9/25/2001 CS 638, Fall 2001

Today

• Shadow Volume Algorithms• Vertex and Pixel Shaders

9/25/2001 CS 638, Fall 2001

Shadow Volumes

• A shadow volume for an object and light is the volume of space that is shadowed– That is, all points in the volume are in shadow for that

light/object pair

• Creating the volume:– Find silhouette edges of shadowing object as seen by the

light source– Extrude these edges away from the light, forming

polygons– Clip the polygons to the view volume

9/25/2001 CS 638, Fall 2001

Shadow Volume

9/25/2001 CS 638, Fall 2001

Practicalities

• Silhouette edges can be found by looking at polygon normals– Silhouette edges are those between a front facing face and back

facing face (from light’s point of view)– The result is a sequence of edges with common vertices

• Assuming convex shadowing objects

• Extend all the vertices of silhouette edges away from the light source

• Clip them to the view volume, and form the polygons that bound the shadow volume

• Final result are a set of shadow volume boundary polygons

9/25/2001 CS 638, Fall 2001

Key Observation

• All points inside a shadow volume are in shadow• Along a ray from the eye, we can track the shadow state by

looking at intersections with shadow volume boundaries– Assume the eye is not in shadow– Each time the ray crosses a front facing shadow polygon, add one to

a counter– Each time the ray crosses a back facing shadow polygon, subtract

one from a counter– Places where the counter is zero are lit, others are shadowed

• We need to count the number of shadow polygons in front of a point. Which buffer can count for us?

9/25/2001 CS 638, Fall 2001

Using Shadow Volumes

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9/25/2001 CS 638, Fall 2001

Real-Time Shadow Volumes

• Compute shadow volumes per frame– Not a problem for only a few moving objects

– Use simplified object geometry to speed things up

• Four pass algorithm– Render scene with ambient light only (everything shadowed)

– Render front facing shadow polygons to increment stencil buffer

– Render back facing shadow polygons to decrement stencil buffer

– Render scene again only for non-shadowed (stencil=0) regions

• Horrible details follow…

9/25/2001 CS 638, Fall 2001

Details

• Turn off any light sources and render scene with ambient light only, depth on, color buffer active

• Disable the color and depth buffers for writing, but leave the depth test active

• Initialize the stencil buffer to 0 or 1 depending on whether the viewer is in shadow or not (ray cast)

• Set the stencil test to always pass and the operation to increment if depth test passes

• Enable back face culling

• Render the shadow volumes - this will increment the stencil for every front facing polygon that is in front of a visible surface

• Cont…

9/25/2001 CS 638, Fall 2001

Details

• Enable front face culling, disable back face culling

• Set the stencil operation to decrement if the depth test passes (and leave the test as always pass)

• Render the shadow volumes - this decrements for all the back facing shadow polygons that are in front of visible objects. The stencil buffer now has positive values for placing in shadow

• Set the stencil function to equality with 0, operations to keep

• Clear the depth buffer, and enable it and the color buffer for writing

• Render the scene with the lights turned on

• Voila, we’re done

9/25/2001 CS 638, Fall 2001

Variant

• Render fully lit, then add shadows (blend a dark polygon) where the stencil buffer is set– http://www.gamasutra.com/features/19991115/bestimt_freitag_01.htm

9/25/2001 CS 638, Fall 2001

Why Use Shadow Volumes?

• They correctly account for shadows of all surfaces on all other surfaces– No shadow where there shouldn’t be shadow– No problem with multiple light sources

• Adaptable quality by creating shadow volumes with approximate geometry

• Shadow volumes for static object/light pairs can be pre-computed and don’t change

• More expensive than light maps, but not too bad on current hardware– Can’t combine the techniques. Why not?

9/25/2001 CS 638, Fall 2001

Other Shadow Algorithms

• There are other algorithms suitable for non-real-time situations, or requiring special hardware

• Two interesting cases:– Derive shadow polygons from light sources

• Compute view from light position

• Transform visible polygons into world space

• Merge them with existing polygons to indicate non-shadowed polygons

– Shadow z-buffer• Compute z-buffer from light viewpoint

• Render normal view, compare world locations of points in the z-buffer and shadow z-buffer – points with same location are lit

9/25/2001 CS 638, Fall 2001

Vertex and Pixel Shaders

• In traditional graphics, a shader is a program that computes something about each vertex or pixel– Renderman shading language, for instance, computes the color of

each pixel with a program

• Trends in graphics APIs and hardware are bringing shaders into real-time contexts– DirectX 8.0– Nvidia GeForce III– Nvidia OpenGL extensions

• http://developer.nvidia.com/ for info

9/25/2001 CS 638, Fall 2001

Figure 1: A vertex shader's position in the DX8 rendering pipeline.

Modified Pipeline (DirectX 8.0)

9/25/2001 CS 638, Fall 2001

Vertex Shaders

• Vertex shaders are implemented in hardware in new generation cards

• They get as input the (x,y,z) vertex location, its texture coordinates (s,t), its color, normal, etc

• They have access to some registers– NOT retained from one vertex to the next

• They have access to some constant memory– Programmer specifies what’s in that memory

• They run a program– A sequence of instructions that compute on the available data

9/25/2001 CS 638, Fall 2001

IO for Vertex Shaders

Figure 2: The inputs and outputs of vertex shaders. Arrows indicate read-only, write-only, or read-write.

9/25/2001 CS 638, Fall 2001

Vertex Program Ops

• All operations work on vectors– Scalars are stored as vectors with the same value in each coordinate

• Nvidia hardware provides an instruction set with 17 operations, including, but not limited to:– Add ADD, Multiply (by scalar, and vector element-wise), Multiply

and add MAD, Reciprocal square root RSQ, Dot product DP3, …– LIT which implements the Phong lighting model in one instruction– Can re-arrange (swizzle) and negate vectors before doing op

• Matrices can be automatically mapped into registers• No branches, but can be done with other instructions

– Set a value to 0/1 based on a comparison, then multiply

9/25/2001 CS 638, Fall 2001

Vertex Programming Example

• Morph between a cube and sphere while doing lighting with a directional light source (gray output)

• Cube position and normal in attributes (input) 0,1• Sphere position and normal in attributes 2,3• Blend factor in attribute 15• Inverse transpose modelview matrix in constants 12-14

– Used to transform normal vectors into eye space

• Composite matrix is in 4-7– Used to convert from object to homogeneous screen space

• Light dir in 20, half-angle vector in 22, specular power, ambient, diffuse and specular coefficients all in 21

9/25/2001 CS 638, Fall 2001

Vertex Program Example

• # blend normal and position v=v1+(1-)v2 MOV R3, v[3] ; MOV R5, v[2] ; ADD R8, v[1], -R3 ; ADD R6, v[0], -R5 ; MAD R8, v[15].x, R8, R3 MAD R6, v[15].x, R6, R5 ;

• # transform normal to eye space DP3 R9.x, R8, c[12] ; DP3 R9.y, R8, c[13] ; DP3 R9.z, R8, c[14] ;

• # transform position and output DP4 o[HPOS].x, R6, c[4] ; DP4 o[HPOS].y, R6, c[5] ; DP4 o[HPOS].z, R6, c[6] ; DP4 o[HPOS].w, R6, c[7] ;

• # normalize normal DP3 R9.w, R9, R9 ; RSQ R9.w, R9.w ; MUL R9, R9.w, R9 ;

• # apply lighting and output color DP3 R0.x, R9, c[20] ; DP3 R0.y, R9, c[22] ; MOV R0.zw, c[21] ; LIT R1, R0 ; DP3 o[COL0], c[21], R1 ;

9/25/2001 CS 638, Fall 2001

Shading Languages

• Programming shading hardware is still a difficult process– Akin to writing assembly language programs

• Shading languages and accompanying compilers allow user to write shaders in high level languages– For example, take the Renderman shading language and

compile it for the Nvidia hardware

– Proudfoot et. al. in SIGGRAPH 2001

9/25/2001 CS 638, Fall 2001

Pixel Shaders

• Pixel shaders operate on fragments in place of the texturing hardware– After rasterization, before any fragment tests or blending

• Compute RGBA values for the fragment:– Fragment color, textures, fog color as inputs in registers– Temporary storage in registers

• Each holds RGBA

– A series of combiners that take the values in registers and perform operations on them

• Referred to as a register combiner architecture• Supported by DirectX 8.0 and OpenGL Extensions

9/25/2001 CS 638, Fall 2001

Overall Architecture

http://developer.nvidia.com/docs/IO/1382/ATT/RegisterCombiners.ppt

The pictures I used in lecture are copyright NvidiaDownload the slides from the link below

Slide 7 from Nvidia

9/25/2001 CS 638, Fall 2001

General Register Combiner

Slide 9 from Nvidia

9/25/2001 CS 638, Fall 2001

General Combiner Operations

Dot is dot productMult is element-wise multiply

Slide 16 from Nvidia

9/25/2001 CS 638, Fall 2001

Final Combiner

RGBOut

Slide 24 from Nvidia

9/25/2001 CS 638, Fall 2001

Multitexture Light Map with Fog

• Diffuse color CD provided per vertex

• Texture0 is diffuse map TD, Texture1 is light map TL

• CF is fog color, AF is fog factor

• Computes AF * CD * TD * TL + (1 – AF) * CF

• Uses two textures, but only one general combiner

• Advantages: performance (single pass, no blending)

• Disadvantages: none

9/25/2001 CS 638, Fall 2001

General Combiner Setup

Slide 40 from Nvidia

9/25/2001 CS 638, Fall 2001

Final Combiner Setup

Slide 41 from Nvidia

9/25/2001 CS 638, Fall 2001

Other Examples

• There are other example uses at the Nvidia web site, including:– Two ways of doing bump mapping with register

combiners

– Shadow volume construction with vertex shaders