OCRmyPDF: [13.4.2] lossy compression of pngs into jpegs when it shouldn't

1. It might be just the older version, but ocrmypdf 12.7.2 seems to compress uncompressed pngs into (lossy) jpegs:

$ ocrmypdf --version
12.7.2
$ wget https://upload.wikimedia.org/wikipedia/commons/7/70/Example.png
$ convert Example.png -define png:compression-level=0 -define png:compression-filter=0 -define png:color-type=2 Example-uncompress.png
$ img2pdf ./Example-uncompress.png -o ./Example-uncompress.pdf
$ ocrmypdf --tesseract-timeout=0 --optimize 1 --skip-text Example-uncompress.pdf Example-uncompress-compress.pdf
$ pdfimages -list ./Example-uncompress-compress.pdf
page   num  type   width height color comp bpc  enc interp  object ID x-ppi y-ppi size ratio
--------------------------------------------------------------------------------------------
   1     0 image     172   178  rgb     3   8  jpeg   no         9  0    96    96 4157B 4.5%

I believe it should be running the image through pngquant instead at optimize level 1.

1. Btw, it’s probably not even worth mentioning since, looking at the changelog, I’m fairly certain you’ve already sorted it out in recent ocrmypdf versions, but small pdfs with small pngs grow instead of shrinking / remaining the same:

$ ocrmypdf --version
12.7.2
$ wget https://upload.wikimedia.org/wikipedia/commons/7/70/Example.png
$ img2pdf ./Example.png -o ./Example.pdf
$ ocrmypdf --tesseract-timeout=0 --optimize 1 --skip-text Example.pdf Example-compress.pdf
$ stat -c "%n,%s" Example*.* | column -t -s,
Example-compress.pdf  7799
Example.pdf           3906
Example.png           2335

Though this might also be the pdf format changing to the archival specs…

1. As a side note, if compute time isn’t a factor, I personally found ‘optipng -o7’ to produce smaller pngs than pngquant and ‘jpegrescan -i -t -v’ to produce the smallest jpeg, even compared to MozJPEG despite the author saying otherwise oddly enough.

p.s. forgot to mention the png-to-jpeg bug also happens with some compressed pngs but I haven’t bothered trying to replicate this since I believe it should never try to convert bitmap images to jpegs to begin with.