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look at the bottom of the two lines

lines

To make the line appear smooth, intermediate colors between the color of the background and the line have been added at its edge. In this case, black has been anti-aliased to white. What if I wanted to crop an anti-aliased shape (like the line on the bottom) but correct the anti-aliasing color to that of its new background? So if the background were blue, how could I change the black-white anti-aliasing to black-blue?

Essentially, I have a red shape that has edges that fade to a white background, and I want to move it onto a black background and correct its edges to fade to black. What tools can I use to do this?

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The simplest way would be to set the blending mode of your layer to "Multiply". This effectively gets rid of the color white in your layer and lets the colors beneath it "shine through".

The best option would to get rid of the white and replace it with transparency. That can become quite complicated, honestly, and I can not remember the "perfect" way of doing it. You might want to look into premultiplication and those sort of things, if you're dead serious about it. ;)

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I'm going to summarize the answer from the related question "Blending Edges in Photoshop to a different color of an extracted object with different background color" Here. Read the original answers for some context on whats really going on. The usecase is a bit dissimilar but not much.

Basically you want to choose remove white matte option after selecting the alpha and copying the contents to a new layer*. How to mask the selection in this case is described in "How can I outline this image with a thick black border".

Matting menu

Image 1: The matting menu

* Altough in this case you can just fill the alpha mask with black for same results.... YMMV tough.

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This really depends on what the image is you are working with. If its truly a black line on a white background @J0hj0h's suggestion of multiply is the best solution. It will turn the white part of your image into a perfect alpha channel.

If you are in a more complex scenario where the line may be red and the background blue, you can use color range to select the background color and turn it into an alpha channel.

https://helpx.adobe.com/photoshop/using/selecting-color-range-image.html

Select the background color, adjust for clarity, then hit mask on the bottom of the layers palette. Now your line should look good on any background. (works for human hair in photos ect as well.)

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