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 "$original_image_file" \ +dither \ -posterize 8 \ -sampling-factor 4:2:0 \ -strip \ -quality 75 \ -interlace Plane \ -gaussian-blur 0.05 \ -colorspace RGB \ -strip \ smaller_file.jpgIf it’s a pic of a person, this processing will likely be a disaster. But for most things where color doesn’t matter too much, it can be quite useful. Play with different
-posterizevalues.If you can do with fewer pixels, adding a
-resizehelps.$ convert "$original_image_file" -resize 215x smaller_file.jpgIf you can get away with black and white, jpeg is terrible. Use PNG instead. E.g.
$ convert "$original_image_file" -threshold 30% -type bilevel smaller_file.pngFor privacy, strip the metadata
The ImageMagick
-stripoption supposedly strips out metadata. But it’s apparently not thorough because the following command yields a slightly smaller file size:$ exiftool -all= image.jpgWhat 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.
I’ve had better luck with webp as far as minimizing size while preserving acceptable quality.
Not sure what other apps do this (they really, really should!), but Tesseract can optionally pre-convert a pasted/uploaded image to webp with a user-selectable quality profile. When I ran an instance, I had a very low (250 KB) upload limit on images, and the pre-conversion made it a non-issue for all but the largest images.
That also works to strip out metadata, though I believe pict-rs (Lemmy’s image “subsystem”) will also strip out metadata (don’t quote me on this and do correct me if I’m wrong).
Edit: Also, whoever runs linkage.ds8.zone has Anubis setup wrong. The
/apipath should not be behind Anubis.I was quite confused when I read your post because Tesseract is an OCR engine. Your link helped sort it out.
I think I have come across various fedi web clients that do conversions. I think peertube shrinks videos, IIRC. The auto conversions are useful but they must be conservative in the extent of their changes. The posterizing that I do w/Imagemagick makes a dramatic change so it could not be done automatically by a client or server, as users need to review the output and decide. So I believe the best compression will always require manual effort in order to judge whether the quality loss is still acceptable for the application.
Regarding Tesseract (the lemmy client) – does that work offline? I’m always looking for a Lemmy client that can briefly connect to sync content and then support reading and writing messages when offline.
No. It’s a PWA and can run offline from cache, but everything is fetched from the API in real time and not stored on-device. I’ve looked into offline support (probably using PouchDB) but haven’t made the first step toward that and have no idea if/when I ever will.
