PNG transparence and 'Mask Simple' filter

I have a question because of this:

The water you can see around the swan is from the original, but it shouldn’t be visible.

As you can see here, the water is visible only with ‘Mask Simple’ filter, because the reworked image do not have any water anymore (all around the swan is transparent):

Is it a bug? Or does transparency depends on a filter…? Or…what?

It is not a bug but rather a matter of the Mask Operation not giving you what you want. You chose “Maximum.” In the part of the top image with the water, the alpha is 255 which is higher than Mask shape’s alpha in the parts where it bleeds through. So, the higher alpha value is used. I am not sure what you want, but maybe Minimum gives it or possibly none of the options.

Yes, you are right, minimum is the right choice to hold transparency, but where from is maximum getting the information? I mean. I deleted all around the swan (you know what I mean?)…, all around the swan is full transparent.

The alpha channel. The mask filter is looking at two images: the video image and the shape it has drawn (with its own alpha channel). Now, it needs to combine the two alpha channels. It does so using some operation. Maximum chooses the larger of the two values. When you removed the water by making it transparent, you set its alpha to 0. In the shape image, inside the shape, the alpha is 255. Do not forget to use filter Alpha Channel: View to help better visualize the results.

Thanks for the hint.

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