The Multiply layer blending mode works in a way that puzzles me. It seems that multiplying a colour by transparency gives the colour and not transparency, as I'd expect from my experience with GIMP.

Here is a composite picture showing the two different behaviours, GIMP on the left and krita on the right.



I've tried grouping in various ways to isolate effects and used Multiply blending mode on the group layer and all sorts of combinations and layer/group structures but the red paint stroke stays in place outside the circle.
(I realise that I can get the 'end result' I'm looking for by using alpha inheritance and multiply blending mode but it's the difference in behaviour that puzzles me.)
Does anyone know the explanation for this?

As far as I understand it, blending modes always compare a pixel A and B. If a pixel B doesn't exist (100% transparent) the software adopt a strategy to do something anyway. In the case of Gimp, it looks like the software prefer to return full transparency and make it invisible. In case of Krita, the result of A only is returned as normal. If you paint on a Normal paint layer with a brush set to Multiply Blending mode, it will paint 'normal' (until painting over previous strokes).

As far as I remember, Painter and Photoshop works the same than Krita regarding this behavior. Gimp one is a bit more exotic, imo.

You get used to what you've known but I'll relearn and adapt Thank you for the explanation Deevad.