Re: Focus measurement (more general image analysis question)

Posted by Gabriel Landini on Mar 07, 2007; 1:52pm
URL: http://imagej.273.s1.nabble.com/Focus-measurement-more-general-image-analysis-question-tp3700123p3700134.html

On Wednesday 07 March 2007 13:10:27 Antje wrote:
> But still I have a question. How shall I compare
> images with different density? Because the standard deviation of an
> image will be dependent on the density of objects. It may happen, that
> there is just one cell within one image and it can also happen, that
> there is no background at all because of the density of cells...

I think it is not possible to get a robust method of sorting
*single* images according to their degree of "in-focusness".

The assumption of an absolute measure of sharpness for a single image most
probably would not hold unless you know already what to expect in the image.

If you look/google/search autofocus, you will find that all focusing
algorithms try to maximise some measure of sharpness across several shots of
the *same scene*.
With a single arbitrary image (as I believe is your case), how do you make
sure that the image is blurry because of bad focus rather than the scene
having originally no sharp edges.
I.e. is this 1) a blurry shot of a sharp scene or 2) is it a sharp shot of a
diffuse-looking scene?

If you look for high frequency contents in the image to make the decision, you
would treat the 2 examples above as the same, while an autofocus algorithm
would find the best solutions for both cases (by taking more shots at various
focal lengths). You then could compare which one was more out of focus (how
far away each image was from the "best focus" shot).
But you cannot do this with a single shot of unknown properties when "in
focus".

Cheers,
G.