Hy anybody could explain me how the heck the isophote filters works ?

I mean what its logic ? what is supposed to do except make most of the image transparent ?

Hope this is of some help. I did a search for the isophotes filter in G'MIC.

http://www.cambridgeincolour.com/forums/thread2595.htm

thank O !

You're welcome. I played with the filter using a color photo and a desaturated layer of the same photo, using the Color selection and changing the other settings. Chose just the default (desaturated) layer to apply the filter.

Got a very interesting effect setting the desat. layer to Value mode over the color layer. It was a little reminiscent of the B&W Pencil filter.

Original image here.

Image layer duplicated and desaturated using luminance setting.

The isophotes filter is in G'MIC > Countours > Isophotes
My settings:
Levels: 17
Smoothness: 1.14
Filling: Colors
Layer Mode Desaturated Layer: Value

The image looks like it has been indexed for a gif. I like the fine, squiggly black lines separating the different colors.

Looks like some sort of graph cut seam detection scheme used in a lot of algorithms including the Liquid Rescale algorithm. Of course I could be wrong.

When I saw O's example, the first thing I thought of was the Paint-by-Numbers pictures. Then after reading the info in the links, I thought of the topographic maps produced by the United States Geological Survey - instead of elevation contours, we get luminosity contours.

Here's a picture of mine which I took a few years ago following an ice storm (took it with my cellphone, no less!). The original image is here. I used a the same image prep (dup layer, desaturate by luminosity) and settings in G'MIC as O did. Once G'MIC finished I set the layer mode to Darken only. What was so interesting for me was that the filter picked out detail in the cloud and snow that I couldn't really get (or see) playing with the regular Brightness/Contrast sliders, and how changing the layer mode really changed the appearance
I probably never would have experimented with this if I had not seen this thread.

nice effect

Yes it does look like a topographic map. I, too, was intrigued about the color detail that the filter picked up.

Bob, I love the zoomed to full size image you did. It has so much detail outlined by the filter. Nice job.

Thanks. I hope one day to own a digital camera that can help me get the kind of photos I used to get with my 35mm - although for a cellphone cam, it did capture more image information than I expected (I know I mentioned that already, it's just such a surprise). Something I didn't realize until I used this filter and the technique.

If David put them in there, they have to be good for something. =)