Ramin Hossaini (blog)

Photoshop tutorial: Creating detailed-masks from channels

What are you on about?

This tutorial will show you how to apply masks to very detailed areas in photos.
For example, if you wanted to modify the sky in the image below, without making the same changes to the foliage, you would have to use masks.

You could duplicate the layer, make changes to the sky on the new layer, apply a mask, and paint areas black or white depending on whether it should be visible or not. But you can imagine how time consuming this would be with the detail of the leaves and flowers. It really isn't a fun (or smart) process.

Here's another way.

Step 2: You have to make a choice here - it won't always be the blue channel. I chose the blue channel because resulted in the foliage becoming really dark, and the sky becoming pretty light. This made it perfect to apply a simple level-correction to get the sky white, and the foliage black.

Step 3: Duplicate the blue channel.

Step 4: Apply levels (as shown below). You can also apply an "S" shaped curve to the image (which would increase the contrast of the image). What you do to the image will differ from image to image, some will need a lot more work, others - not so much (like the one used here).

Step 5: Select the RGB channel again and go back to the layers-palette. Duplicate the layer and make the changes to the image - keeping in mind that only the sky in this case will retain the changes.
In my case, I simply changed the blending mode to multiply to darken the sky. You could also change the colours of the sky to a more reddish colour for example.

Step 6: Next, we need to select the channel. Do this by going back into the channels tab and CTRL-left click the new channel you made.

Step 7: Go back to the layers tab and click on the mask button. This will automatically make a mask based on your selected channel.
Black in the channel, will be black in the mask - and will reveal the layer below it.

Step 8: If you press the "\" key, you will see how the mask is being applied.

Step 9: Now grab a brush and pain the sky white. Note: You could also have done this when originally creating the channel - it's completely up to you.

Step 10: Press the "\" key again to see what it looks like.

Before and after in detail:

This shows how only the sky has been affected.

Before:

After:

Conclusion
This method can also be used when creating masks involving hair - which is a tricky thing to do otherwise.
The key is to get a decent channel to work with - after that, things become a breeze!