Projective Textures and Shadow Maps Prof. Aaron Lanterman School of Electrical and Computer Engineering Georgia Institute of Technology2 What is projective texturing? • An intuition for projective texturing – The slide projector analogy From Stanford CS448A: Real-Time Graphics Architectures lecture 11; see graphics.stanford.edu/courses/cs448a-01-fall3 “Slide projector” in different locations Images from C. Everitt, “Projective Texture Mapping,” developer.nvidia.com/object/Projective_Texture_Mapping.html4 Texture matrix € strq⎡ ⎣ ⎢ ⎢ ⎢ ⎢ ⎤ ⎦ ⎥ ⎥ ⎥ ⎥ =120 0120120120 012120 0 0 1⎡ ⎣ ⎢ ⎢ ⎢ ⎢ ⎢ ⎢ ⎢ ⎤ ⎦ ⎥ ⎥ ⎥ ⎥ ⎥ ⎥ ⎥ LightFrustrum( projection)Matrix⎡ ⎣ ⎢ ⎢ ⎢ ⎢ ⎤ ⎦ ⎥ ⎥ ⎥ ⎥ LightView(lookat)Matrix⎡ ⎣ ⎢ ⎢ ⎢ ⎢ ⎤ ⎦ ⎥ ⎥ ⎥ ⎥ ModelingMatrix⎡ ⎣ ⎢ ⎤ ⎦ ⎥ x0y0z0w0⎡ ⎣ ⎢ ⎢ ⎢ ⎢ ⎤ ⎦ ⎥ ⎥ ⎥ ⎥ From “The Cg Tutorial,” p. 252.5 Projective texturing vertex shader void C9E4v_projTexturing(float4 position : POSITION, float3 normal : NORMAL, out float4 oPosition : POSITION, out float4 texCoordProj : TEXCOORD0, out float4 diffuseLighting : TEXCOORD1, uniform float Kd, uniform float4x4 modelViewProj, uniform float3 lightPosition, uniform float4x4 textureMatrix) { oPosition = mul(modelViewProj, position); // Compute texture coordinates for // querying the projective texture texCoordProj = mul(textureMatrix, position); // Compute diffuse lighting float3 N = normalize(normal); float3 L = normalize(lightPosition - position.xyz); diffuseLighting = Kd * max(dot(L, N), 0); } From “The Cg Tutorial”6 Projective texturing pixel shader void C9E5f_projTexturing (float4 texCoordProj : TEXCOORD0, float4 diffuseLighting : TEXCOORD1, out float4 color : COLOR, uniform sampler2D projectiveMap) { // Fetch color from the projective texture float4 projColor = tex2Dproj(projectiveMap, texCoordProj); color = projColor * diffuseLighting; } From “The Cg Tutorial”7 Watch out for reverse projection! Images from C. Everitt, “Projective Texture Mapping,” developer.nvidia.com/object/Projective_Texture_Mapping.html8 A dramatic shadow in 2K Games’ BioShock From www.wired.com/gaming/gamingreviews/multimedia/2007/08/pl_bioshock?slide=16&slideView=69 The shadow mapping concept (1) • Depth testing from the light’s point-of-view – Two pass algorithm • First, render depth buffer from the light’s point-of-view – the result is a “depth map” or “shadow map” – essentially a 2D function indicating the depth of the closest pixels to the light – This depth map is used in the second pass Slide from C. Everitt, “Shadow Mapping,” Powerpoint presentation, developer.nvidia.com/attach/639210 The shadow mapping concept (2) • Shadow determination with the depth map – Second, render scene from the eye’s point-of-view – For each rasterized fragment • determine fragment’s XYZ position relative to the light • this light position should be setup to match the frustum used to create the depth map • compare the depth value at light position XY in the depth map to fragment’s light position Z Slide from C. Everitt, “Shadow Mapping,” Powerpoint presentation, developer.nvidia.com/attach/639211 The shadow mapping concept (3) • The Shadow Map Comparison – Two values • A = Z value from depth map at fragment’s light XY position • B = Z value of fragment’s XYZ light position – If B is greater than A, then there must be something closer to the light than the fragment • then the fragment is shadowed – If A and B are approximately equal, the fragment is lit Slide from C. Everitt, “Shadow Mapping,” Powerpoint presentation, developer.nvidia.com/attach/639212 Shadow mapping with a picture in 2D (1) light source eye position depth map Z = A fragment’s light Z = B depth map image plane eye view image plane, a.k.a. the frame buffer The A < B shadowed fragment case Slide from C. Everitt, “Shadow Mapping,” Powerpoint presentation, developer.nvidia.com/attach/639213 Shadow mapping with a picture in 2D (2) light source eye position depth map Z = A fragment’s light Z = B depth map image plane eye view image plane, a.k.a. the frame buffer The A ≅ B unshadowed fragment case Slide from C. Everitt, “Shadow Mapping,” Powerpoint presentation, developer.nvidia.com/attach/639214 Shadow mapping with a picture in 2D (3) Note image precision mismatch! Slide from C. Everitt, “Shadow Mapping,” Powerpoint presentation, developer.nvidia.com/attach/639215 Visualizing the shadow mapping technique (1) • A fairly complex scene with shadows Slide from C. Everitt, “Shadow Mapping,” Powerpoint presentation, developer.nvidia.com/attach/639216 • Compare with and without shadows Slide from C. Everitt, “Shadow Mapping,” Powerpoint presentation, developer.nvidia.com/attach/6392 Visualizing the shadow mapping technique (2)17 • The scene from the light’s point-of-view Slide from C. Everitt, “Shadow Mapping,” Powerpoint presentation, developer.nvidia.com/attach/6392 Visualizing the shadow mapping technique (3)18 • The depth buffer from the light’s point-of-view Slide from C. Everitt, “Shadow Mapping,” Powerpoint presentation, developer.nvidia.com/attach/6392 Visualizing the shadow mapping technique (4)19 • Projecting the depth map onto the eye’s view Slide from C. Everitt, “Shadow Mapping,” Powerpoint presentation, developer.nvidia.com/attach/6392 Visualizing the shadow mapping technique (5)20 • Projecting light’s planar distance onto eye’s view Slide from C. Everitt, “Shadow Mapping,” Powerpoint presentation, developer.nvidia.com/attach/6392 Visualizing the shadow mapping technique (6)21 • Comparing light distance to light depth map Slide from C. Everitt, “Shadow Mapping,” Powerpoint presentation, developer.nvidia.com/attach/6392 Visualizing the shadow mapping technique (7)22 • Scene with shadows Slide from C. Everitt, “Shadow Mapping,” Powerpoint presentation, developer.nvidia.com/attach/6392 Visualizing the
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