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Image Graphics |
  
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Image Graphics |
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The ImageGraphic hierarchy is shown below.
Classes in the ImageGraphic hierarchy will be of interest to developers who do any kind of image processing, as it is the class that owns the pixel data. It also has all the properties that are required to properly interpret the pixel data, such as Rows, Columns, BitsPerPixel, etc.
The GrayscaleImageGraphic class represents images whose pixel data can be represented by a single sample per pixel. This is the case for DICOM images whose photometric interpretation is MONOCHROME1 or MONOCHROME2. The pixel values are indices into a lookup table (LUT) pipeline that consists of three LUTs. The first two LUTs are MODALITY LUT and VOI LUT (value of interest) transformations. We won’t get into the details of exactly what those transformations are—that is better addressed by DICOM Supplements 33 & 100—but sufficed to say that they control what parts of an image’s dynamic range is emphasized. (Example: the familiar parameters Window Width and Window Center completely specify a linear VOI LUT.) The final LUT, known as the colour map, transforms output of those first two LUTs into 32-bit ARGB (alpha, red, green and blue channel) values suitable for display.
The ColorImageGraphic class is used to represent colour images in ARGB format. Colour DICOM images – which typically have RGB or YBR photometric interpretations – must be converted to ARGB format before they can be represented using ColorImageGraphics. Fortunately, to aid this, the SopDataSource normalizes colour pixel data to ARGB format automatically.
A noteworthy feature about ColorImageGraphic is that because each pixel contains an alpha channel, it is possible to do alpha compositing. This is often useful when certain regions of a grayscale image need to be marked with a partially transparent colour. This can be done by creating a ColorImageGraphic, setting certain pixels on it to the desired colour and transparency, adding it to the scene graph and redrawing the PresentationImage. The default renderer will automatically take care of the alpha compositing.
IImageGraphic, GrayscaleImageGraphic and ColorImageGraphic are generic image objects in that they are not strongly tied to any DICOM specific concepts, such as Sop Instance UIDs and the like. Thus, it is possible to use them in conjunction with non-DICOM images.