Transparency...

How to use the netpbm utilities

Getting the netpbm package

To create a GIF with a transparent color, you need a few utilities from a fairly recent version of the netpbm package, which is available from ftp.x.org, ftp.cs.ubs.ca, and sunsite.unc.edu (and probably other sites too). If you FTP this, pick a site close to where you are, geographically.

You're welcome to ask me for help in compiling these, but I don't promise to be of much help. I've only recently started to learn the tricks of compiling things on odd platforms.

Here at MIT, Seth Finkelstein compiled this package for AIX, Solaris, and Ultrix, and I compiled it for SunOS:

(note, if you get the binaries for SunOS 4.1.3, you'll be using giftopnm instead of giftoppm; it should work just the same, though.)

How do I make a picture transparent?

The process of creating such an image requires several steps.

You have to have a version of ppmtogif which accepts the -transparent flag.
You need to be able to convert to ppm format the picture with which you wish to endow the GIF89 transparent extension. If you're starting with a GIF image, the simplest path requires only the giftoppm program.
You need to edit your original image so that all pixels which you wish to turn into the transparent "color" are the same color, and that this color appears only in these pixels. I found it easiest to choose some vivid color which couldn't possibly be mistaken for anything else.
Convert the image to ppm format. If you're starting with a GIF, and working on a UNIX system, it's likely that the command
```
giftoppm foobar.gif > barquux.ppm
```
will do what you want, if you replace "foobar.gif" and "barquux.ppm" with the filenames you'd like to use.
Convert the image back to gif format, specifying the color which is to become the transparent "color". This command is likely to be similar to:
```
ppmtogif -transparent #rgb barquux.ppm > fooquux.gif
```
(where, again, "barquux.ppm" and "fooquux.gif" are replaced with the appropriate filenames, and "#rgb" is replaced with the name of the color which you are converting to a transparent color). Most frequently, #rgb will be a hexidecimal number like "#e95cef" (a shade of purple) or "#ffffff" (white) , though you could conceiveably use any of the acceptable color specifications which are listed in the ppmmake (1) manpage. I usually use white for the color which I am going to make transparent - it stands out in most pictures fairly well. Another color you might want to use is #bfbfbf, which is the default background color of Mosaic for X.
If some of these commands that contain a hash (or pound) symbol ("#") don't work, try preceding the "#" with a backslash ("\"). For example:
```
ppmtogif -transparent \#bfbfbf foobar.ppm
```
This is necessary because some unix shells (apparently sh and ksh, and perhaps others) interpret a "#" as the beginning of a comment.
You're done. fooquux.gif is the file you want.

You could have done all this conversion in one step, also:

giftoppm foobar.gif | ppmtogif -transparent #rgb > fooquux.gif

If you have any questions about this process, please feel free to contact me for help, although I make no claims as to my actual helpfulness. :-)

Please feel free to let me know what you think.

-Eric Mumpower (nocturne@mit.edu)