Making an image semi-transparent… Or is that semi-opaque?

[Ofnuts]

(09-26-2018, 09:48 PM)HavingTooMuchFun Wrote: FANTASTIC! I experimented a little bit, and by having the “from” color be black or white, almost any color can be made “semi-transparent.” The very few exceptions can also be transformed by adding an infinitesimal amount of black or white, so little that you can’t even see it. Thank you so much for explaining this, this will be a wonderful addition to my artwork!

I’m still struggling with some of the original issues of this post, though:

Why are images that I have made semi-transparent using the opacity control on the layers palette copying or cloning as opaque unless I export and re-import them?

Why are semi-transparent images that are in the clipboard of the brushes palette being applied as opaque?

How do I go from using the brushes palette back to normal pencil function? Currently I’m having to restart Gimp to make that happen.

Edit>Copy copies you layer data, and your layer is opaque.

Warning, pseudo-math ahead.

Layer composition can be seen as an mathematical operator (you have modes like Add, Subtract, Multiply, Divide...), the most frequent operator being "Over" (aka, Normal mode) (often noted ⊕). So basically your "visible" image is computed as:

TopLayer ⊕ Layer2 ⊕ Layer3 ⊕ ... ⊕ LayerN ⊕Background

Which is really computed bottom up, ie:

TopLayer ⊕ (Layer2 ⊕ (Layer3 ⊕ (... ⊕( LayerN ⊕Background)...)))

Layer groups are really just a way to move these parentheses, and so force a different order of composition, for instance making a group with Layers 2 and 3:

TopLayer ⊕ ((Layer2 ⊕ Layer3) ⊕ (... ⊕( LayerN ⊕Background)...)))

That group behaves like another layer, this would be strictly equivalent to computing the group on another image, and then inserting the resulting layer:

Group = Layer2 ⊕ Layer3                              

TopLayer ⊕ (Group ⊕ (... ⊕( LayerN ⊕Background)...)))

So where  does the Opacity slider come in? It is used to add weights/coefficients to the layers (or the groups):

Ot*TopLayer ⊕ (O2*Layer2 ⊕ (O3*Layer3 ⊕ (... ⊕( On*LayerN ⊕ Ob*Background)...)))

But really it does not change the layer's data (which is why you can change it later, if the layer data was changed round-off errors would creep in quickly).

In other words there are two ways for a layer to be transparent: because it is actually transparent (with or without a layer mask) and because you put a small weight on it. If your layer is the only visible on in the image and you set the opacity for it, your visible image is:

Ol*Layer

Which is why Edit>Copy visible copies the layer with its opacity as mentioned by Espermaschine.