Bash Batch Resize Images File Size Problem

Question

I have a small script that I use to resize all of the images in a directory. I run this script in cygwin and it uses "convert" to do the image resizing. The images change their resolution just fine, but I am having problems with file sizes after the script is run.

I typically use this script to resize images dumped out from a Powerpoint presentation to use in a little web presentation app that I wrote. When I dump out gif's and run the script, the files more than double in size (ex. 8KB to 18KB; 14KB to 50KB)

The pertinent lines of the script are as follows:

/usr/bin/convert $holdfile -thumbnail x480 temp.GIF
mv temp.GIF $i

Is there a switch to prevent the file sizes from growing so much? I know that the file sizes are not huge, but when I have a good number of people connecting to a presentation or the unavoidable dialup users, I just want to make their experience as nice as possible.

Edit: I should have specified that the files start at a 960px x 720px resolution and are being resized to 640px x 480px.

Answer 1

Well, this can happen if convert compresses worse than the input files. Since the exact same compression scheme might yield different results depending how good the compressing code is this can happen.

Another, more likely option here would probably be that you are resizing the images which will probably be done with bicubic resizing. This causes the edges of text or drawings to become a little bit blurry. This means they use up more colors and compress worse.

Also likely would be that your original images use an optimized color palette, maybe just with a few colors and after resizing they need the full 256 colors which are supported by a single GIF frame, due to smoothing done by the resizing.

In any case, you probably should see better performance using PNG instead of GIF. PNG was designed as a modern replacement for GIF and no (graphical) browser in use today has problems displaying PNGs (without an alpha channel). PNG compresses much better than GIF, and allows more colors at the same time. Also there are tools like optipng which will compress PNG images even further.

Answer 2

Convert automatically optimizes the palette however the palette might be growing due to colors being blended during resize. You should be able to inspect the source and resultant images in a graphics program and see the number of colors.

GIF only supports LZW compression but due to patent restraints that have since expired (the last was 2004) it was once necessary to manually enable LZW compression. I'm not sure if that is still the case however it's worth looking into.

If LZW compression is specified but LZW compression has not been enabled, the image data is written in an uncompressed LZW format that can be read by LZW decoders. This may result in larger-than-expected GIF files. - imagemagick.org