Emerging Technologies for Games
Alpha Sorting and “Soft” Particles
CO3303
Week 15
Today’s Lecture
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Alpha Sorting Problems
Run-time Depth Sorting
Hard Flat Particles
Depth Particles
Soft Particles
Depth-Soft Particles
Further Possibilities
Alpha Sorting Problems
• Saw alpha blending in the second year
– A very attractive blending technique
• However, it causes sorting issues
– Without resolving these, method is not very useful
• Problem is depth buffer
ignores transparency
– Transparent pixels drawn in
nearby polygons “obscure”
distant polygons drawn later
• Avoid problem by drawing
polygons from back to front
Run-time Depth Sorting 1
• One solution is to sort all
alpha blended polys in
back-to-front order
– Every frame (can be slow)
• If all polygons face camera
(e.g. a particle system):
– Sort polygons based on
camera-space z distance
– Camera space z =
CP●ZC
CP is vector from camera to poly,
ZC is camera z-axis
Run-time Depth Sorting 2
• How to sort polys that face in any direction?
• Might think to sort on
camera-space z again
– But distance to which point on
the polygon?
– Nearest?
– Furthest?
– Average?
• Will any of these work?
• No, a more complex
approach is needed
Run-time Depth Sorting 3
• First, assume that polygons don’t intersect
• Then, given two polygons, one of them will be entirely
on one side of the plane of the other
• Identify this polygon, and
see if it is on the side nearer
the camera or not
• First step is to get a face
normal for each polygon
– Reverse normal if it faces
away from the camera
– Do this prior to sorting
Run-time Depth Sorting 4
• Join either point of polygon 2 to
each of the points of polygon 1
• Calculate dot products of these
vectors with normal of polygon 2:
– Results all +ve: poly 1 is nearer
– Results all -ve: poly 1 is further
– Results mixed: poly 1 is split by
plane of poly 2. So repeat test the
other way round (2nd diagram)
– If split both ways, then the polygons
are intersecting
Run-time Sorting Practicalities
• Must ensure this sorting is efficient as possible
– Sort pointers to polygons not polygon data itself (less to sort)
– Retain previous sorting and use a sort algorithm that is good with
almost sorted data (e.g. insertion sort)
– Don’t sort every frame, only every two or three
– Run the sorting concurrently
– Reduce density of particle systems based on distance
– Use partitions to help:
• Sort partitions from back to from, then particles in one partition at a
time – reducing number polygons in each sort
• In practice, another technique (not covered here),
alpha-to-coverage is often used as an alternative
Hard Flat Particles
• Alpha blending is as useful as other blending methods
once the polygons are sorted
– Note: just like other blending (additive, multiplicative etc.), we
need to render all the opaque polygons first
• However, all blending
methods exhibit hard
edges if they intersect
other polygons
– Makes dense particle
systems look poor
– Particularly large particles
like smoke (indoors is
particularly bad)
Depth Particles
• A simple improvement is to give alpha blended particles
some depth
– Simulated bumpiness, like normal / parallax mapping
– Store it in the alpha channel of the texture
• Adjust the depth used for
the depth-buffer by this
value
– Improves hard edges by
making them bumpy
• Must hand calculate depth
value in shader
– Not done this before
Soft Particles
• To improve further, note that visual problems mainly
occur when a particle intersects with an opaque object
• We know the opaque objects have already been drawn…
– So they are already in
depth buffer
• Can compare depth of
particle with depth
already in buffer
• Fade pixels out when the
difference is small
– Adjust alpha towards 0
Depth-Soft Particles
• This method can be combined with the depth particles idea
presented earlier
– Or can just be used on flat particles
• We must do some detailed work with depth buffer
– Rather unfamiliar
• But almost completely
removes hard edges where
alpha particles intersect
solid objects
– Can work with other blending
modes too
Further Possibilities
• The SoftParticles DirectX10 sample in the DirectX SDK
takes this idea a little further
– Explores volumetric particles - consider the volume of particle that
camera is looking through