Strange color conversion in Affinity Photo

[SergioRus]

Hello,

I played with conversion from different color spaces in Affinity Photo and have found that it seems strange.

I have made test file in ROMM RGB color space and tried to correctly convert one to sRGB color space with all four existed methods. 

I wanted to see difference between Perceptual , Relative colorimetric, Absolute colorimetric, and Saturation methods, but I cant see difference and it seems for me strange especially for perceptual method.

I have made video and atteched here my test file.

Could somebody explain me is it normal or not.

Excuse me for bad English.

ProPhoto_RGB.afphoto

[v_kyr]

When converting to sRGB using relative intent there should not normally be any colour loss or shift on colours that are within sRGB's gamut.  Colours outside of the sRGB (saturated colours - especially some cyan, green and yellow) gamut may be clipped to the nearest colour that is within sRGB. Though there could be some slight colour shift when using perceptual rendering intent.

Usually there also shouldn't be any change in contrast by simply changing from ProPhoto to sRGB.  However, if changing from 16-bit ProPhoto to 8-bit sRGB (or any colour space in 8-bit) then the dynamic range is be reduced.  With decent jpeg encoding this is not normally noticeable unless you try to do further processing (such as boosting shadows) after  converting to 8-bit.

You have to check if the left side rectangle used ProPhoto RGB colors are part of the sRGB gamut (color space), if yes then conversion might have some glitches here, if not (aka if those colors aren't part of the sRGB gamut) then they are possibly converted to their nearest sRGB counterparts. - The whole also depends highly on the accuracy of the underlayed color engine and the accuracy and working mode of the Affinity color picker here!

BTW, if you just reassign the docs ICC profile (from ProPhoto RGB to sRGB) the colors of question do get obviously different values.

[lacerto]

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[kirkt]

Working color spaces typically convert with only one intent, regardless of the choices presented to you during conversion.  Seeing no difference amongst the different rendering intents is, therefore, expected.

In contrast, conversion from a working color space to a printer color profile (or any profile configured with various rendering intent options) will provide different results for the various rendering intents, permitting you to control out of gamut color mapping and its relation to in gamut colors.  The attached image is an example of a ProPhoto color image being converted to sRGB and to Epson Velvet Fine Art paper via their respective ICC color profiles.  In each conversion, I used two rendering intents: Perceptual and Relative Colorimetric.  As you know, Perceptual will map the out of gamut colors to the destination profile's gamut volume boundary, while also scaling the in gamut colors to maintain perceptual relationships amongst the colors in the image.  Relative colorimetric will scale the out of gamut colors to the boundary of the destination profile, without scaling the in gamut colors.

As you can see in the attached image, for the conversion to sRGB there is no difference between the two rendering intents; in contrast, the conversion to the Epson profile exhibits differences in the mapping - that is, you can see the scaling in the Perceptual intent, especially in the blue colors in the image.  The images depict the color distribution in a test image I constructed in ProPhoto, and the lines depict the vectors of the displacement of the color from its original position in the ProPhoto gamut volume to its remapped location (onto or into the destination gamut volume) in the source profile's gamut volume.  The images are shown as a 2D projection in the Lab color space.  When you examine the mappings of ProPhoto to sRGB, there is no scaling taking place within the sRGB gamut volume, implying that the rendering intent for working space conversion is relative colorimetric (it is easier to visualize this with a 3D volume you can rotate and examine).

kirk

 

 

I made my own test image, but here is a test image you can use to try this out for yourself:

link text "http://www.brucelindbloom.com/downloads/RGB16Million.png"

[SergioRus]

3 hours ago, kirkt said:

Thank you, friends.

I only want to understand exactly how things work. So please confirm my understanding:

1. Affinity use relative colorimetric conversion to render monitor color if working color spoce is wider that monitor color space.

2. Affinity use relative colorimetric conversion only to convert between RGB working spaces, regardless of the choices presented  during conversion.

3. Affinity use only one conversion method (what method?) to convert from RGB to CMYK  regardless of the choices presented  during conversion. Because I tried to convert my test file  also to  CMYK (US Web coated (SWOP) v.2), and all four conversion methods provide identical results. 

4. The difference between color conversion methods will only be seen if we will convert picture to device specific color profile?

 

[lacerto]

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[kirkt]

I was speaking in a general sense, related to the ICC profiles.  How AP handles conversion specifically, I do not know.  This is a question to ask the Affinity folks, like @James Ritson.

Kirk

[lacerto]

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