Selection to Mask Workflow
Selecting is a key aspect of the compositing workflow and one that is not without a good deal of complexity. There are many selection tools, many reasons to select and many things you should know about it. In the compositing skill-set selection is most vital when it is used in order to separate a figure or object from its background and it is in this context that I shall adress it. If you get good at this then all other selecting tasks shall seem trivial.
The selection to mask workflow is, in principle, simple enough. Here is the general workflow:
|Review||Review the image to determine what region needs to be masked, and if this region is naturally defined by a single color.|
|Select||Select the region.|
|Make the mask||Press the mask button. This will automatically make a mask based on the selection.|
|Edit non-bordering regions||The mask that you have just created will almost certainly require editing: refining and correcting.
There are separate process for removing junk, editing soft and hard masks and painting.
|Edit hard edges|
|Edit soft edges|
|Edit by painting|
At this stage, you should review the image to determine exactly where the mask is to be located. Is the object, or its background, defined by a single flat color? If yes, then you are in luck. This will make the whole process easier.
At this stage, you simply make the selection. This you do using one of the selection tools. For a hard edge object the Quick Selection tool is sufficient. For regular objects like boxes, buildings etc the Polygonal Lasso Tool is a good choice. For things like hair the Select / Color Range is the only option. Using the Color Range is simple, but also critical.
With the selection made, press the Add Layer Mask button at the bottom of the Layers Palette. This will make a Layer Mask from the selection. Its possible that the mask will now require inverting. Simply 'Command i' whilst in the layer mask.
Its possible that you now have the perfect mask. However, most likely, the mask will require editing. To get a better idea of how good your mask is, 'Alt click' on the mask in the Layers Pallet. This will show the mask in the window. In this case, the face needs some editing.
To 'stress test' the mask, press 'Command L' to summon the Levels adjustment. Move the middle slider (the 'gamma') to either side. This reveals any flaws in the mask. This temporary adjustment is only for testing purposes. Do not press the 'OK' button. This test is known as a 'Gamma Slam'. Moving the gamma slider to the left will test the black of the mask, and to the right the white of the mask. In this case, the background looks a bit noisy... however, the background is probably good enough to require no more work.
Edit the mask
Having made the preliminary mask you now need to get rid of the obvious noise and imperfections. The white 'keep' area of the mask is called the holdout and the black 'remove' section is called the junk. Both might require attention.
Edit non-bordering regions
Editing regions of the mask that do not include any border is simple. Just painting them out with a brush. In the case below, the girls face does not include any border.
Edit hard edges
For refining a hard edge object, Clamping may be employed.
Edit soft edges
In the masking of a figure, the hair will require a very soft mask. The problem is that any severe adjustment to a mask is likely to damage these soft regions. Therefore, in the example case of the image of the boy, any edit to the hard-edged cloth will damage the hair.
There is a way of combining a junk and holdout adjustment in such a way as to leave the bordering edges untouched. However, it is for advanced users. Details are here: Dilate Erode Combine.
For newbies, the simplest approach is to first make a 'pre-selection' using a Lasso or Marquee. This should encompass only the hard-edged regions. By doing so, any edit made to the mask will not include its soft-edged regions.
Edit by painting
For painting into tight regions of the mask, the cutting-in method may be used:
Layer Clipping and Masking
In the example below, the hair of the masked girl has a white fringe. This cannot be fixed using a clamp as it would damage the softness of the hair. Instead it is fixed after the mask process is complete. This is done by first clipping a layer to the masked layer. Then, using a Clone Tool, the hair is patched. Remember to set the 'clone sample mode' to 'Current & Below'.
The following are some miscellaneous observation reading the art and craft of masking.
Masking as drawing
Masking is a drawing activity. Organic shapes in particular can be subject to a lot of creative refinement in the process of masking. In the before (left) and after (right) below the shape of the girl has been clarified and the unfortunate wiggly pattern on the outline of her right sleeve has been removed.
Where are you?
Whilst masking, be aware of where you are. After having made the mask from a selection you will be taken automatically to the mask component of the layer (B). Any painting or filling will be done to the mask. If you accidentally navigate to the layer contents component then you may well accidentally painting into the layer contents component of the layer (A). This is a BAD thing. Option Click on the mask to get into and out of it.