[okular] [Bug 408222] Okular is bad in displaying of scanned PDF ! Please fix this !

[Tobias Deiminger]

https://bugs.kde.org/show_bug.cgi?id=408222

--- Comment #22 from Tobias Deiminger <haxtibal at posteo.de> ---
Found a quite on-topic paper [0], where it's suggested to compute local indexes
for luminance comparison, contrast comparison and structural comparison to
assess the quality of downsampling algorithm.

If I understand correctly, structural comparison refers to SSIM [1] which can
be calculated using latest ImageMagick 7. For example:

$ compare -metric SSIM test-1-original.jpg test-1-cairo-200dpi.png null: 2>&1
0.826331

$ compare -metric SSIM test-1-original.jpg test-1-splash-200dpi.png null: 2>&1
0.824194

Higher is better, so cairo slightly wins in that structural comparison example.

For the test, test-1-cairo-200dpi.png and test-1-splash-200dpi.png have been
generated locally from test-1.pdf using poppler_page_render and renderToImage
respectively in a standalone application.

If no one disagrees (in the sense that observations wouldn't be meaningful
enough or lack too much context), I'm going to follow this track and do broader
tests with more metrics and try to gain better understand of the results.

[0] Korneta, 2017
[1] https://en.wikipedia.org/wiki/Structural_similarity

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