Image Synthesis
Transparency
computer graphics & visualization
Inter-Object realism
• Covers different kinds of interactions between
objects
– Increasing realism in the scene
– Relationships between objects easier to understand
• Shadows, Reflections, Transparency
Image Synthesis – WS 10/11
Kai Bürger – Computer Graphics and Visualization Group, TU München
computer graphics & visualization
Transparency
How to handle non-opaque (semi-transparent) objects
– Multiple fragments contribute to a pixel
– Visibility order of fragments is important
– Depth buffer can be used for hidden surface
removal but not for semi-transparent objects
Image Synthesis – WS 10/11
Kai Bürger – Computer Graphics and Visualization Group, TU München
computer graphics & visualization
Transparency
See-through objects
Depth test
discards objects
Object order affects final color
Blending non commutative
1C1 + (1-1)0C0  0C0 + (1-0)1C1
Image Synthesis – WS 10/11
Kai Bürger – Computer Graphics and Visualization Group, TU München
computer graphics & visualization
Transparency
General solution
– Visibility sort all semi-transparent objects in the
scene (back to front)
– Render opaque objects first and update the depth
values
– Render semi-transparent objects in visibility order
• Apply depth test but do not alter depth values
– Use alpha-blending in order to obtain linear
combination of src and dst color
CDst = (1 - Src) CDst + SrcCSrc
Image Synthesis – WS 10/11
Kai Bürger – Computer Graphics and Visualization Group, TU München
computer graphics & visualization
Transparency
Opaque
Semi-transparent
Image Synthesis – WS 10/11
Kai Bürger – Computer Graphics and Visualization Group, TU München
computer graphics & visualization
Transparency
without visibility sorting
with visibility sorting
Image Synthesis – WS 10/11
Kai Bürger – Computer Graphics and Visualization Group, TU München
computer graphics & visualization
Transparency
Correct transparency by sorting and alpha-blending
Image Synthesis – WS 10/11
Kai Bürger – Computer Graphics and Visualization Group, TU München
computer graphics & visualization
Transparency
Image Synthesis – WS 10/11
Kai Bürger – Computer Graphics and Visualization Group, TU München
computer graphics & visualization
Depth peeling
Recall: standard depth test gives nearest fragment at each
pixel without imposing any order
But: there is also a second, third, … fragment. Depth test
does not provide a means to render the nth nearest surface.
Depth peeling solves this problem (in a pixel shader):
• With n passes over a scene, n layers deeper into the scene are generated
• E.g. with 2 passes, the nearest and second nearest surfaces are extracted
• Both the depth and RGBA information is obtained for each layer
Image Synthesis – WS 10/11
Kai Bürger – Computer Graphics and Visualization Group, TU München
computer graphics & visualization
Depth peeling
• The method peels different depth layers in frontto-back order
• Drawback: as many rendering passes as objects
depth complexity: maximum number of surface
points (fragments) mapping to one pixel
Image Synthesis – WS 10/11
Kai Bürger – Computer Graphics and Visualization Group, TU München
computer graphics & visualization
Depth peeling – Reduce/Readback
How to determine the depth complexity?
– Render the scene without depth test and increment the
stencil bit (or entries in a texture render target using
additive blend) whenever a fragment is generated
– „Reduce“ the buffer to obtain the maximum
• In each pass, maximum of 4 texels is computed and stored into 1
texel of output texture
Reduce m x m region in fragment shader
1 pass: render quad that covers 1/4 pixels
2 pass: render quad that covers 1/16 pixels
…
Image Synthesis – WS 10/11
Kai Bürger – Computer Graphics and Visualization Group, TU München
computer graphics & visualization
Depth peeling - Queries
How to determine the depth complexity while peeling?
– Employ GPU occlusion query to obtain information
(i.e. ask GPU if fragments were rasterized into pixels in the last draw call)
– Pseudocode
bool bKeepPeeling = true
while(bKeepPeeling)
{
Initialize/Issue occlusion query (to the GPU)
Depth peel the next layer (as described in the following)
bKeepPeeling = GetQueryResult();
}
– Peels one “empty” layer, but avoids reduce/readback pass
– Application has to wait for query result after each peel pass
Image Synthesis – WS 10/11
Kai Bürger – Computer Graphics and Visualization Group, TU München
computer graphics & visualization
Depth peeling: Algorithm
Image Synthesis – WS 10/11
Kai Bürger – Computer Graphics and Visualization Group, TU München
computer graphics & visualization
Depth peeling issues
Image Synthesis – WS 10/11
Kai Bürger – Computer Graphics and Visualization Group, TU München
computer graphics & visualization
Depth peeling
Example
Image Synthesis – WS 10/11
Kai Bürger – Computer Graphics and Visualization Group, TU München
computer graphics & visualization
Depth peeling – back to front
• Back to front order:
– Use standard depth test (with op: greater)
– Discard fragments with depth greater or equal last pass
(initialize ping pong depth buffers with depth = 0)
– (optionally) test with less equal against depth buffer storing
opaque geometry (rendered before transparent objects)
– Blend result onto frame buffer
Pro: does not need to store RGBA values for each peel layer
Image Synthesis – WS 10/11
Kai Bürger – Computer Graphics and Visualization Group, TU München
computer graphics & visualization
Depth Peeling: Drawbacks
• As many rendering passes as
objects depth complexity
• Solution
– Multisampling render target
– Create up to 8 samples per fragment
• Each sample has individual stencil value
Stencil-routed k-buffer
(requires #depth layers / 8 rendering passes)
Ideal for complex geometry, much faster than depth peeling
Image Synthesis – WS 10/11
Kai Bürger – Computer Graphics and Visualization Group, TU München
computer graphics & visualization
Stencil Routed A-Buffer
• What we need:
– “A fragment list per pixel”
• MSAA (multi-sampled anti-aliasing)
– List of samples per pixel
– Samples store coverage
• Fragments are rasterized at
“higher frame buffer resolution”
– 8xMSAA = 8 x aliased resolution
• Pixel shader is run once per-pixel
• Frame buffer storage is at sample resolution
Image Synthesis – WS 10/11
Kai Bürger – Computer Graphics and Visualization Group, TU München
computer graphics & visualization
K-buffer
• But:
– Need to write only 1 sample at once
– Need all MSAA samples at pixel center
• MSAA sample patterns don’t help
• Solution:
– Turn off multisampling but still render
to MSAA render target
– Pixel shader output stored in all sub-samples
 now writing 8 samples per pixel
(but all samples contain the same value)
Image Synthesis – WS 10/11
Kai Bürger – Computer Graphics and Visualization Group, TU München
computer graphics & visualization
Stencil Routing and MSAA
7
6
5
4
5
4
3
2
9
8
7
6
4
3
2
1
2
1
0
3
2
1
0
6
5
4
3
8
7
6
5
• stencil always decrements (stencil pass and fail op = decrement)
• stencil passes when 2 (stencil reference value)
Image Synthesis – WS 10/11
Kai Bürger – Computer Graphics and Visualization Group, TU München
computer graphics & visualization
Stencil buffer initialization
• Clear stencil buffer to pass value (2)
• initializes sample 0 to 2
• Use sample mask to selectively set
other subsamples (to reference value)
5 samples filled
• Why start at 2 ?
• When all n subsamples are filled: (n-1) stencil
values will be 0 and one stencil value 1
• Overflow occured if all stencil values are 0 !
8 samples filled
overflow occured
Image Synthesis – WS 10/11
Kai Bürger – Computer Graphics and Visualization Group, TU München
computer graphics & visualization
K-buffer
• Now 8 samples per pixel
– If we need more:
multipass rendering (or render to texture array)
• Start with different initial stencil mask to skip first 8n
fragments
• Up to 254 layers (due to 8 bit stencil values)
• Sort these samples in depth order and blend
them
– Can be done in one fragment shader pass
Image Synthesis – WS 10/11
Kai Bürger – Computer Graphics and Visualization Group, TU München
computer graphics & visualization