle, WA 98104              Or Call (206) 622 - 5500 for info

I've ordered one, but it hasn't come in yet.

2. lighing and direction of normal vectors

3. GIF to PS, GIF viewer for X, VAX/VMS

4. Hot Wax Coating problem

5. GIF Help:where is a GIF-Viewer for the ATARI ST

6. Windows Equivalent of Graphic Converter?

7. Hi, I am doing a paper on 3D-boards, and I am looking for definitions

8. HELP! i-Mate, iMac & Calcomp NOT WORKING!

9. Is it me or BMRT who is inconsistent?

10. POV parse inconsistent

11. clone tool inconsistent?

12. Inconsistent Print Quality: PSP problem maybe?

13. inconsistent Text Spacing Prob in PS5