Clamping

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Revision as of 15:21, 18 January 2012 by WikiSysopPS (talk | contribs) (What is the use of clamping?)
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What is clamping?

Consider the simple black and white image below:

Clamping 01.png

Let it represent the edge of a mask. Its Histogram will look like this (below):

Clamping 02.png

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):

Clamping 03.png

The histogram (below) shows that the blur has resulted in a full set of grey values.

Clamping 04.png

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.

Clamping 05.png

This will give us (below):

Clamping 06.png

What is the use of clamping?

Clamping can be used to gently move the mask edge in (erode) or out (dilate) one or two pixels.

Erode 01.png

Sometimes, if the mask is soft, then a blur isn't even necessary.

A use for a hard edged dilated or eroded mask is this:

Problem
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.
Solution
Combine the two!
Procedure
  1. Make two masks: a soft one (using a Color Range) and a hard one (using whatever you want).
  2. Duplicate the hard one and erode one by a few pixels and dilate the other.
  3. 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.
  4. 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.