As you will soon discover as you get to grips with Photoshop, knowing how to 'select' items is one of the most important techniques you need to know. It will enable you to separate items such as a figure from its background and do many manipulation techniques.
Every image and its background will be different, and will require different selection techniques and tools. Here we look at one of the more difficult tasks - selecting the bare branches of a tree from its background (this technique will also be useful for flyaway hair etc.)
You can right-click on this image and copy it, and then paste it into a new Photoshop document to practice.
So how do we isolate this tree from its background?
We are going to use Channels. Firstly, make sure your Channel palette is visible (WINDOW...Channels).
Click on the small downward-pointing arrow and select Panel Options.
The panel on the left will appear. Select the radio button for the largest thumbnail - this will let you see the content of the channels more clearly.
A simple observation of the three channels tell us that the blue channel has the highest contrast between the sky and tree. We are going to exploit this property to separate the two. Go to the Layers palette and duplicate (Ctrl. + J) the Layer. Now switch to the Channels palette and click on the Blue channel of the copied layer. Click IMAGE...Adjustments...Curves (or press Ctrl. + M) to apply curves modification on the current channel.
Select the point closer to the dark grays of the channel and drag it (2) forward in the x-axis (horizontally) keeping y-axis at 0 (shown by the small circle). This will make the darker pixels uniformly more darker. It shouldn’t affect the background, drag it a little back to left if it does.
Next, select another point closer to the White Point, drag it (3) back towards the midtones keeping the y-axis(vertical) at maximum (shown by the small circle). What we are doing is saturating the dragged range of blue up to 100%. Make sure the whole background turns white or almost so, and there is a very high contrast between the tree and sky.
When you click OK in the Curves panel, your image should now look like the one above.
Using the Channel as Mask
Ctrl-click on the channel thumbnail to load the channel into the selection. Areas which are predominantly blue will be selected totally and the rest according to respective degrees. Areas which have no blue (the black parts) will be left from the selection.
Adding a Layer Mask
With the selection in place (i.e. with he 'marching ants' going) go to the Layers tab and click the Add Layer Mask icon to save the selection as a Mask.
The Layer Mask is the second thumbnail as seen above. Now we'll invert this mask. Press Ctrl + I.
See how the second thumbnail now looks like a 'negative'? It's because we've inverted the layer mask.
Looking at your main image, it doesn't seem to have changed does it? Just go to the Layers palette and 'turn off' the visibility of the original layer (remember we duplicated this layer?) by clicking the 'eye' icon off.
The image below shows the result - a brilliant way to isolate complex objects from their backgrounds. The gray/white checkerboard denotes transparency and we can create a new layer, fill it with anything we want and drag it below the copy layer.