What is clamping?
Consider the simple black and white image below:
Let it represent the edge of a mask. Its Histogram will look like this (below):
A histogram is a graphical representation of data. PS's histogram charts the distribution of the light values of an image. The histogram above shows a spike at white, a spike at black and nothing in between. Now lets blur the image (below):
The histogram (below) shows that the blur has resulted in a full set of grey values.
As well as charting data, the histogram can also be used to move the data around. If the black and grey values are moved towards the right, everything from light grey to dark gray will be converted to black.
This will give us (below):
So... a clamp can move a simple black shape out or in.
What is the use of clamping?
Clamping used to combine hard and soft masks
- A hard mask is clean but its edges, especially on hair, are too hard.
- A soft mask has great edges but is noisy in the black and white ares.
- Combine the two!
- Make two masks: a soft one (using a Color Range) and a hard one (using whatever you want).
- Duplicate the hard one and erode one by a few pixels and dilate the other.
- Copy the erode and paste it into soft. Then go to Edit / Fade and set the blend mode to Screen. This will render the black of the pasted erode invisible as it is the neutral color for the screen mode (edit Fade dialogue box below).
- Copy the dilate and paste it into soft. Then go to Edit / Fade and set the blend mode to Multiply. This will render the white of the pasted erode mask invisible as it is the neutral color for the multiply mode.
The image below is a graphical representation of this process. The green area represents the hard, interior eroded mask (applied via a screen blend), the red area represents the hard, exterior dilated mask (applied via a multiply blend). Between the two the original edge of the soft mask can be seen.
Clamping used to shift the perimeter of a mask
Often a mask is fine but is too small / big by one or two pixels. A clamp / dilate / erode can be used to shift this shape. Be warned! it will also likely soften or destroy detail (see below)
Clamping used to de-noise a mask
A gentle clamp can be used to 'convert' grey noise into black or white. Keep an eye on your edges as you do this to ensure that they are not damaged in the process.
What is the problem with clamping?
One problem with clamping is that it is so darn difficult to do in the interface hell that is Photoshop. Another problem is that, on its own, it can destroy detail such as fine hairs, corners, detail etc. The dilate in the image below on the right has resulted in softened corners.