The grayscale to color blending transformation model applies only to a pair of grayscale values, one of which is first mapped to color and then superimposed upon the other. The resulting values are device independent color PCS-Values. This process is illustrated in Figure N.2-3.
For the purpose of this section, pixels are referred to as stored pixel values and transformations are defined as point operations on these values. However, it is likely that pixels from either or both the superimposed and underlying image sets will have been spatially resampled and hence interpolated or replicated. Such operations do not affect the conceptual pipeline.
Figure N.2-3 Grayscale to Color Blending Transformation Model
The Modality LUT and VOI LUT transformations are applied to the stored pixel values of the underlying image.
The output range of the VOI LUT transformation depends either on the width of the linear window or the range of output values of the LUT defined by the LUT Descriptor. Conceptually, for the purpose of describing the succeeding blending operation, the smallest pixel value from the range is mapped to 0.0 and the largest pixel value is mapped to 1.0 and all intermediate values are linearly mapped to the [0.0..1.0] interval.
The Modality LUT and VOI LUT transformations are applied to the stored pixel values of the superimposed image.
The full output range of the preceding VOI LUT transformation is implicitly scaled to the entire input range of the Palette Color LUT Transformation.
The output range of the RGB values in the Palette Color LUT Transformation depends on the range of output values of the LUT defined by the LUT Descriptors. Conceptually, for the purpose of describing the succeeding blending operation, a LUT entry of 0 is mapped to 0.0 and the largest LUT entry possible is mapped to 1.0 and all intermediate values are linearly mapped to the [0.0..1.0] interval.
Note: In practice, the Palette Color LUT output for the superimposed images is encoded in 8 or 16 bits and hence will have a range of 0 to 0xFF or 0xFFFF.
The Palette Color LUT used is that encoded in the Blending Presentation State; any Palette Color LUTs or Supplemental Palette Color LUTs in the image instances are ignored.
The inputs to the blending operation are grayscale values from 0.0 to 1.0 from the underlying image (Y u ) and RGB values from 0.0 to 1.0 from the superimposed image (RGB s ), and an opacity value from 0.0 to 1.0 (A).
The output is a single image containing RGB values (RGB o ) blended as:
R o = R s * A + Y u * (1-A)
G o = G s * A + Y u * (1-A)
B o = B s * A + Y u * (1-A)
The output of the blending operation is implicitly scaled to the gamut of the hypothetical device described by the ICC Input Profile, resulting in PCS-Values.