Re: [GD-DEVEL] RGB to Lab colorspace conversion

hi,

Color conversions would rock. If you can find implementation under a
BSD license (or similar but not GPL), we will happy to port them.

Cheers,

On Mon, Jan 25, 2010 at 11:41 PM, Joe Lencioni <[email protected]> wrote:
> First of all, thank you for developing GD. It is an indispensable tool for
> working with images in PHP.
>
> I am working on an image resizing and cropping script that attempts to
> detect which areas of an image are more important when making cropping
> decisions--essentially asking the computer to determine perceptual color
> differences. To accomplish this, I convert pixels from the RGB colorspace
> into Lab colorspace and compare the differences. (
> http://en.wikipedia.org/wiki/Lab_color_space#Advantages_of_Lab). While I can
> make this happen in PHP, it is slow and uses too much memory. I would love
> to see functions to facilitate this conversion and comparison directly in
> GD.
>
> To convert from RGB to Lab, you first need to convert from RGB to XYZ
> colorspace and then from XYZ to Lab colorspace. Here are the functions that
> I use for conversion in PHP (which have been derived from the formulas on
> the EasyRGB website http://www.easyrgb.com/index.php?X=MATH):
>
> GD color index to RGB:
> http://code.google.com/p/smart-lencioni-image-resizer/source/browse/trunk/slirimage.class.php?spec=svn59&r=46#755
> RGB to XYZ:
> http://code.google.com/p/smart-lencioni-image-resizer/source/browse/trunk/slirimage.class.php?spec=svn59&r=46#770
>
> XYZ to CIE-L*a*b* (more complex):
> http://code.google.com/p/smart-lencioni-image-resizer/source/browse/trunk/slirimage.class.php?spec=svn59&r=46#825
> or
> XYZ to Hunter-Lab (less complex):
> http://code.google.com/p/smart-lencioni-image-resizer/source/browse/trunk/slirimage.class.php?spec=svn59&r=46#806
>
> And comparing the colors (more info at
> http://en.wikipedia.org/wiki/Color_difference):
>
> deltaE 2000 (complex):
> http://code.google.com/p/smart-lencioni-image-resizer/source/browse/trunk/slirimage.class.php?spec=svn59&r=46#874
> or
> deltaE 76 (simple):
> http://code.google.com/p/smart-lencioni-image-resizer/source/browse/trunk/slirimage.class.php?spec=svn59&r=46#857
> or
> deltaE CMC:
> http://code.google.com/p/smart-lencioni-image-resizer/source/browse/trunk/slirimage.class.php?spec=svn59&r=46#967
>
> For accuracy purposes, I would prefer to be able to use the more complex
> equations. However, if they prove to be too slow, it would be nice to fall
> back on the simpler versions.
>
> So my question is this: does this kind of functionality make sense to
> include in GD (I hope so)? If so, will you add it? I am unfamiliar with C,
> otherwise I would submit the code, but hopefully it won't be too much work
> to convert the PHP functions I have provided here.
>
> Thanks!
> Joe
>



-- 
Pierre

@pierrejoye | http://blog.thepimp.net | http://www.libgd.org