TIFF 6.0 Specification
Final—June 3, 1992
Section 18: Associated Alpha Handling
This section describes a scheme for handling images with alpha data.
Introduction
A common technique in computer graphics is to assemble an image from one or
more elements that are rendered separately. When elements are combined using
compositing techniques, matte or coverage information must be present for each
pixel to create a properly anti-aliased accumulation of the full image [Porter84].
This matting information is an example of additional per-pixel data that must be
maintained with an image. This section describes how to use the ExtraSamples
field to store the requisite matting information, commonly called the associated
alpha or just alpha. This scheme enables efficient manipulation of image data
during compositing operations.
Images with matting information are stored in their natural format but with an
additional component per pixel. The ExtraSample field is included with the image
to indicate that an extra component of each pixel contains associated alpha data. In
addition, when associated alpha data are included with RGB data, the RGB com-
ponents must be stored premultiplied by the associated alpha component and
component values in the range [0,2**BitsPerSample-1] are implicitly mapped
onto the [0,1] interval. That is, for each pixel (r,g,b) and opacity A, where r, g, b,
and A are in the range [0,1], (A*r,A*g,A*b,A) must be stored in the file. If A is
zero, then the color components should be interpreted as zero. Storing data in this
pre-multiplied format, allows compositing operations to be implemented most
efficiently. In addition, storing pre-multiplied data makes it possible to specify
colors with components outside the normal [0,1] interval. The latter is useful for
defining certain operations that effect only the luminescence [Porter84].
Fields
ExtraSamples
Tag
= 338 (152.H)
Type = SHORT
N
=1
This field must have a value of 1 (associated alpha data with pre-multiplied color
components). The associated alpha data stored in component SamplesPerPixel-1
of each pixel contains the opacity of that pixel, and the color information is pre-
multiplied by alpha.
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