nVIDIA Shadow Mapping - Lecture Slides | CS 5610, Exams of Computer Science

Download nVIDIA Shadow Mapping - Lecture Slides | CS 5610 and more Exams Computer Science in PDF only on Docsity! 1 Cass Everitt NVIDIA Corporation cass@nvidia.com Shadow Mapping 2 Render Scene and Access the Depth Texture • Realizing the theory in practice • Fragment’s light position can be generated using eye-linear texture coordinate generation • specifically OpenGL’s GL_EYE_LINEAR texgen • generate homogenous (s, t, r, q) texture coordinates as light-space (x, y, z, w) • T&L engines such as GeForce accelerate texgen! • relies on projective texturing 3 What is Projective Texturing? • An intuition for projective texturing • The slide projector analogy Source: Wolfgang Heidrich [99] 4 About Projective Texturing (1) • First, what is perspective-correct texturing? • Normal 2D texture mapping uses (s, t) coordinates • 2D perspective-correct texture mapping • means (s, t) should be interpolated linearly in eye- space • so compute per-vertex s/w, t/w, and 1/w • linearly interpolate these three parameters over polygon • per-fragment compute s’ = (s/w) / (1/w) and t’ = (t/w) / (1/w) • results in per-fragment perspective correct (s’, t’) 5 About Projective Texturing (2) • So what is projective texturing? • Now consider homogeneous texture coordinates • (s, t, r, q) --> (s/q, t/q, r/q) • Similar to homogeneous clip coordinates where (x, y, z, w) = (x/w, y/w, z/w) • Idea is to have (s/q, t/q, r/q) be projected per- fragment • This requires a per-fragment divider • yikes, dividers in hardware are fairly expensive 6 About Projective Texturing (3) • Hardware designer’s view of texturing • Perspective-correct texturing is a practical requirement • otherwise, textures “swim” • perspective-correct texturing already requires the hardware expense of a per-fragment divider • Clever idea [Segal, et.al. ‘92] • interpolate q/w instead of simply 1/w • so projective texturing is practically free if you already do perspective-correct texturing! 2 7 About Projective Texturing (4) • Tricking hardware into doing projective textures • By interpolating q/w, hardware computes per- fragment • (s/w) / (q/w) = s/q • (t/w) / (q/w) = t/q • Net result: projective texturing • OpenGL specifies projective texturing • only overhead is multiplying 1/w by q • but this is per-vertex 8 Back to the Shadow Mapping Discussion . . . • Assign light-space texture coordinates via texgen • Transform eye-space (x, y, z, w) coordinates to the light’s view frustum (match how the light’s depth map is generated) • Further transform these coordinates to map directly into the light view’s depth map • Expressible as a projective transform • load this transform into the 4 eye linear plane equations for S, T, and Q coordinates • (s/q, t/q) will map to light’s depth map texture 9 Shadow Map Operation • Automatic depth map lookups • After the eye linear texgen with the proper transform loaded • (s/q, t/q) is the fragment’s corresponding location within the light’s depth texture • r/q is the Z planar distance of the fragment relative to the light’s frustum, scaled and biased to [0,1] range • Next compare texture value at (s/q, t/q) to value r/q • if texture[s/q, t/q] ≅ r/q then not shadowed • if texture[s/q, t/q] < r/q then shadowed 10 Dedicated Hardware Shadow Mapping Support • SGI RealityEngine, InfiniteReality, and GeForce3 Hardware • Performs the shadow test as a texture filtering operation • looks up texel at (s/q, t/q) in a 2D texture • compares lookup value to r/q • if texel is greater than or equal to r/q, then generate 1.0 • if texel is less than r/q, then generate 0.0 • Modulate color with result • zero if fragment is shadowed or unchanged color if not 11 OpenGL Extensions for Shadow Map Hardware (See Page 445) • Two extensions work together • SGIX_depth_texture (see page 445) • supports high-precision depth texture formats • copy from depth buffer to texture memory supported • SGIX_shadow (see page 445) • adds “shadow comparison” texture filtering mode • compares r/q to texel value at (s/q, t/q) • Multi-vendor support: SGI, NVIDIA, others? • Brian Paul has implemented these extensions in Mesa! 12 New Depth Texture Internal Texture Formats • SGIX_depth_texture supports textures containing depth values for shadow mapping • Three new internal formats • GL_DEPTH_COMPONENT16_SGIX • GL_DEPTH_COMPONENT24_SGIX • GL_DEPTH_COMPONENT32_SGIX (same as 24-bit on GeForce3) • Use GL_DEPTH_COMPONENT for your external format • Work with glCopySubTexImage2D for fast copies from depth buffer to texture • NVIDIA optimizes these copy texture paths