Resize bug in most image processing programs?

[eL_PuSHeR]

Hello all.

A friend of mine sent this page to me yesterday.

Although old, the mentioned issue seems to remain on most image processing programs.

So far XnView 1.97.2 (even after trying the 32bits workaround), Irfanview 4.25 (zooming is almost fine) and Paint Shop Pro X are affected. The only one that worked (after the workaround) was Adobe Photoshop CS3 (I don't have CS4 or CS5 so I can't say for them).

What do you think?. I think this is an interesting issue which I never thought about. Is there any other possible workaround that rings a bell?

[obelisk]

read that before, really isn't a big issue.

[XnTriq]

Gamma Correct Resizing

[eL_PuSHeR]

Ok. This is old news. The current workaround is:

1. Decrease gamma to 0.45
2. Resize
3. Increase gamma to 2.2

[DOS386]

...

[DOS386]

read that before, really isn't a big issue.

I did read it and it is a BIG issue

The current workaround is:

1. Decrease gamma to 0.45
2. Resize
3. Increase gamma to 2.2

NO it isn't. It is theoretically correct, in practise it's invalid. Why ? "Decrease gamma to 0.45" will cause massive damage by truncation to the 8 bits, actually much worse than the gamma error that you are trying to avoid. The method works with 16 bppc (not yet (?) supported in XN/NC), but not with 8 bppc.

Actually it's even more complicated. I had some discussion with Eric Brasseur recently (and he improved his article a bit), the "correct" meaning of the 8bppc RGB values is:

http://en.wikipedia.org/wiki/SRGB



There is a "special" formula (see above), "similar" to y=x^2.2 for larger values, but very different for very dark values.

So what's missing in XN/NC is not only the 8 bppc -> 16 bppc conversion, but also the sRGB Gamma conversion.





Download now: http://ompldr.org/vNzltZQ/TESTPICT.ZIP (6'914 Byte's)

See also :

http://newsgroup.xnview.com/viewtopic.php?t=21739 "Gamma correct resizing (english, français)" - Leonidas - 2011-Jan-28

http://newsgroup.xnview.com/viewtopic.php?t=21441 "Resize bug (Bilinear and nearest neighbour)" 2010-Dec (valid about bilinear, invalid about NN)

http://newsgroup.xnview.com/viewtopic.php?t=20095 "16 bppc - supported or not ?" - 2010-May

http://newsgroup.xnview.com/viewtopic.php?t=14372 "Gamma Correct Resizing" - robofunk - 2008-Feb-08

http://www.4p8.com/eric.brasseur/gamma.html "Gamma error in picture scaling"

EDIT : this post has been updated many times, last 2011-Feb-05