N.2.3 Common Spatial and Annotation Transformations

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Figure N.2-2 Common Spatial and Annotation Transformation Model

The common spatial and annotation transformations apply to any device-independent values, whether they be grayscale P-Values or color PCS-Values, for any type of presentation state.

The values with which to render annotations are encoded as device-independent values, either as grayscale P-Values or as color PCS-Values. In the case of PCS-Values, CIELab values are encoded, and defined by reference to a D50 illuminant.

Grayscale presentation states may specify annotations in color for rendering on a color output device.

The mechanism for mapping grayscale P-Values and color PCS-values to the same display is implementation-dependent and not defined by the standard.

N.2.3.1 Shutter

The Shutter transformation provides the ability to exclude the perimeter outside a region of an image. A gray level may be specified to replace the area under the shutter.

One form of this transformation uses the Display Shutter Module as used in the X-Ray Angiography Image Storage SOP Class, though it may be applied to any Image Storage SOP Instance, including single frame images.

Another form uses a bit-mapped overlay to indicate arbitrary areas of the image that should be excluded from display by replacement with a specified gray level, as described in the Bitmap Display Shutter Module.

Notes: 1. Since annotations follow the shutter operation in the pipeline, annotations in shuttered regions are not obscured and are visible.

2. Any shutter present in the referenced image object is ignored (i.e. not applied).

N.2.3.2 Pre-Spatial Transformation Annotation

The Pre-Spatial Transformation Annotation transformation includes the application of bit-mapped overlays as defined in the Overlay Plane Module, and free unformatted text or vector graphics as described in the Graphic Annotation Module that are defined in the image pixel space (as opposed to the displayed area space).

N.2.3.3 Spatial Transformation

Some modalities may not deliver the image in the desired rotation and need to specify a rotation into the desired position for presentation. This transformation, specified in the Spatial Transformation Module, includes a rotation of 90, 180, 270 degrees clockwise followed by a horizontal flip (L <--> R). Rotation by an arbitrary angle is not supported.

In addition, selection of a region of the image pixel space to be displayed is specified in the Displayed Area Module. This may have the effect of magnifying (or minifying) that region depending on what physical size the display is instructed to render the selected region. If so, the method of interpolation (or sub-sampling) is implementation dependent.

Note: In particular the number of displayed pixels may be different from the number of image pixels as a result of:

- minification (e.g. 1 display pixel for 4 image pixels),

- magnification (4 display pixels for each image pixel) ,

- interpolation (display pixels derived from values other than those in the image pixels), and

- sub-sampling.

N.2.3.4 Post-Spatial Transformation Annotation

The Post-Spatial Transformation Annotation transformation includes the application of free unformatted text or vector graphics as described in the Graphic Annotation Module that are defined in the displayed area space (as opposed to the image pixel space).

This implies that the displayed area space is defined as being the image after all Spatial Transformations have been applied.

These annotations are rendered in the displayed space, though they may be anchored to points in either the displayed area or image pixel space.