We just released a new iPhone app that quantises photos (StencilPic-- buy it :)). There is a postscript on this post about terminology, but the subject is preconditioning. Suppose we do a two-color stencil of the following image:
|Two color stencilization from StencilPic app|
|After sharpen filter in Effects Studio App|
|Two color stencilization of Effects Studio app sharpened image from StencilPic app|
PS-- an note on terminology.
We originally called our app "PosterPic" and said it did "posterization". However, many people thought that mean what graphics people would probably call "quantized color table", which we think is now the dominant terminology because that is a named Photoshop effect:
|Source image (courtesy Parker Night at flickr)|
|Output from StencilPic|
However, what our app does is something more like the the famous Fairey Obama poster which a graphics person would call "quantized pseudo-color display" (note Fairey also did the Andre the Giant poster that went viral in graphics thanks to Brown students). That doesn't have a ring to it, so we went with the term "stencil". This is often black and white, but it's whatever spray paint you want. The google image search of "stencil portrait" is what convinced us this was good terminology:
Another postscript based on the interesting comment. Here is Adelson's shadow with and without sharpening on source image. I just use a screen shot of the last screen in the app so you can see different thresholds.