opj_decompress is 3x slow on Windows for PGM

[krasin]

Hi there,

Thank you for maintaining OpenJPEG!

Today, I profiled opj_decompress on Windows and Linux and mentioned that it's 3x slower on Windows. After a deeper look (for which I had to add some printfs and compile opj_decompress.exe), I realized that imagetopnm takes 165ms, while decompressor just 62ms:

$ time -p bin/opj_decompress.exe -i lala.jp2 -o lala2.pgm

load image + initialize: 0.390500 ms
[INFO] Start to read j2k main header (85).
[INFO] Main header has been correctly decoded.
[INFO] No decoded area parameters, set the decoded area to the whole image
[INFO] Header of tile 1 / 1 has been read.
[INFO] Stream reached its end !
[INFO] Generated Outfile lala2.pgm
imagetopnm: 164.228699 ms
decode time: 62 ms
total time: 228.419403 ms
real 0.28
user 0.00
sys 0.01

The reason for this is the fact that imagetopng calls fprintf for every pixel. For text-based variants (such as P2), it could be justified, but for binary PGM (P5), it can be way faster.

My quick and dirty experiment, where I just generate the output array for a 16-bit PGM in a loop (shuffling the bytes, so that they are in bigendian mode, as required for PGM) and then call a single fwrite, the time for imagetopnm goes down to ~7 ms.

Would you be open for a patch that will have a (much) faster path for saving binary PGM files?

I will be totally cool to work on that, but would appreciate an early signal that this is an acceptable approach.

Thank you in advance,
Ivan Krasin