Re: Transparent GIF's

[Matt Ion <soundy -at- NEXTLEVEL -dot- COM>]

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
settings:

       These options determine how PMView saves GIF
       files.

       *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
you.

>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,
 Matt
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