Confusion about transparency

[u/frankstonshart]

Something I have always struggled with in GIMP is getting the layers to all simply be equally transparent and equally visible, such that it wouldn't even matter what order the layers are stacked in.

I find that when I set all the layers to an equal transparency, even if the layers have similar images, one will stand out the most, usually whichever is at the top of the stack.

Additionally, sending a colour to Alpha on all the layers doesn't make them equally visible to one another, even if I add a layer beneath them that's just a bucket fill of whatever colour I Alpha'd. It seems like the intensity/amount of certain colours that are shared among the layers adds or multiplies, and dominates to the point where other details are concealed behind it.

Let's say you have a stack of 4 photos of the process of someone turning their head from left to right and you'd like to see all four positions of the face overlaid with equal visibility so it's like having four faces on one head. What would you do?

[u/Stratelier]

Try setting all layers to a transparency that totals 100% between them (e.g. if five layers, then 20% each) and set their blendmodes to "Merge".

Merge sums the opacity values between layers (unlike Normal which "screens" them) and is actually commutative until the total is 100%.

[u/frankstonshart → u/Stratelier]

Thank you for this advice.

[u/ofnuts]

The rule is: opacity is 1/N where N is the position in the stack, from the bottom. So:

• Bottom is 1/1: 100% opacity
• Next up is 1/2: 50% opacity
• Next is 1/3: 33% opacity
• Then 25%, 20%, etc ..

A nice property here is that if you add a layer on top, you don't need to change the opacities of the layers below it.

[u/Stratelier → u/ofnuts]

The one downside is how you'll need to adjust alpha values if you reorder the layers.

But you'd need to set alpha values by hand somewhere anyway, so...

[u/frankstonshart → u/ofnuts]

Thank you for this advice.