Differences

This shows you the differences between two versions of the page.

--- stacker:docs:tutorials:tutorial003 [2012/11/18 05:30]
rjlittlefield [How DMap Works On The Inside]
+++ stacker:docs:tutorials:tutorial003 [2012/11/18 05:46]
rjlittlefield [How DMap Works On The Inside]
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 In practice, this simple concept needs some refinement to work well.  There are two main issues:
-  - It turns out that there's no way to determine how well an image is focused by looking at just one pixel -- the software has to look at nearby pixels also.  Estimation Radius controls the definition of "nearby".
+  - It turns out that there's no way to determine how well an image is focused by looking at just one pixel -- the software has to look at nearby pixels also.  Estimation Radius controls the definition of "nearby". \\ \\
-  - Most scenes have some areas where there is not enough detail to tell reliably which input image has the best focus.  Those areas have to be identified so that the proper image to use can be determined from other information.  In Zerene Stacker, this identification is carried out mostly by using the "contrast threshold".  Pixel positions that contain only low contrast information are less trustworthy to determine focus, because the calculation gets disturbed by random pixel noise and by out-of-focus blurs from nearby strong edges.  By setting the threshold, you use your human judgment to specify which locations can be trusted and which cannot.  At locations that cannot be trusted, DMap ignores its estimate of depth based on image content, and instead switches to a different scheme that bases the estimate on surrounding pixel positions that can be trusted.  In the terminology that we used earlier, the locations that can be trusted are described as being where you care about detail, and the locations that cannot be trusted are described as being where you prefer smooth gradations.  There's only one mechanism, but it can be used for two different purposes -- either to emphasize smooth gradations or to help guide the software to make proper decisions for the images at hand.
+  - Most scenes have some areas where there is not enough detail to tell reliably which input image has the best focus.  Those areas have to be identified so that the proper image to use can be determined from other information.  In Zerene Stacker, this identification is carried out mostly by using the "contrast threshold" slider.  Pixel positions that contain only low contrast information are less trustworthy to determine focus, because the calculation gets disturbed by random pixel noise and by out-of-focus blurs from nearby strong edges.  By setting the threshold, you use your human judgment to specify which locations can be trusted and which cannot.  At locations that cannot be trusted, DMap ignores its estimate of depth based on image content, and instead switches to a different scheme that bases the estimate on surrounding pixel positions that can be trusted.  In the terminology that we used earlier, the locations that can be trusted are described as being where you care about detail, and the locations that cannot be trusted are described as being where you prefer smooth gradations.  There's only one mechanism, but it can be used for two different purposes -- either to emphasize smooth gradations or to help guide the software to make proper decisions for the images at hand.
 The final refinement is "smoothing".  It turns out that even with both of the above refinements, there is still a bit of noise in the estimates of which image has the best focus.  To reduce this error, a weighted average is taken of estimates at nearby pixel positions.  Again, there's a numeric definition of "nearby", and this time it's determined by Smoothing Radius.