How can I get linear output from darktable for texture work?
Asked 7/23/2019
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2 answers
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I need exported images that remain linear with scene irradiance/exposure, for use as textures in other software that expects non-tonemapped linear images. In darktable, does the base curve compensate for camera nonlinearity, or is the input already treated as linear and the base curve mainly used to create a display/look? If I want final pixel values to stay proportional to exposure, should I disable the base curve?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
6y ago
2 Answers
1
It should be quite easy to measure. Take a series of pictures with different exposure time t, using a constant light source. Then the input to the system will be linear, because the number of photons that will hit the sensor is proportional to E and t. This will give measure points of the imaging system, which can be used to create a calibration profile.
Taking pictures against a white wall in a dark room gave the following result:
As you can see, the output from the camera is pretty linear, with the basecurve turned off.
Originally by user85952. Source · Licensed CC BY-SA 4.0
user85952
6y ago
0
Generated from our catalog & community — verify before relying on it.
For a linear workflow, disable the base curve. The evidence in the community test suggests the camera output is already essentially linear when the base curve is turned off, and the base curve is mainly for applying a tone/rendering look rather than preserving linear scene data.
A practical way to verify this is to photograph a constant-lit target while varying only exposure time. Since captured light is proportional to irradiance × time, the output should scale linearly if the pipeline is linear. The reported test against a white wall showed that with base curve disabled, the output was pretty linear.
So if you need non-tonemapped images for textures or other linear-aware programs, turn off the base curve and validate with an exposure series if needed.
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UniqueBot
AI6y ago
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