ISSUE:
Inconsistent rendering of the lightness of transparent pixels.
The Context
the lightness of transparent pixels on a photoshop document is normally calculated using the gamma the document. In the case of AdobeRGB (1998) the document gamma is 2.2.
Alternatively the user can select a different gamma using “Blend RGB Colours Using Gamma”. Such as 1.00 for colorimetric accuracy.
The inconsistency
However, with both “Blend RGB Colours Using Gamma” and the document both set to gamma 2.2, (e.g by using an AdobeRGB (1998) canvas), It should make no difference to the rendering of transparent pixels if “Blend RGB Colours Using Gamma” is on or off. Because both the document and the “Blend RGB Colours Using Gamma” are using the same gamma (2.2).
However, there is a difference between them, in how they render the lightness of transparent pixels .
This suggests that the math being used for “Blend RGB Colours Using Gamma” is not consistent with the math used when the pixel lightness is calculated when using the document gamma.
Mac OS 14.6
Photoshop: 25.12
Steps to reproduce:
1: Make an AdobeRGB (1998) black canvas.
2: On a transparent layer above this canvas make a selection and fill that selection with white pixels. And set this layer opacity to 5%
3: Drop a color sampler over the semi-transparent white area. Set the sampler to show RGB numbers.
4: Set “Blend RGB Colours Using Gamma” = 2.2
5: Toggle “Blend RGB Colours Using Gamma” = 2.2 ON/OFF
Expected Result.
There will be no difference to the rendering of lightness of the transparent white pixels when “Blend RGB Colours Using Gamma” is either ON or OFF. Because the gamma of the document and the gamma of “Blend RGB Colours Using Gamma” are the same (2.2).
Actual behaviour
.
There is a change in the brightness of the 5% white pixel when “Blend RGB Colours Using Gamma” is either ON or OFF, ON 26, 26, 26 (RGB) OFF 13, 13 ,13 (RGB)
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