"Ard" <ard### [at] waikatoacnz> wrote:
> Yep.  When you start messing with gamma, particularly investigating the GAMA
> chunks in PNGs, the number 0.45455 comes up a lot because it is 1/2.2.  POV
> essentially raises every pixel value to the power of 0.45455 before writing
> it to disk and placing it it in the preview.

One thing that everybody should keep in mind when looking at gamma-corrected
images such as PNG is that many applications do *not* handle the PNG gAMA
chunk according to the specs.  Two of the main culprits are:
- All versions of Internet Explorer (including the beta version 7.0)
- Old versions of Photoshop (older than 7.0 or CS)

MSIE is known for having very bad support for transparency in PNG images
(this is fixed in MSIE 7.0) but also for its bad support for gamma
correction.  According to the PNG specs, the browser should apply the
correction when displaying a PNG image including a gAMA chunk and should
not apply it when the image does not include it.  Or to be more consistent,
if the browser applies a gamma correction to other colors such as those
from HTML, CSS or other images (JPEG, GIF) then it should do the same
for PNG images that have no gAMA chunk.  MSIE fails to do that consistently.
In addition, MSIE seems to assume that the display gamma is a bit below 2.0.
According to http://www.libpng.org/pub/png/pngapbr.html#msie-win-unix MSIE
uses an implicit display gamma of approximately 1.93.

Other browsers such as Firefox or Opera (version 7 or later) process the PNG
gAMA chunk correctly.  However, they may rely on the operating system for
some display functions so it is possible that a PNG image does not look the
same in Firefox for Windows, for Linux or for OS X.

So given that the most common applications for displaying PNG images do not
handle the gamma information correctly, one should be careful before stating
that POV-Ray handles this in the "right way" or not.

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