This is a very interesting topic, and I usually find in Photograph forums some people showing others how to adjust contrast in their pictures using the Curves tool (or the Levels tool), usually without caring at all about the fusion mode used to preserve tone. Glad to see some people really care about this.
I have done the following experiment with a photograph to find out how well PS keeps tones depending on the colour profile in which the picture is. I am not sure if what I did provides any conclusion, but I will explain.
One of my programs (
Tone Hacker) plots a Hue histogram (it's rare to find them, although I consider them really useful that's why I put it there) based on the RGB->HSV conversion formulas found here:
RGB to HSV formulas.
- I have opened a photograph in Photoshop in sRGB, saved it (JPG) and calculated its Hue histogram
- Then I have added a strong contrast 'S' shaped curve in Normal fusion mode and saved again (JPG) and calculated its Hue histogram
- Then I have switched the curve from Normal to Luminance fusion mode and saved again (JPG) and calculated its Hue histogram
- And I have repetead the 3 steps for the image converted first to AdobeRGB, obtaining the histograms and...
- Repeated again the process converting from the original sRGB to ProPhoto obtaining the histograms too.
Finally I show you here the original image and the curve applied:
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And here I compare (the upper is the Hue histogram resulting when having applied the curve, and the lower histogram is always the original in each colour profile, just upside down for easy difference detection):
[span style=\'font-size:14pt;line-height:100%\']sRGB[/span]Normal fusion mode (left) vs Luminance fusion mode (right)
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[span style=\'font-size:14pt;line-height:100%\']AdobeRGB[/span]Normal fusion mode (left) vs Luminance fusion mode (right)
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[span style=\'font-size:14pt;line-height:100%\']ProPhoto[/span]Normal fusion mode (left) vs Luminance fusion mode (right)
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The curve is very severe. It's easy to see that when the Normal fusion mode in the curve clearly modifies the Hue in the whole scene, however the Luminance fusion mode preserves the Hue and does it independently of which colour profile is being used.
Of course the effect of the curve in each colour profile is different, as histograms are, but the important conclusion is that HUE IS ALWAYS PRESERVED IF LUMINANCE FUSION MODE IS USED.
Am I right or I am talking bullshit?
Regards.
PS: I have to write one day the saturation histogram routine and repeat the tests.