Specifying option --oversample tends to introduce upsampling in rendering
by rasterizing page to an higher DPI.
This upsampling improves OCR results, but a correct choice of interpolation
method can increase even more the OCR quality.
Ghostscript seems to use a nearest interpolation as default choice for pdf.
This method doesn't average new introduced pixels with original pixels
resulting in an almost similar image but with more pixels.
Providing -dInterpolateControl=-1 force switching interpolation on.
In this commit the above option is passed to all ghostscript rendering
calls.
After testing, rendering a page at same DPI with interpolation
enabled does not introduce significant time overhead.
time (repeat 40 gs -dQUIET -dSAFER -dBATCH -dNOPAUSE -sDEVICE=png16m \
-dFirstPage=1 -dLastPage=1 -r100.000000x100.000000 \
-dInterpolateControl=-1 -o /dev/null -dAutoRotatePages=/None -f pzII.pdf)
7,66s user 0,33s system 99% cpu 8,012 total
time (repeat 40 gs -dQUIET -dSAFER -dBATCH -dNOPAUSE -sDEVICE=png16m \
-dFirstPage=1 -dLastPage=1 -r100.000000x100.000000 \
-o /dev/null -dAutoRotatePages=/None -f pzII.pdf)
7,42s user 0,39s system 99% cpu 7,808 total
Ghostscript interpolation control reference:
https://www.ghostscript.com/doc/current/Use.htm
James R. Barlow
James Barlow
Tristan PorteriesandGitHub