Shadows

1. Shadows David Luebke University of Virginia

2. More On Textures • Multitexturing • Modern hardware can read from multiple textures at once, even with mipmapping • Detail texturing • Light mapping • Bump mapping • Normal maps versus bump maps • Can have detail bump maps • Environment mapping

3. Shadows • An important visual cue, traditionally hard to do in real-time rendering • Outline: • Notation • Planar shadows • Soft shadows • Projective shadows • Shadow volumes • Shadow maps • Shadow optimizations

4. Notation • Light source • Point vs area • Occluders & receivers • Identify ahead of time? • Self-shadowing? • Shadow • Umbra • Penumbra • Soft vs hard shadows

5. Planar Shadows • Old trick: project the occluder geometry to a plane and render over ground plane • Can do with a matrix • Z-bias issues • Semiopaque shadows harder • Stencil and Z-buffer tricks • Another option: generate textured rectangle • Problems • Light source inside object (Antishadows) • Only planar receivers  no self-shadowing

6. Planar Soft Shadows • Basic idea: • Sample light source multiple times, average results (accumulation buffer) into a texture • Gooch et al: move plane up and down • Nested shadows fewer passes

7. Projective Shadows • Render scene from light’s point of view • Render all occluders as black • Can turn off depth buffer etc • Project onto receiver polygons using projective texture mapping • Works for curved surfaces • Designer separates occluders and receivers  no self-shadowing

8. Shadow Volumes • Basic idea: • Create polygonal objects to represent shadowed volumes • Make clever use of stencil buffer so that these objects affect what lighting is done

9. Shadow Volumes • Details of the basic algorithm: • Compute shadow volumes • View-independent! • Clear stencil buffer • Render the scene without diffuse/spec lighting • “Render” front faces of shadow volumes • Turn off color, depth updates (but leave depth test on) • Visible polygons increment pixel stencil buffer count • “Render” back faces of shadow volumes • Turn off color, depth updates (but leave depth test on) • Visible polygons decrement pixel stencil buffer count • Render scene with only diffuse/spec lighting • Only update pixels where stencil = 0

10. Shadow Volumes • Refinements (see book) • NV30, XBox supports signed stencil addition • Two-sided lighting determines whether polygon adds or subtracts 1 from stencil buffer • One-pass algorithm! But a little slower in practice? • What if you’re inside a shadow volume? • Invert meaning of stencil test • What if near clip intersects shadow plane? • Carmack, others: use z-fail test • Clever extensions in NV2X help this idea out • Creating shadow volumes: vertex program! • ATI: clever degenerate-edge trick again

11. Shadow Volumes • Advantages: • Robust • Self-shadowing • GPU • Disadvantages: • Geometry limited • Stencil polys • Multi-pass scene geometry • Stencil test – per pixel expense • Hard shadows

12. Shadow Maps • Idea: • Render scene from light source, read Z-buffer • Result: discretized image (shadow map) telling distance of objects to light source • Render scene normally • At each pixel, calculate distance D to light • Compare to distance S stored in shadow map • If D=S, surface lit by light, else in shadow

13. Shadow Maps • The basic algorithm • Render scene with ambient lighting only • Read Z-buffer, transform values into coordinate system of light • Use comparison to set alpha buffer • Render w/ diffuse and specular components, multiplying by alpha

14. Shadow Maps • Advantages: • Hardware-accelerated general shadow algorithm • Supports self-shadowing • Cost is linear in # lights and # polygons • Disadvantages: • Self-shadow aliasing • Biasing and other techniques can help, but not fix • Shadow map resolution critical! • Solution: perspective shadow maps