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Converting Luminance to Transparency in Photoshop

A while back I signed my name on a postcard with a black sharpie. I scanned it, desaturated it, and adjusted the levels in Photoshop to white out the background and make the darkest black true black. All is well.

Now, I'd like to use the signature as a semi-transparent watermark on some of my images, but I'm not sure how to convert the luminance to an alpha mask. Essentially, the parts that aren't true black should let some of the background color through.

How can this be done? I'm sure the answer is simple, but I've not discovered the method.

3 Responses to "Converting Luminance to Transparency in Photoshop"

  1. Can't you just invert it and use it as an alpha mask on another layer? Create a new layer and fill it with the color you'd like the signature to be in. Then select the layer with your signature, Invert it so the black becomes white and command+click on the thumbnail of the layer to select the whiteish area. Then select the new layer you created in the layers palette and hit the "New mask" button which will create a mask out of your selection. Then you'll have a properly masked signature which you can create a transparent PNG or similar from for easy re-use.

    Or you could probably just let the signature layer be as it is, black on white, and then use the "Multiply" layer mode when overlaying it on a photo. And optionally adjust the opacity.

  2. Jesper helped me figure it out. Here's the steps I followed:

    1. Select and copy the signature.
    2. Create a layer mask on the signature layer.
    3. Go to the channel mixer and make the mask layer visible (click to make the eyeball appear).
    4. Paste into the mask channel.
    5. Switch back to layers, select the mask, and invert it.

    That seemed to do the trick, and is pretty similar to what you suggested, Frode.

  3. I was going to say, apply the signature as a mask to a dynamic color layer, so you can make it black, green, red, or whatever color you like at any time, but you seem to have figured it out.