Re: Transparent GIF's

Subject: Re: Transparent GIF's
From: Matt Ion <soundy -at- NEXTLEVEL -dot- COM>
Date: Sun, 18 May 1997 15:17:54 -0800

On Thu, 15 May 1997 23:55:51 -0500, Christine Bain wrote:

>1) From what I understand in the case of HTML images, it is not enough to
>turn an image transparent in a graphics tool. You have to use a utility
>that transposes the GIF into what is called an 87a or 89a. This is so that
>the browser sees the image as transparent and allows the background color or
>background image to come through.

I believe that one particular color must be designated as the
"transparent" color; the browser then knows to display the background
through that color. It's usually, but not always, the most prevalent
or surrounding color, but that exact color may vary as well.

>Does anyone know what the difference between an 87a and an 89a is?

Here's a snippet from the PMView online help:

General Description

The GIF format was designed by Compuserve in
1987. Since then it has become very popular for
general image exchange. There are two types
of GIF files; GIF87a, the original standard of 1987
and GIF89a, the revised standard of 1989 that
adds features for including comments, animation
and text.

The GIF format is a lossless format and is good
for saving any type of image that has 256
colors (or shades of gray), or fewer.

The GIF Save Options dialog includes "Always Write GIF89a Format" and
"Interlace" checkboxes, as well as settings for "Transparent Color
Index" and "Background Color Index". Further information on those

These options determine how PMView saves GIF

*Always Write GIF89a Format* If on, the file
will always be written in the newer GIF89a
format. If off, the file will be written in GIF87a
format, unless the Transparent Color Index
option is used. The GIF89a format has a new
color table flag that specifies whether the color
table is sorted or not. If you know that the
target application can take advantage of this
information, you may turn on this option and
the sort flag will be set (PMView always keeps
the palette sorted). The flag will not be set for
GIF87a files in order to preserve compability
with older applications that (erroneously) will
not accept GIF87a files with the sort flag set.

*Interlace* If on, the image will be saved using
4-pass interlacing. If off, the image will be
saved sequentially in one pass. Using interlacing
only makes sense with programs that
progressively update the display while loading.
For instance, when loading an interlaced GIF file
the image will gradually be displayed during
four passes. At the end of the first pass each
eighth row of the image is visible. Already at
this point of loading you'll probably see what
the image looks like. With each pass the image
quality will gradually improve as missing lines
are added.

*Transparent Color Index* If on, transparency
information will be written to the GIF file. Use
this option if you would like one of the colors
in your image to be transparent with the
background. Typically you'll need this if you
make images for WWW-browsers. Note that
transparency information requires the newer
GIF-variant; GIF89a. Thus, if this option is on,
the file will be written in the GIF89a format
instead of GIF87a. Note that PMView will not use
transparency information when loading GIF files.

*Background Color Index* If on, the specified
background color index will be written to the
GIF file. Use this option if the target application
fills unused space with the color specified in the
GIF file. Typically may need this if you make
images for WWW-browsers. If off, index zero
(0) will be written to the file (in PMView index
zero will always correspond to the most used
color in the image). Note that PMView will not
use this information when loading GIF files.

>2) I have been searching for a utility that can do batch conversions of
>GIF's to transparent 87a or 89a. I have not had any luck.

I have at least two or three different utilities that will do batch
conversions, including PMView 1.00 and a plug-in for ColorWorks v2.
However, these are all OS/2 applications, so they probably won't help

>I don't think it is possible to do a batch conversion. The reason is that
>each image has to be opened in the conversion utility and then you must
>click on the area that is to be transparent.

That depends on the software. PMView, as noted, allows you to define a
color (0-255 in the GIF color palette) as transparent. Once GIF Save
Options are set, those options are used for all GIF saves (and format
conversions) until changed again.

>3) Is this the same for screen captures? I'm not sure, but would assume it
>is. Does anyone know?

Screen capture is just a source of an image. Transparency is specific
to the GIF storage format. Any image, regardless of how it was
created, can be saved as a transparent GIF.

>4) The procedure can go pretty quick if you have all of your images in a
>separate graphics directory from your html files.

Why would that make a difference? Just work on a *.GIF filemask.

Your friend and mine,
<insert standard disclaimer here>

And you can see them there, on Sunday mornning
They stand up and sing about, what it's like up there
They call it Paradise, I don't know why
You call someplace Paradise... kiss it goodbye...
- Eagles, "The Last Resort"

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