opencv - Isolate the non blurred part of foucsed image

I have an image taken by a mobile phone camera with focus on a part of it let say a face or whatever. I want to know -approximately- where is the focused area. For example, a bounding box around the focused area. example :

Answer

In general, this is an image segmentation problem (http://en.wikipedia.org/wiki/Image_segmentation) into which you would be trying to isolate the focused to the non-focused regions of the image.

Optical lenses are equivalent to low pass filters anyway and the effect of a low pass filter on a signal is to smooth it out by limiting the higher frequency content.

In digital image processing, low frequencies correspond to big objects (general form) and higher frequencies correspond to small objects (details).

Therefore, to isolate the focused region, you could be looking for areas that are at a higher standard deviation relative to the blurred areas which would be at a lower standard deviation because of the smoothing operation of the out-of-focus lens.

To distinguish 'blurred' vs 'non-blurred' regions you can setup a "window" of dimensions $M \times N$ and slide it over your image.

At each position of the window compute the variance of the "patch" values as if they were a one-dimensional vector of $M \times N$ values. This will result in another "image" (or more generally a two-dimensional array) depicting the variation of the standard deviation at each location on your image. You can then collect all of these values and obtain their histogram. The histogram will exhibit a multimodal distribution (For more information please see http://en.wikipedia.org/wiki/Multimodal_distribution ).

In the case of the image in your post, the histogram is likely to exhibit two modes. One corresponding to relatively high values of standard deviation (focused area) and one corresponding to relatively low values of standard deviation (blurred areas).

You can then position a threshold, for example just between the two modes to establish a criterion between your focused and unfocused regions and then select all "patches" (or MxN window areas) whose standard deviation is higher than your threshold. (For more information please see: http://en.wikipedia.org/wiki/Thresholding_%28image_processing%29)

This will isolate the focused parts of the image for you.

Hope this helps.