I was curious about the issue raised by forum member TRANTOR about getting different results using the same numbers in Photoshop when working in different color spaces.
http://forum.luminous-landscape.com/index.php?topic=105479.msg870588#msg870588I'm not referring to RGB values, since it is obvious that the same numbers will lead to different results depending on the color space, but to Hue rotation. I though (and I guess TRANTOR did too) that changing Hue should lead to the same result as long as you stay inside the color space gamut.
Well, it turns out that is not the case, and Hue rotation is relative to the color space, which might affect those who work by numbers (I don't, but know some people in the graphic arts industry that do)
I started with the same color sample L=70; a=-30; b=45 and rotated the Hue all the way from -180 to +180 in three color spaces, sRGB, Adobe RGN and Lab, and plotted the resulting color in Lab coordinates.
Here are two views from a 3D plot (Y=L; X=a; Y=b. White=Lab; Green=sRGB; Red=Adobe RGB. The small yellow circle is the starting point)
The surprises (at least to me) are:
- The difference in the L value between RGB spaces and Lab, especially for large Hue rotations
- The shape in Lab is square (I was expecting a circle)
The following image shows the difference in the three color spaces compared with the original with a hue rotation of 45. The upper part is the original color, then from left to right sRGB, Adobe RGB and Lab.
And the following with a Hue rotation of 180 (again, the upper part is the original color, then from left to right sRGB, Adobe RGB and Lab).
Intrigued by the rectangular shape of the results in Lab, I calculated the DeltaE between each sample (Hue was increased in steps of 15) and got the following results:
Even if the Lab shape is rectangular, the DeltaE between samples remains between 13 and 16, while in the RGB cases the variation is higher.
Is this expected? Does it makes sense to work by numbers or do you have to work in Lab for this? Or is HSL wrongly implemented in Photoshop?