Extracting subjects from their background using channels in Photoshop

The most difficult part in post-processing, is replacing backgrounds the proper way, specially if you subject has long hair and the background is not a seamless color. In this episode, we will look at an old but still used technique for better cutout of subjects and how to extract them from the background and place them on another.  

First open your photo in Photoshop, duplicate the background layer (Cmd + J on Mac) / (Ctrl + J on PC).

Go to Channels (next to the Layers palette) and select the channel with the most contrast between the subject and its background. In our case, it is the Blue Channel. 

Hit Cmd + L (Mac) / Ctrl + L (PC) to bring on the Levels dialogue box.

We want to increase the contrast between the background and the subject, therefore, we want to whiten the background and blacken the subject. As a first step, select the "Sample in image to select white point" - the right most eye dropper icon. Next, sample from a very close area from the background immediately next to the subject.

Bring the black slider to the right and the white slider to the left to increase the contrast even more. See fig.1. below.


Next, we need to blacked the subject completely before we invert the mask. For that, select a soft edge brush and change its Mode to "Overlay". Make sure your have pure black as your foreground color (hit the D key). Set Opacity to 100% and Flow to 100%.

Start by brushing the contour of the subject. The Overlay mode will make sure not to spill the brush strokes outside the subject. Keep on painting until the back contour is completely defining the subject. When done, change the mode to "Normal" and paint inside the subject edges. 

Fig.2 - Look at the detailed selection of the stray hair...

Now press Cmd + I (Mac) / Ctrl + I (PC) to inverse the mask. Press X to invert the brush foreground and background. Using Black as your brush strokes color, start painting over the background. Don't be afraid to paint over stray hair as the Overlay mode will handle things for you.

Make a rough selection around the subject using any Lasso tool and fill with black.  Fig.3.

Fig.3. Fill the whole subject with black

Now the time has come to inverse the mask so that your subject is white and the background is black. Next Cmd / Ctrl click on the blue channel to create a selection. Go to layers and duplicate the active layer. This will create a new layer with your subject cutout on a transparent layer. What you can do next is to refine the edge of the selection by feathering it 1 pixel before adding an underneath white layer to change the background.

It is now your turn to practice.