Advanced Masking Using Channels

In this lesson I will cover the advanced technique of creating masks using channels. This method allows you to define a mask that will capture very small and precise details. The only requirement is that at least on of the channels (including CMYK) will need to show clear separation of the item you want to crop.

Step One

Assuming you have already opened your file in Photoshop, click on the channels tab. This is usually located next to the layers pallet on the left. If you do not see it there you can enable it by clicking Window/Show Channels from the top menu.

What you are looking for is a channel that has a clear definition between the background and the object you want to create a mask for. In my example I am going to replace the sky in the background. The blue channel offers the clearest contrast of the trees and the sky.

Step Two

Make a copy of the channel by dragging it over the (create new channel) icon in the lower right of the channels dialogue box.

Step Three

Select the new channel and open up the levels dialogue box by clicking Image/Adjust/Levels (ctrl+L).

Drag the black and clear arrows below the histogram inwards until you get close to a pure black and white channel.

At this point, if there are any areas where the black is in the white or visa vera, use your paint brush (B) tool to fill them in.

Step Four

Copy or drag and drop your new background image into the file. This should appear as a new layer. This layer can be on top of your working layers.

In my photo, there is some water that reflects the sky that I will want to simulate in the new background. I do this by using the select tool to select the top half of the sky. I then copy and past this as a new layer (ctrl+C) (ctrl+V).

On the new layer enable the free transform tool by clicking Edit/Free Transform (ctrl+T). Then drag the top middle toggle block to the bottom of the screen. Double click the center of the selection to finalize. Now press (ctrl+E) to merge the layer to the layer below. You should now see the desired reflection.

Step Five

Return to the channel pallet and (ctrl+click) the new layer mask you have created. This will create a selection of the new mask.

Return to your layers pallet. You should still have your new background layer selected. Press the (Add Layer Mask) in the bottom left of the layers pallet. This will add your mask to the selected layer.

You should now see a completed image

I have adjusted the colors to match the back ground. I will cover this step in a future tutorial.