Bruckner und Seemann

Besides producing good looking pictures volume rendering widens its field of application by improving performance. Fast transformations enable users to observe volumes from different perspectives. In computer aided surgery (CAS) volume rendering is used to visualize geometric relations between tools and organs. CAS gives birth to several minimal invasive surgical interventions. Short reaction times needed may be achieved through fast volume rendering.
Although today's graphic boards speed up rendering algorithms, sophisticated algorithms are needed, if volume rendering has to be really fast. To circumvent expensive hardware accelerated boards the shear warp algorithm may be the right alternative as the fastest software renderer. The possibility transport some of its calculations to the graphics board via OpenGL, makes it even faster. The next section summarizes the algorithm. For a detailed description see Philippe Lacroute's and Marc Levoy's "Fast Volume Rendering Using a Shear-Warp Factorization of the Viewing Transformation".

When using the most basic Image Order algorithm, the data access is not linear: Since the sampled slices should be parallel to the viewing plane, the sampled voxels are an actual plane in the volume only for straight viewing directions. Commonly, they are not continuous in the data-set.

When we are using parallel projection, a simple shear transformation may be used to transform the sampled slices into actual planes in the volume data, making the sampling linear and much more efficient for arbitrary viewing direction.
However, a warping transform must be used to maintain the original viewing matrix, or the image is “smeared”.

Using the appropriate shear transform, an efficient volume rendering algorithm may be used:
1. Transform the volume data to object space by translation and sampling each slice according the shear transform.
2. Composite the sampled slices together into an intermediate 2D image.
3. Transpose the intermediate image to image space using the warp transform.
 

Intermediate sheared image

Final warped image

The following features are implemented:

  • Shear warp as a VTK class with:
    • Alpha compositing with shading
    • Maximum intensity projection
    • Isosurface rendering
    • Bilinear interpolation
    • Nearest neighbour interpolation
    • Run length encoding
    • Parallel projection
  • Transfer function editor
  • Saving and loading of actors / viewports / transfer functions
  • Slicer in 3 principal viewing axis

dataset: skull CT scan
dimensions: 184x256x170
alpha compositing
dataset: skull CT scan
dimensions: 256x256x256
alpha compositing
raycaster with nearest neightbour interpolation
raycaster with linear interpolation
 
dataset: maschner lobb function
dimensions: 41x41x41
isosurface 192
  
shear warp with nearest neighbour interpolation
shear warp with linear interpolation
dataset: skull CT scan
dimensions: 256x256x256
left upper: isovalue at 55
right upper: mip
right lower: alpha compositing

dataset: mummy CT scan
dimensions: 128x128x128
alpha compositing

dataset: feet CT scan
dimensions: 128x128x128
maximum intensity projection

dataset: x-mas tree CT scan
dimensions: 256x256x249
maximum intensity projection

description: values lower than 40 are transparent. Stem and candles are similar in its opacity whilest the decoration is more radioopaque and can be differently colored in red.

dataset: human feet CT scan
dimensions: 128x128x128
alpha compositing

description: the transfer function segments bony structures and a smooth shadow of soft tissue

The following section is a step-by-step guideline:

1. File -> New -> Actor
2. Now select the appropriate file by typing the file name or via file selection menu on right click

2a. Select a file via file chooser

 

3. Adjust advanced properties:

  • run length encoding (true/false)
  • shading (true/false)
  • function type:
    • alpha compositing
    • mip
    • isosurface
  • interpolation type:
    • nearest heighbour
    • bilinear
  • isosurface value

4. Adjust expert properties (lighting parameters as specified in the Phong shading model):

  • ambient term
  • diffuse term
  • specular highlight
  • specular power
5. File -> New -> Add a render viewport
6. Set the name in the property viewer
7. File -> New -> Add a slicer viewport
8. Select the viewing axis in the property viewer

9. Add the actor to the vieports via right click

10. Set the transfer functions in the editor dialogs via right click on the actor property viewer

Special functions of the viewer window

Zoom: hold right mouse button and move up and down
Rotate: hold left mouse button and move up, down, left or right
Translate: hold middle mouse button and move up, down, left or right