ExtraSamples

Description of extra components.

Tag = 338 (152.H)
Type = SHORT
N = m

Specifies that each pixel has m extra components whose interpretation is defined
by one of the values listed below. When this field is used, the SamplesPerPixel
field has a value greater than the PhotometricInterpretation field suggests.
For example, full-color RGB data normally has SamplesPerPixel=3. If
SamplesPerPixel is greater than 3, then the ExtraSamples field describes the
meaning of the extra samples. If SamplesPerPixel is, say, 5 then ExtraSamples
will contain 2 values, one for each extra sample.

ExtraSamples is typically used to include non-color information, such as opacity,
in an image. The possible values for each item in the field's value are:

0 = Unspecified data
1 = Associated alpha data (with pre-multiplied color)
2 = Unassociated alpha data