Interaction between group blending and layer transparency?
Christian Davideck May 23, 2009 4:34 PM
Hi folks,
I’d be very grateful, in case someone’s willing to explain this issue to me. I’ve been trying to understand this for a long time, but I can’t find any information on the Internet and the Photoshop manual is mute on that point. (All it says about groups/blending modes is here: http://livedocs.adobe.com/en_US/Photoshop/10.0/WSfd1234e1c4b69f30ea53e41001031ab64-78a4.ht ml)
The question is:
What are the exact rules governing the interaction between group blending and layer transparency?
The issue apparently not as trivial as it looks at first sight. Because when I first brought up this issue, even such respected experts as Mark Reynolds said:
Mark Reynolds:"I think you are pulling apart and spotting some interesting oversights. Hard mix behaves completely differently to the way I would expect it to within a Hard mix group. Maybe these are genuine oversights and bugs. […] I think you have brought up some interesting things here, and it would be really interesting to have some other input.”
It all started out with this consideration:
How does a semi-transparent layer in “Vivid Light” appear compared to a “Vivid Light” group containing (only) this layer ?
Answer: Completely different. But why ?
Mark Reynold’s explanation for this was:
OK, You are right.
[…] Its explainable in this way .. layer groups when you apply a blend mode to them take their VISIBLE contents and apply the mode to the whole area, so its similar to applying a neutral 50% grey behind transparent pixels.
THEY DO behave differently but that gives more flexibility”
You can see for yourself by using the attached images (at the end of this posting). Blend the two layers as shown in the following two examples. Scroll to the right to see the layer configuration. All layers and groups have 100% opacity and fill.
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By the way, Vivid Light is not the only mode that produces different results with the 2 different methods above. This also holds true for (at least): Color Dodge, Hard Mix and Difference.
Ok, so where does that leave us now ? If it’s true what Mark said (which I do believe) and applying a blend mode to a group visually amounts to applying the neutral color for the group’s blend mode behind non-opaque pixels, then why is this most fundamental rule nowhere documented in the 674 pages of the CS3 manual ? Why is something as basic and essential as blend mode calculation within a group an undocumented feature ? How the heck are users supposed to find out?
And what about all the other blend modes for which no neutral colour exists (hard light, etc.) ? Since there no neutral colour exists, there must be another rule which comes into play, but what is it ?
I'd be very, very thankful for anybody explaining the abovementioned behaviour. I searched quite a bit on the net and couldn't find any documentation on it, so I'm sure there are lots of users in here who would find an explanation equally useful for them too.
Cheers.
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