A client of mine was illustrating an article about the World Economic Forum whose annual meeting is usually held in scenic Davos in Switzerland. My client was keen to stress the importance of interconnectivity within both the forum and the wider business community and gave me an image of connected wires of light to marry with the snowy village shot.
To cut out the sky of this pic I adjusted one of the RGB channels. This is a specially useful technique if the area you want to cut out is clearly differentiated by the tones in the picture already – like a figure shot against a white background for example. In these situations you already have a bit of information there that can do the job for you.
Have a look at the channels and select the one which is closest to the black and white mask you want to create – it’s usually the blue channel in RGB. Copy this channel by dragging it to the ‘Create a new channel’ icon at the bottom of the Channels palette. Firstly, create a stark channel of just white and black with no greys. One way to do this is to go Image > Adjust > Threshold… but you may want to use Curves…
This will never be enough to create the cut-out mask in itself but it should be possible to create a channel that closely fits the outline you want. It’ll then be necessary to select the Brush Tool (B) to fill in the areas of white or black as desired. Black hides; white reveals.
When you are happy with the channel, simply cmd/ctrl-click on the channel in the Channels palette go to the appropriate layer in the Layers palette and click the “Add layer mask” icon at the bottom of the palette (it looks like a washing machine). The channel copy will be applied to the layer as a mask.