Re: Image Scaling

On Sun, Jun 16, 2013 at 10:52:57PM +0200, Sebastian Geerken wrote:
> [...]
> > > > Nice! BTW, I have this page bookmarked for quite some time:
> > > > http://www.4p8.com/eric.brasseur/gamma.html
> > > > I don't know whether this makes a difference in real life, but the
> > > > examples are interesting.
> > > 
> > > Interesting!
> > > 
> > > I didn't read the whole article, but it made me wonder what effect a
> > > gamma correction has on image scaling. See the test page
> > > <http://www.4p8.com/eric.brasseur/gamma_dalai_lama.html> with:
> > > 
> > > 1. dillo 3.0.3,
> > > 2. current version from the hg repository, and
> > > 3. latter with the attached patch applied.
> > > 
> > > The patch uses floating point arithmetics; perhaps there is a faster
> > > algorithm based on integer arithmetics.
> > 
> > I don't know, but we could cache the values as in attached patch.
> 
> Yes, I also thought about this, and probably the fastest way is to
> calculate all values once, at the beginning, and later only access the
> array.
> 
> I've been working on the other side: from what I've understood, image
> data is in most cases already gamma-corrected. I'd like to extend
> ImgBuf by the gamma value, and the different parsers (GIF, JPEG, PNG)
> should extract the factor from the image.
> 
> So far, I'm working on JPEG, and came upon the field "output_gamma" in
> "struct jpeg_decompress_struct". Unfortunately, this value seems
> always 1. Can someone give me a hint?

http://www.cambridgeincolour.com/tutorials/gamma-correction.htm
has some info on the topic.
I think we can completely ignore gamma unless we actually touch the
color values. I think the only place we do that is in the image
scaling code.
For the web I think it's safe to simply assume a gamma value of 2.2.
To improve local image browsing we might check for embedded color profiles
and use the gamma value provided there.
That's how I understand it at least.

  Johannes