Engine Basics:
Optimizing Geometry
CS 446: Real-Time Rendering
& Game Technology
David Luebke
University of Virginia
Demo Time
• SIGN UP SHEET
• Today: Fight Night videos
Real-Time Rendering
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Homework
• The pitch: “due” Tuesday, keep pitching
• The game: due today at midnight
Real-Time Rendering
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Recap: Scene Graph Node Examples
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Objects
Groups of objects
LOD selectors
Animation “effectors”
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• Transforms
• Useful state
– Bounding boxes, partitions
• Graphics state
– Which textures, shaders bound
– Material
– Other state - glEnable(GL_FOO)
“Flipbook” animation selectors
Transform drivers
Deformers
Skeletal animation players
• “Raw” geometry
– Vertices and triangles
• Lights!
• Cameras!
Note: often actual geometry only exists at leaves
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Scene Graph Traverals
• Classical use of scene graph:
– Traverse in depth-first fashion
– Apply effects of “action” nodes when reached
• LOD selectors, animation effectors
– Push/pop matrices when reach/leave xform nodes
– Issue geometry as reached
• Not very efficient for complex scenes (why?)
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Unnecessary pushing/popping
Constantly switching graphics state
No spatial organization
Lots of recalculation for multiple passes
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Scene Graph Traversals
• Can optimize traversal logic several ways, e.g.:
– Cache per-frame computations: bbox, matrices
– Flatten hierarchy for unchanging subgraphs
– Call animation effectors first & update bboxes bottom-up
• Visibility computations affect traversal
– View-frustum culling: top-down, separate visibility grid
– Cells & portals: portals can be special arcs
– Other: BSP trees, 2.5D from-region, fog-of-war
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Scene Graph Traversals
• Common traversals:
– Simple depth-first. Optional:
• View-frustum culling or other visibility scheme
• Different LOD biases
• Cache bbox/visibility/matrix info for later passes
– Rough front-to-back sort
– Sorted by state
– Geometry-only
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Efficient Rendering
• So leaves of the scene graph represent geometry…
• How to efficiently represent that geometry?
– Hint: not like this:
typedef float Point[3];
typedef Point Triangle[3];
Triangle tris[] = new Triangle[ntris];
...
glBegin(GL_TRIANGLES);
for (i=0;i<nverts;i++) {
glVertex3fv(tris[i][0]);
glVertex3fv(tris[i][1]);
glVertex3fv(tris[i][2]);
}
glEnd();
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David
5 Steps to Efficient Rendering
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Step 1: reduce #vertices/triangle
Step 2: reduce overhead of each vertex
Step 3: recognize the existence of the vertex cache
Step 4: display lists (deprecated re: performance)
Step 5: learn about the hardware
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David
Other things you should know
• Most interesting stuff in OpenGL isn’t in red book
– Extension registry
• http://oss.sgi.com/projects/ogl-sample/registry/
• Extensions you should know:
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Real-Time Rendering
ARB_multitexture
EXT_framebuffer_object
ARB_occlusion_query
ARB_point_sprite
ARB_texture_rectangle
ARB_draw_buffers
ARB_texture_float (_framebuffer_float, _half_float_pixel, etc)
ARB_depth_texture
ARB_vertex_buffer_object (_pixel_buffer_object)
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Coming Up
• Next topic: programmable graphics hardware
– We’ll use NVIDIA’s Cg language because I know it
– Similar alternatives:
• Microsoft’s HLSL for DirectX
• OpenGL Shading Language (GLSL, once Glslang)
– Different approach: Sh
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David