How to get layer to only have colors where mask is bright in GIMP?

Question

I would like to create a reflection of a room on a window. My specific goal is to make sure that only the brighter parts are reflected. For this, I created a higher contrast copy of the picture of the room, where only the bright parts are visible, then I made it grayscale, than made the black color transparent. Now I have two layers, a map, where the info is where the basic picture should be transparent (the black parts), and the original picture.

How do I transform the original colored picture to be transparent on the darker parts as well?

Thank you!

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Answer 1

Use a high contrast BW version as a layer mask for the colored version. In practice, when you add the layer mask, select option "greyscale copy of the image". Then select the mask icon at the layers panel and apply "Curves":

Layer mask can be added by right clicking the wanted layer in the layers panel. You get a list where one option is to add a layer mask.