Footprint Evaluation for Volume Rendering

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Footprint Evaluation for Volume Rendering

• Presentation by
• Lian Jiang
• Gaozhu Peng
• Sandeep Gupta

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• INTRODUCTION
• Forward Mapping
• Easy to Parallel
• ENHANCEMENTS
• Supports Perspective without excessive cost
• Adaptive Re-sampling

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• Problem
• How to fill the pixel values between
  the volume project sample?

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Rendering Pipeline

• Transformation
• Shading
• Reconstruction
• Visibility

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• Transformation
• 1. Original unit sphere (U)
• 2. Grid scale transformation ( Matrix S)
• Viewing transformation (V)
• Shading
• Pre-shaded pipeline

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Footprint definition
voxel
3D extent
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If we place D at the origin, the footprint
function is
This footprint function defines how much voxel(D)
will deposit its value to pixel(Dxx, Dyy)
?(Dx, Dy, Dz)footprint(x,y)
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Example Sphere Kernel
Approximate the 3D kernel h(x,y,z) extent by a
sphere
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Why we need Footprint Table?
Answer

• Evaluating footprint(x,y) on the fly is too time
  Consuming involves integration of the kernel
  function h(x,y,z)
• We can build the footprint table at
  preprocessing time
• The kernel function can be any cone, Gaussian
  and sphere (depends on the renderer)

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How to use Footprint Table?
Answer Footprint value lookup

• For rectilinear meshes, the footprint of each
  sample is
• identical except for an image-space
  offset.
• The renderer only needs to calculate
  footprint function
• once for each orthographical view.
• Weight is calculated by table lookup at the
  footprint
• function value at each pixel that lies
  within the
• footprints extent

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Effect of number of entries of the table

• Time versus space tradeoff
• If a lot entries, nearest neighbor works fine
• If coarse, interpolate from the nearest
• samples.
• For smaller table size, interpolation gives
• much better results.
• Produce intuitive images

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How to deal with Visibility
Answer

• Splatting uses the compositing operator to
  perform
• visibility
• Either front to back or back to front
  compositing
• The problem for simply composite samples
  footprint
• onto the accumulation buffer sample by sample

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• ENHANCEMENTS
• Supports Perspective without excessive cost
• Adaptive Re-sampling

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Volume Rendering of a high potential iron
protein (rendered by VTK)