[Pkg-phototools-devel] Bug#668476: openimageio-tools: handling of gamma for JPEG files in "iv" is weird

[Miles Bader]

Package: openimageio-tools
Version: 1.0.0+dfsg0-1
Severity: normal

[Sorry if this is a bit long; issues involving image gamma are always
kind of confusing, so I'm trying to be as explicit and verbose as
possible...]

Given:

 1. A typical PNG image file, "foo.png", which was encoded using a
    standard encoding gamma correction value of 1/2.2 (0.4545), and
    has an explicit PNG gAMA chunk recording that value.

 2. A JPEG file, "foo.jpg", produced from "foo.png" file using the
    imagemagick command "convert foo.png foo.jpg".

 3. Another PNG file, "foo2.png", produced from "foo.jpg" using the 
    imagemagick command "convert foo.jpg foo2.png"; note that foo2.png
    _doesn't_ have an explicit gAMA chunk.

Then:

 * Most image display programs (imagemagick, eog, geeqie, qiv, ...)
   display all of the files identically.  In other words, they assume
   that the encoding gamma correction value is 1/2.2 (0.4545) for both
   JPEG files, and PNG files without an explicit gAMA chunk.

 * "iv foo.png" and "iv foo2.png" displays the image "correctly"
   (matching what other image-display programs do), suggesting that it
   assumes a default encoding gamma correction value of 1/2.2 for PNG
   files.

 * _However_, "iv iv.jpg" displays the image much too dark, unlike all
   other programs.  By using the ")" key-command to change the "gamma"
   value to 2.2, the image then looks correct.

=> Thus the bug: it appears that iv assumes an encoding gamma
correction of 1.0 for JPEG files, which does not match standard
practice.


[As an aside, the term "gamma" value displayed in the iv GUI is a bit
confusing; it appears to actually be "encoding gamma correction
divided by display gamma correction."]

Thanks,

-Miles


-- System Information:
Debian Release: wheezy/sid
  APT prefers unstable
  APT policy: (500, 'unstable'), (500, 'testing'), (500, 'stable'), (50, 'experimental')
Architecture: amd64 (x86_64)

Kernel: Linux 3.2.0-2-amd64 (SMP w/2 CPU cores)
Locale: LANG=ja_JP.utf8, LC_CTYPE=ja_JP.utf8 (charmap=UTF-8)
Shell: /bin/sh linked to /bin/dash

Versions of packages openimageio-tools depends on:
ii  libboost-filesystem1.49.0  1.49.0-2
ii  libboost-regex1.49.0       1.49.0-2
ii  libboost-system1.49.0      1.49.0-2
ii  libboost-thread1.49.0      1.49.0-2
ii  libc6                      2.13-27
ii  libgcc1                    1:4.7.0-3
ii  libgl1-mesa-glx [libgl1]   7.11.2-1
ii  libglew1.7                 1.7.0-3
ii  libglu1-mesa [libglu1]     7.11.2-1
ii  libilmbase6                1.0.1-4
ii  libjpeg8                   8d-1
ii  libopenexr6                1.6.1-5
ii  libopenimageio1.0          1.0.0+dfsg0-1
ii  libpng12-0                 1.2.49-1
ii  libqt4-opengl              4:4.7.4-3
ii  libqtcore4                 4:4.7.4-3
ii  libqtgui4                  4:4.7.4-3
ii  libstdc++6                 4.7.0-3
ii  libtiff4                   3.9.6-2
ii  libx11-6                   2:1.4.4-4
ii  libxext6                   2:1.3.0-3
ii  zlib1g                     1:1.2.6.dfsg-2

openimageio-tools recommends no packages.

openimageio-tools suggests no packages.

-- no debconf information