Presentation State Compositor Component Sequence Attribute
| Tag | (0070,1805) |
|---|---|
| Type | Required, Empty if Unknown (2) |
| Keyword | PresentationStateCompositorComponentSequence |
| Value Multiplicity | 1 |
| Value Representation | Sequence (SQ) |
RGBA Compositor components. Each RGBA Compositor component combines pairs of RGBA values to produce a single RGBA value.
The order of Items is significant. If there are more than one compositor component, the components are chained such that the output of one compositor component is an input to the next compositor component.
The number of Items in this Sequence shall be the one less than the number of Items in Volume Stream Sequence (0070,1A08).
See Section C.11.27.3.
Section C.11.27.3
C.11.27.3 Compositor Component Usage
If Pixel Presentation (0008,9205) is TRUE_COLOR and there are more than one Item in the Presentation State Classification Component Sequence (0070,1801), the Presentation State Compositor Component Sequence (0070,1805) defines the transformations of the multiple RGB data streams produced by the Classification components into one RGB output stream, a process known as "compositing".
Note
If there is only one Item in Presentation State Classification Sequence (0070,1801), Presentation State Compositor Sequence (0070,1801) is empty since there is only one RGB data stream so there is no need to composite.
Each Item in Presentation State Compositor Component Sequence (0070,1805) describes one two-input "compositing component" in which two RGBA inputs are composited into one RGB output. The first compositor component in the Sequence combines the outputs of the first two classification components to produce one RGB output. Subsequent compositor components combine the output of the previous compositor component with the output of the next classification component to produce one RGB output. This process continues until the RGB outputs of all classification components are composited into a single RGB output.
Each classification component produces a RGBA output, while each compositor component has two RGBA inputs to produce a RGB output. This means that all compositor components after the first in the Sequence have only one Alpha input available, that is the Alpha value from the next classification component output to be composited. In this case, (1-Alpha) is used in place of the Alpha input missing from the previous compositing component.
Note
Through the use of (1-Alpha 2) as Alpha 1for the input from the previous Compositor, the Compositor performs standard Porter-Duff "A over B" compositing in all but the first compositor [Porter and Duff 1984].
The internal structure and usage of the Compositor component is described in Section FF.2.3.3.2 “Internal Structure of RGB and RGBA Compositor Components” in PS3.4.