There is an insane amount of confusion between linear and non linear response to color mappings, it depends on the drivers, the software, the hardware, the settings in your applications, what kind of algorithms are being implemented and probably and infinite set of other things.

Assuming that I can open an image or I'm in an image editor, there is a test I can do to test what kind of mapping I get from that environment ?


The subject is a bit challenging*. The short answer is:

  • You look up the image profile, which contains the mapping to what it should look like in a known reference space.

However this look-up tells you what its intended to look like, not what it originally was. So if you want to measure the values you need to have an untainted source. Basically that means having control of all stages from capture to use. Remember your camera may be doing white balance, you need to account for that in addition.

A quick mental test to perform is:

  • If the image does not look dark, then its not linear. Because ive never seen anybody but myself calibrate a monitor to linear.

The reverse however is not a truth test. So dark images are no more likely to be linear than light ones. There is an exception to the rule above, and that is synthetic images. They can be bright even if linear.

The real issue is that you can not really know whether or not the info you have is true. A image may have wrong metadata, or some processes may have mangled he image unknown to you. This is frequently the case for say bump maps, which can report srgb although intended as linear.

So to actually know the answer you need to know under what circumstances the image was made, by whom and what was done and intended with the image. or then just make naive assumptions.

  • Maybe its just easier to say you can not do this very easily.

For hardware.

You simply need to purchase a calibration system for your hardware to control the pipeline. Or you can outsource this to a outside standards agency that does it periodically for you. In real manufacturing processes you also need to do quality control and quite frequent recalibration of systems.

The process is a bit tedious, so if you have more than 3 devices you need to dedicate one person for doing the calibration and workflow instructions.

But if all you need is somewhat better resuilts than before then byall means just buy a calibrator and run it periodically (or better yet constantly)

* So much that there is an entire industry for ensuring this quality control, So may be easier to hire a QC engineer.

  • Is that topic about gamma correction? Does the raw camera format do any correction or modification of what comes from a ccd/cmos? – user29318 Nov 21 '14 at 12:11
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    @Anphiteoth Gammacorrection is one simplified model to compress the data meaningfully in the signal. Profiles are just vessels for more complex correction info than merely gamma curves. The camera raw tool manipulates the raw sensor data. But it might still not be linear, depends on sensor. Remember you want to simulate human perceptive components when processing the image so the primary goal is to accenuate the world like the eye would. Showing a blue or orange cast thus wouldnt convey the real situation ever because now the eye does it twice. – joojaa Nov 21 '14 at 15:11

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