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I have iphone photos which are non-linear sRGB format. I need linear RGB. I couldnt find a way in opencv(python) to convert them to linear RGB.

Also tried ImageMagick convert : 

convert output0.jpg  -colorspace RGB output0.jpg

Still same sRGB no effect.

Any ideas ?

fmw42
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2adnielsenx xx
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  • Are you referring to Adobe RGB? As in, wider colour gamut? – jmkmay May 18 '18 at 18:17
  • I think your **ImageMagick** command does convert to RGB colourspace, but then saving as a JPEG converts it back to SRGB again because it assumes JPEGs must be in sRGB colourspace. If you used `convert iPhonePic.jpg -colorspace RGB result.png` and load that into **OpenCV** you should do better. – Mark Setchell May 18 '18 at 19:00

2 Answers2

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You can use the conversion given in link below to convert sRGB to XYZ

Matlab Color Conversion

Then use cv::COLOR_XYZ2BGR or cv::COLOR_XYZ2RGB to get RGB.

Garvita Tiwari
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Only a very few image formats truly support linear RGB. PNG does, but not JPG. You can tell if the gamma value is 1, it is linear. You can process in linear colorspace with ImageMagick, but saving to JPG may show the proper linear pixel values, but the JPG meta data does not store the fact that it is linear RGB. You can tell if the pixel data has been changed to linear if the image is darker. You could assign a linear profile to the JPG, but I am not sure where you would find one. Note also in very old version of ImageMagick -colorspace RGB was actually producing sRGB and vice-versa

For example, you command makes no change, since JPG does not truly support linear RGB.

Input:

enter image description here

convert logo.jpg -colorspace RGB logoRGB.jpg

enter image description here


But PNG does:

convert logo.jpg -colorspace RGB logoRGB.png

enter image description here

Colorspace: RGB
Gamma: 1

In order to force the conversion to linear RGB in JPG, you can do the following to make the pixel data linear. But the meta data will still show non-linear sRGB, gamma=0.4545

convert logo.jpg -colorspace RGB -set colorspace sRGB logoRGB2.jpg

enter image description here

Colorspace: sRGB
Gamma: 0.454545

EDIT: The following link may be useful if one wants a Linear RGB profile. See http://fnordware.blogspot.com/2008/05/making-linear-icc-profile.html

fmw42
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