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NX Gamma Tests |
Gamma encoded color space: Capture NX (1.3.2) takes a NEF,
converts it to a
gamma encoded color space,
and presents that to you within NX for editing.
The only problem with this is that some (all?) adjustments
within Capture NX are not gamma aware.
Why do we care? Not being 'gamma aware' means color shifts --
and since you are editing a NEF in the first place, I am going to
assume that you care about color accuracy. Not being gamma aware
means color shifts.
An Example: Consider the beetle picture to the right.
Looks nice? It was resized in a gamma aware program. Click on the
link under the picture to see the result in Capture NX, caused by
it not being gamma aware.
Most people will never notice that NX Adjustments are not gamma aware. Most
photos are well exposed and rarely heavily modified. But if your photo contains
abrupt changes in color or contrast or needs heavy editing, you will notice subtle
shifts in color. I have noticed color shifts in several spots. (1) With
D-Lighting, (2) in the recovery of extremely
underexposed areas of photos, and (3) resizing photos smaller for the web.
Gamma Test Image
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Classic Gamma Test Image: A classic gamma test is to create the image that
you see to the right (green square over purple background) in your favorite image
editing program and manipulate it (in NX, use two colorize steps to produce it).
The colors in this test image are very carefully chosen. The purple is RGB (255,0,255)
and the green is RGB (0,255,0). These colors stay the same before/after
gamma encoding/decoding. So we can then examine the averaged area between
the purple and green to see if an photo editor is gamma aware.
GOOD gamma aware result
BAD non-gamma aware result
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Background: What happens when you rotate this test image? Some purple
and green will have be 'averaged' together. Take the very simple case of one
purple pixel (255,0,255) and one green pixel (0,255,0) that need to be averaged
together. The result in a non-gamma aware program will be (128,128,128), which
is half of 0+255 for red, green, and blue. But a gamma aware program would need
to convert to linear (which is why the colors were chosen carefully -- the result
is the same color), then averaged, which is (128,128,128), and then gamma encode
that, resulting in (186,186,186). The results are obvious to see. (128,128,128)
is dark color, whereas (186,186,186) is much lighter.
The Test: Rotate the test image by 5° and compare the results to
what is expected from a gamma aware vs non-gamma aware program. A gamma aware
program will have a pleasing white'ish blending. A non-gamma aware program will
have an incorrect dark blending color.
Capture NX Test Results: Here are the test results for various Adjustments
within Capture NX.
Distortion Control
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Gaussian Blur
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Resize
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Straighten
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Given the dark blending seen, we can conclude the adjustments are not
gamma aware. Given that these adjustments fail, I need to question if there
are any adjustments within NX that are gamma aware.
Capture NX needs to be fixed: Presenting gamma encoded RGB to edit
with non-gamma aware Adjustments is a design flaw, which only leads to shifts
in color when the RGB values are edited -- as can be clearly seen in the agove NX
tests. Since many (all?) adjustments within NX are non-gamma aware in the first
place, moving when gamma encoding takes place is the best way to fix NX.
Currently gamma encoding appears to be a hidden step within RAW Adjustments.
How Nikon must fix Capture NX: Nikon needs to change when gamma encoding
takes place within NX. And the solution to fix all of NX at once is deceptively simple:
Gamma encoding in NX is currently a (hidden) automatic step within RAW Adjustments.
Gamma encoding must instead be an automatic last step in the edit list.
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Another very clever Gamma Test: Thanks to 'Eric Brasseur' for the following
test image. Save the following specially created
258x222 Dalai Lama photo to your computer (right click on the picture, select
'Save Picture As...') and open in your favorite image editing program, then resize
to 50% (129x111). What do you see as a result? A non-gamma aware program like
Capture NX will produce a gray square. Something that you can see is
incorrect. A gamma-aware program will produce the result that we can see
is correct:
Original test image
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NON-gamma aware resize
Gamma AWARE resize
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