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I have a BMP image without an alpha channel. For a game texture, I would like to add a "mask map" which is a black / white image which tells which regions should be transparent.

To do this, I first opened the BMP image and added a new channel to it and called it Alpha.

Then I opened the mask image, pressed Ctrl A to select everything and copied it.

Then back to the RGB image, I de-ticked the eyes for each channel except the Alpha channel.

Then I pasted the copied image.

However, I don't see any change in the resulting image when I enable the eyes of each channel.

How could I achieve this in the proper way?

Thank you.

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You can't paste directly into the alpha channel, but there are ways to work around that.

Here's one method that might work for you.

  1. Add an alpha channel to the image you want to mask

  2. Right click the image layer in the layers panel, and hit the Add Layer Mask icon. The fill you choose for the mask doesn't matter at this stage - white or black will do.

  3. Open the other image you want to use as a mask, and select all, and copy it (CTRL+A, CTRL+C)

  4. Back in the other image, select the layer mask thumbnail by clicking on it in the layers panel

  5. Paste the mask you copied earlier (CTRL+V)

  6. Hit the Anchor icon (or CTRL+H), to anchor the floating selection to the layer mask.

You can also right click the layer, and choose Apply Layer Mask. This will remove the layer mask, but retain the mask information in the alpha channel.

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