Ivan Louette wrote:
I can't figure how I could extract a saturation map (as a greyscale image) with SVG filters. My idea is using it to replace any hue of an image with only one like it's done with cyan in the joined png created with Xara Xtreme Linux Edition. I am searching for a long time already but without success. Perhaps not possible.
My guess would be that it should be possible to at least compute a rough approximation of the operation by simply trying to approximate the function(s) involved. I've done such things to simulate conical gradients, and it can work reasonably well (although the quality of the approximation is often dependent on the renderer): http://wiki.inkscape.org/wiki/index.php/Advanced_Gradients
In Xara I only put a cyan rectangle above the image and apply the "Hue" transparency type. I was able to synthezize all Xara's color effects but this one.
I'm afraid I'm too busy at the moment to give it a go myself, but I hope you'll succeed :) I think it's quite interesting what can be done within the confines of SVG filters, and I really hope the possibilities will be improved in later versions of the standard (and that the renderers catch up).