cross-posted from: https://linkage.ds8.zone/post/523771

Before posting an image to the fedi, I want to be mindful about the network burden it will cause. I’m only uploading the image once but potentially thousands of people could end up downloading it.

If it’s a color image, then JPG is typically best. This #ImageMagick command reduces the filesize quite a bit, trading off quality:

  $ convert "$original_image_file" \
    +dither \
    -posterize 8 \
    -sampling-factor 4:2:0 \
    -strip \
    -quality 75 \
    -interlace Plane \
    -gaussian-blur 0.05 \
    -colorspace RGB \
    -strip \
    smaller_file.jpg

If it’s a pic of a person, this processing will likely be a disaster. But for most things where color doesn’t matter too much, it can be quite useful. Play with different -posterize values.

If you can do with fewer pixels, adding a -resize helps.

  $ convert "$original_image_file" -resize 215x smaller_file.jpg

If you can get away with black and white, jpeg is terrible. Use PNG instead. E.g.

  $ convert "$original_image_file" -threshold 30% -type bilevel smaller_file.png

For privacy, strip the metadata

The ImageMagick -strip option supposedly strips out metadata. But it’s apparently not thorough because the following command yields a slightly smaller file size:

  $ exiftool -all= image.jpg

What else?

Did I miss anything? Any opportunities to shrink images further? In principle the DjVu format would be more compact but it’s not mainstream and apparently not accepted by Lemmy.