Fred Tedsen wrote:
Is there a way to check for problems using a Mac? There should be a way to see problems in images directly if something is obviously wrong, but what conditions will most reliably show them?
Yes, in fact I just thought of a method that should work and is reasonably simple.
2) Take a test image of some tonally smooth object, make it somewhat OOF so that there is no texture in the image itself (for instance a colorchecker chart is a good choice; the different colors enable one to diagnose the problem better, should it occur; for instance, in skibum\'s bad copy the problem was most pronounced in yellows and not so much in blues).
3)Convert it two different ways using dcraw, once as a three color image and once as a four color image. For instance in a Mac terminal window go to the directory with the test image and execute the two commands
(Notes: I had to convert the test image IMG_5707 to dng because I haven\'t yet updated to the latest version, which now supports the 7D; and obviously, you should substitute the name of your test file. Also, the end of the command \" > IMG_5707-4color.tiff \" pipes the output to a file of that name; the syntax might be a bit different on windoze, I wouldn\'t know.)
4) load both images into Photoshop and overlay one on top of the other; set the blending mode to difference. Make a levels adjustment layer and bring the difference image up a few stops to look for any patterns (it\'s the difference image, so it should have been black if the two conversions agreed with one another). You\'ll want to try both layer orderings (4-color either above or below 3-color) since a priori one doesn\'t know which if either might be brighter if there\'s a channel imbalance.
Here\'s what I got with skibum\'s bad copy color checker test image (this is at 200%, and two stops brightening of the difference image, for visualization purposes):
The mazing is directly the result of the green channel imbalance; with 3-color mode, dcraw does its standard interpolation, while with 4-color mode it averages the two green channels from diagonally adjacent pixels before interpolating. Since in the OOF image there is no texture, the difference image shows only artifacts that are generated by the mismatch in greens.
Fred Tedsen wrote:
Is there a way to check for problems using a Mac? There should be a way to see problems in images directly if something is obviously wrong, but what conditions will most reliably show them?
Yes, in fact I just thought of a method that should work and is reasonably simple.
2) Take a test image of some tonally smooth object, make it somewhat OOF so that there is no texture in the image itself (for instance a colorchecker chart is a good choice; the different colors enable one to diagnose the problem better, should it occur; for instance, in skibum\'s bad copy the problem was most pronounced in yellows and not so much in blues).
3)Convert it two different ways using dcraw, once as a three color image and once as a four color image. For instance in a Mac terminal window go to the directory with the test image and execute the two commands
(Notes: I had to convert the test image IMG_5707 to dng because I haven\'t yet updated to the latest version, which now supports the 7D; and obviously, you should substitute the name of your test file. Also, the end of the command \" > IMG_5707-4color.tiff \" pipes the output to a file of that name; the syntax might be a bit different on windoze, I wouldn\'t know.)
4) load both images into Photoshop and overlay one on top of the other; set the blending mode to difference. Make a levels adjustment layer and bring the difference image up a few stops to look for any patterns (it\'s the difference image, so it should have been black if the two conversions agreed with one another).
Here\'s what I got with skibum\'s bad copy color checker test image (this is at 200%, and two stops brightening of the difference image, for visualization purposes):
The mazing is directly the result of the green channel imbalance; with 3-color mode, dcraw does its standard interpolation, while with 4-color mode it averages the two green channels from diagonally adjacent pixels before interpolating. Since in the OOF image there is no texture, the difference image shows only artifacts that are generated by the mismatch in greens.
Fred Tedsen wrote:
Is there a way to check for problems using a Mac? There should be a way to see problems in images directly if something is obviously wrong, but what conditions will most reliably show them?
Yes, in fact I just thought of a method that should work and is reasonably simple.
2) Take a test image of some tonally smooth object, make it somewhat OOF so that there is no texture in the image itself (for instance a colorchecker chart is a good choice; the different colors enable one to diagnose the problem better, should it occur; for instance, in skibum\'s bad copy the problem was most pronounced in yellows and not so much in blues).
3)Convert it two different ways using dcraw, once as a three color image and once as a four color image. For instance in a Mac terminal window go to the directory with the test image and execute the two commands
4) load both images into Photoshop and overlay one on top of the other; set the blending mode to difference. Make a levels adjustment layer and bring the difference image up a few stops to look for any patterns (it\'s the difference image, so it should have been black if the two conversions agreed with one another).
Here\'s what I got with skibum\'s bad copy color checker test image (this is at 200%, and two stops brightening of the difference image, for visualization purposes):
The mazing is directly the result of the green channel imbalance; with 3-color mode, dcraw does its standard interpolation, while with 4-color mode it averages the two green channels from diagonally adjacent pixels before interpolating. Since in the OOF image there is no texture, the difference image shows only artifacts that are generated by the mismatch in greens.
Oct 23, 2009 at 01:26 PM
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