Question

I have an optimally compressed png that I’m rotating by 1 degree using ImageMagick –

convert -rotate 1 crab.png crab-rotated.png

The size goes from 74 KB to 167 KB. How do I minimize that increase?

Orginal:

Rotated:

Answer 1

The increase in file size is probably due to less efficient compression. You won’t be able to do anything about that unless you decrease the compression level (-quality option) or use a more efficient but lossy compression method (e.g. JPEG).

Here’s the reason why I think this happens (I hope somebody can correct me if I am wrong). By rotating the image, you are introducing spatial frequencies that were not present in the original image. If these frequencies are not suppressed during compression, than the file size will inevitably increase. However, suppressing these frequencies may degrade the quality of your image. It’s a delicate balance.

The amount of increase (or decrease) in filesize after rotation depends on the frequencies already present in the image, i.e. it is image-specific.