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This is an excerpt of an image I work within Inkscape:

enter image description here

There is a blueish bitmap to the left and a red bitmap to the right. Currently both are wet to an opacity of 65 %, but I want their opacity to be 100 % where they don't overlap and maybe 50 % where the overlap (most of the vertical rectangular selection in the middle).

How do you do that?

2 Answers 2

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You could do this by arranging how the items are stacked in Inkscape. If you duplicate the blue shape, make sure the original shape is at the bottom of the stack with the opacity set to 100%, the red object at 100% above that, and a copy of the blue object set to 50% at the top of the stack, sort of like a sandwich with the red as the filling.

It doesn't matter if you use vector objects or bitmap imges for this, as long as the bitmap image is a transparent png obviously (with no white background).

Then you can just select both blue pieces together and move them around. Where there's an overlap it will be at 50%, elsewhere it will remain at 100%.

An example. The object sandwich arrangement is shown right

enter image description here

The Layers & Object Panel looks like this, with the top object set to 50% opacity

enter image description here

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You can insert opacity masks to the objects.

enter image description here

A can be a shape or an image or a group. Now it's only a blue rectangle.

B is a group of shapes which is opaque and has different greyshade areas. They can overlap, the topmost greyshade is meaningful. It could as well be an image or a shape with a gradient fill.

In the bottom B is placed on A, both are selected and Object > Mask > Set Mask is applied. Areas below Black are now fully transparent and areas below white are as they were originally. Mid grey causes 50% transparency.

The process can be taken back by applying Object > Mask > Release mask.

Mask can also be set as inverted. But I do not recommend it. An example:

Warning: It may look like the next is just what you need:

enter image description here

In the left a grey shape is drawn on the blue shape. In the right both are selected and Object > Mask > Set Inverse Mask is applied.

Unfortunately it doesn't work with images. Not at least in Inkscape 1.3. The blue shape must be a vector. Use only a full area mask with images and do not try the mask inversion if the target contains an image.

BTW. There's another answer which shows how you could overlay a half-transparent copy of the other image. I guess it creates exactly the effect you actually wanted but didn't ask.

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