Re: Image Scaling

On Mo, Jun 17, 2013, Johannes Hofmann wrote:
> 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.

Thanks.

> 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.

I've just added gamma information to the whole chain (from image
decoders to FltkImgBuf), without using it at all. Currently, all image
decoders pass a value of 1/2.2, ignoring the possible values in the
file. May be fixed. I found it useful just to have this value stored.

I do not plan to correct any image values (assuming that the display
has a gamma of 2.2), but just to use gamma for scaling.

Sebastian