In this 12-minute tutorial by PiXimperfect, learn how to use alpha channels in Photoshop to make “insanely difficult” selections. An example situation for which you may need to employ this technique could be selecting pieces of the sky from behind a tree on the horizon.
Photoshop’s Channels allow you to split your image up into red, green, and blue colors.
Each channel you view appears black and white, but that’s because it’s actually a representation of the presence of different colors. The darker areas have less of the color in question, and the brighter areas have more. You can utilize this to select only the sky, thanks to the difference in color between it and the trees.
Chances are the areas you’re selecting aren’t completely black or completely white at the moment, so now you need to perfect this “mask” so that you can select only the sky.
Make a copy of the channel layer with the most contrast. The first step to try is adjusting the levels. Pull the white and black sliders inwards, but not too far or you’ll “fry up the edges” like this:
Now you’ve made a pretty good start, but things are far from perfect. Next, grab the brush tool and make sure to select the “overlay” blend mode for the brush.
Once you’ve done that, you can start to paint white, or black, onto the image. White will not paint onto the darker areas, and black will not paint onto the whiter areas. Still try and be careful with the brush though, or you’ll burn out the intermediate regions.
Once your channel layer is looking full of contrast, press and hold “Cntrl / Cmd” and click on the channel layer in the “Channels” window. Photoshop will then make a selection for you of the bright areas. Go back to the layers tab and create a mask from it. Select the mask and press “Cntrl / Cmd + I” and you will invert the mask to get rid of the sky.
Once you’re left with just the foreground, you can then paste in your new sky and make any further tweaks necessary.
Check out the full video above to see exactly how it’s done.