Siggraph. Graphics Gems for Games

Graphics Gems for Games
Emil Persson
Senior Graphics Programmer
@_Humus_
Findings from Avalanche Studios

Graphics Gems for Games
Particle trimming
Merge-instancing
Phone-wire Anti-Aliasing
Second-Depth Anti-Aliasing

Graphics Gems for Games
Particle trimming

Particle trimming
GPUs increasingly more powerful
ALU – Through the roof
TEX – Pretty decent increase
BW – Kind of sluggish
ROP – Glacial speed
ROP bound?
Draw fewer pixels
???
Goto 1
Source: Wikipedia [1]

Particle trimming
Typical ROP bound cases
Particles
Clouds
Billboards
GUI elements
Solutions
Render to low-res render target
Abuse MSAA
Our solution
Trim particle polygon to reduce waste

Particle trimming
Common with large alpha=0 areas
Wasted fillrate
Adjust particle geometry to minimize waste
Automated tool [2]

Particle trimming
Huge fillrate savings
More vertices ⇒ Bigger saving
Diminishing returns
Just Cause 2 used 4 for clouds, 8 for particle effects

Particle trimming
First attempt: Manual trimming
Tedious, but proved the concept
OK for our cloud atlas
> 2x performance
Dozens of atlased particle textures
Automatic tool [3]
Input:
Texture and Alpha threshold
Vertex count
Output:
Optimized enclosing polygon

Particle trimming
Algorithm
Threshold alpha
Add each solid pixel to convex hull
Optimize with potential-corner test
Reduce hull vertex count
Replace least important edge
Repeat until max hull vertex count
Brute-force through all valid edge permutations
Select smallest area polygon

Particle trimming
Original texture

Particle trimming
Thresholded texture

Particle trimming
Convex hull

Particle trimming
Reduced convex hull

Particle trimming
Final 4 vertex polygon
(60.16%)

Particle trimming
Final 6 vertex polygon (53.94%)

Particle trimming
Final 8 vertex polygon (51.90%)

Particle trimming
Issues
Polygon extending outside original quad
No problem for regular textures. Use CLAMP.
May cut into neighboring atlas tiles …
Compute all hulls first, reject solutions that intersect another hull.
Revert to aligned rect if no valid solution remains
Performance
Brute-force
Keep convex hull vertex count reasonably low
Filtering
Add all four corners of a pixel (faster), or interpolate subpixel alpha values (accurate)
Handling ”weird” textures
I.e. alpha != 0 at texture edge

Graphics Gems for Games
Merge-Instancing

Merge-Instancing
Instancing
One mesh, multiple instances
Merging
Multiple meshes, one instance of each
What about: Multiple meshes, multiple instances of each?
Instancing: Multiple draw-calls
Merging: Duplication of vertex data
Merge-Instancing: One draw-call, no vertex duplication

Merge-Instancing

Merge-Instancing
Instancing
Merge-Instancing
for (int instance = 0; instance < instance_count; instance++)
for (int index = 0; index < index_count; index++)
VertexShader( VertexBuffer[IndexBuffer[index]], InstanceBuffer[instance] );
for (int vertex = 0; vertex < vertex_count; vertex++)
{
int instance = vertex / freq;
int instance_subindex = vertex % freq;
int indexbuffer_offset = InstanceBuffer[instance].IndexOffset;
int index = IndexBuffer[indexbuffer_offset + instance_subindex];
VertexShader( VertexBuffer[index], InstanceBuffer[instance] );
}

Merge-Instancing
Implemented in Just Cause 2 on Xbox360
Draw-calls less of a problem on PS3 / PC
Xbox360 HW lends itself to this approach
No hardware Input Assembly unit
Vertex shader does vertex fetching
Accessible through inline assembly

Merge-Instancing
Merging odd sized meshes
Choose common frequency
Duplicate instance data as needed
Pad with degenerate triangles as needed
Example
Mesh0: 39 vertices, Mesh1: 90 vertices
Choose frequency = 45
Pad Mesh0 with 2 degenerate triangles (6 vertices)
Instances[] = {
( Mesh0, InstanceData[0] ),
( Mesh1, InstanceData[1] ),
( Mesh1 + 45, InstanceData[1] ) }

Graphics Gems for Games
Phone-wire Anti-Aliasing

Phone-wire AA
Sources of aliasing
Geometric edges
Mostly solved by MSAA
Post-AA usually works too
Breaks down with thin geometry
Shading
Sort of solved by mipmapping
Poorly researched / understood
Few practical techniques for games
LEAN mapping [4]

Phone-wire AA
Sources of aliasing
Geometric edges
Mostly solved by MSAA
Post-AA usually works too
Breaks down with thin geometry
Shading
Poorly researched / understood
Few practical techniques for games
LEAN mapping

Phone-wire AA
Phone-wires
Common game content
Often sub-pixel sized
MSAA helps
… but not that much
Breaks at sub-sample size
Idea
Let’s not be sub-pixel sized!

Phone-wire AA
Phone-wires
Long cylinder shapes
Defined by center points, normal and radius
Avoid going sub-pixel
Clamp radius to half-pixel size
Fade with radius reduction ratio

Phone-wire AA
Demo + source available! [5]
// Compute view-space w
float w = dot(ViewProj[3], float4(In.Position.xyz, 1.0f));
// Compute what radius a pixel wide wire would have
float pixel_radius = w * PixelScale;
// Clamp radius to pixel size. Fade with reduction in radius vs original.
float radius = max(actual_radius, pixel_radius);
float fade = actual_radius / radius;
// Compute final position
float3 position = In.Position + radius * normalize(In.Normal);

Phone-wire AA off, MSAA 4x

Phone-wire AA on, MSAA 4x

Graphics Gems for Games
Second-Depth Anti-Aliasing

Second-Depth Anti-Aliasing
Filtering AA approaches
SIGGRAPH 2011 - ”Filtering Approaches for Real-Time Anti-Aliasing” [6]
Post-AA
MLAA
SMAA
FXAA
DLAA
Analytical approaches
GPAA
GBAA
DEAA
SDAA [7]

Second-Depth Anti-Aliasing
Depth buffer and second-depth buffer
Depth is linear in screen-space
Simplifies edge detection
Enables prediction of original geometry
Two types of edges
Creases
Silhouettes
Silhouettes require second-depth buffer
Do pre-z pass with front-face culling
Alternatively, output depth to render target for back-facing geometry

Second-Depth Anti-Aliasing
Attempt crease case first
Look at depth slopes
Compute intersection point
Valid if distance < one pixel
Used if distance < half pixel
If invalid, try silhouette

Second-Depth Anti-Aliasing
Try as silhouette
Neighbor depths useless
Look at second-depths
Compute intersection point
Used if distance < half pixel

Second-Depth Anti-Aliasing
Results
Demo + source available! [7]

References
[1] http://en.wikipedia.org/wiki/Comparison_of_AMD_graphics_processing_units
[2] http://www.humus.name/index.php?page=News&ID=266
[3] http://www.humus.name/index.php?page=Cool&ID=8
[4] http://www.csee.umbc.edu/~olano/papers/lean/
[5] http://www.humus.name/index.php?page=3D&ID=89
[6] http://iryoku.com/aacourse/
[7] http://www.humus.name/index.php?page=3D&ID=88