Implementing Photoshop's "Poster edges" -filter

Question

I'm trying to find out what effects the Photoshop "Poster edges" filter is composed of. It seems it's a combination of edge detection and posterization, but I haven't been able to duplicate it, not even close, with these so I guess I'm missing something. The image below shows the same image before and after the Poster edges filter:

I've tried performing posterization (and quantization) on the image, along with edge detection using Sobel, but apparently Photoshop is doing something different as the results are very different. Basically the posterization looks very different and edges are very weak compared to the photoshop filter.

So does anybody know how the Poster edges filter is implemented, or have any idea what image-processing should be done to achieve the latter image from the former.

Not that it really matters, but I'm using Java, and my image filtering code is based for the most part on the filters found here: http://www.jhlabs.com/ip/filters/index.html

Edit Description of the filter from adobe.com:

Poster Edges Reduces the number of colors in an image (posterizes it) according to the posterization option you set, and finds the edges of the image and draws black lines on them. Large broad areas have simple shading, and fine dark detail is distributed throughout the image.

Answer 1

Regarding the edges: I would assume that Photoshop uses something more sophisticated than a simple derivative filter (like Sobel) for edge detection. There are edge detection algorithms that try to find only "salient" edges, i.e. those that are relevant to human vision, edges a human artist would draw if he does a line sketch. An old and (rather) simple algorithm that goes in this direction (at least a bit) is the Canny edge detector. You should be able to find an implementation of this one. Google for "salient edges" for current research literature, but don't expect implementations or nice pseudocode in research papers.

Regarding posterization: Given their talks at SIGGRAPH, the Adobe guys are very much into bilateral filtering (please Google, I can't link any more), a smoothing technique that preserves important edges. I think if you apply the bilateral filter and posterize afterwards you should come closer to the look you want. Unfortunately, efficiently implementing the bilateral filter is not trivial.

Update for anyone still interested in this topic

The Bilateral filter, which I suggested above, is increasingly replaced with the Guided filter, at least in the Computer Vision community (the graphics people don't seem to have realized the Guided filter yet). The Guided filter achieves similar results, but is much easier to implement efficiently. The exact algorithm for the Guided filter is highly efficient, while efficient Bilateral filtering requires approximations or insanely complicated algorithms.