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 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.