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Shadow Mapping

Shadow Mapping. Chun-Fa Chang National Taiwan Normal University. Advanced Texture Mapping. Using multiple textures Multi-Pass textures 1 st Pass: render the scenes as usual Create textures from the output images 2 nd Pass: render the scenes again using the created texture. Shadow Map.

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Shadow Mapping

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  1. Shadow Mapping Chun-Fa Chang National Taiwan Normal University

  2. Advanced Texture Mapping • Using multiple textures • Multi-Pass textures • 1st Pass: render the scenes as usual • Create textures from the output images • 2nd Pass: render the scenes again using the created texture

  3. Shadow Map • Using two textures: color and depth • Relatively straightforward design using pixel (fragment) shaders on GPUs.

  4. Eye’s View Light’s View Depth/Shadow Map Image Source: Cass Everitt et al., “Hardware Shadow Mapping” NVIDIA SDK White Paper

  5. Basic Steps of Shadow Maps • Render the scene from the light’s point of view, • Use the light’s depth buffer as a texture (shadow map), • Projectively texture the shadow map onto the scene,  Use “TexGen” or shader • Use “texture color” (comparison result) in fragment shading.

  6. What’re in the Example Code? • A C++ class for storing matrix state: class OpenGL_Matrix_State { void Save_Matrix_State(); void Restore_Matrix_State(); void Set_Texture_Matrix(); } • A proxy rectangle for debug

  7. (1) Rendering from Light’s View • Set the camera to the light position. • Viewport set to the same size as the texture. • To avoid the floating point precision problem (casting its own shadow to a surface) , depth must be shifted: • glPolygonOffset(..., ...); • glEnable(GL_POLYGON_OFFSET_FILL); • Shading could be turned off • We only care about the depth!

  8. (2) Creation of Shadow Map (Texture) • Draw the objects (from light’s view) • To create a depth texture, use: • glTexImage2D( GL_TEXTURE_2D, 0, GL_DEPTH_COMPONENT, shadowMapSize, shadowMapSize, 0, GL_DEPTH_COMPONENT, GL_UNSIGNED_BYTE, 0); • Then use glCopyTexSubImage2D()to copy the frame buffer to the depth texture.

  9. (3) Generation of Texture Coordinates • When we render the scene again from the normal camera view: • We store the light’s view to the texture matrix. • The texture matrix is then passed to the GLSL shaders. • gl_TextureMatrix[0] * vertex gives us the homogeneous coordinates in light space • Divide by w to obtain the texture coordinates. • Watch out! Must shift from [-1, 1] to [0,1]

  10. Normalized Coordinates • Independent of the screen resolution or window size. • Clip coordinates: after Model-View and Projection transformation. • Normalized Device Coordinates (NDC): after division by w.

  11. OpenGL Transformations • Detailed definitions and equation derivations at: http://www.songho.ca/opengl/gl_transform.html

  12. (4) Depth Comparison in Fragment Shader • Compare two depths: • Depth read from the shadow map • Depth by transformation to the light space • In the shadow if ____?_(your exercise)____ • Set a darker color for shadowed surfaces

  13. More GPU Programming and GPGPU Chun-Fa Chang National Taiwan Normal University

  14. Calculator vs. Computer • What is the difference between a calculator and a computer? • Doesn’t a compute-r just “compute”? • The Casio fx3600p calculated can be programmed (38 steps allowed).

  15. Turing Machine • Can be adapted to simulates the logic of any computer that could possibly be constructed. • von Neumann architecture implements a universal Turing machine. • Look them up at Wikipedia!

  16. Simplified View • The Data Flow: 3D Polygons (+Colors, Lights, Normals, Texture Coordinates…etc.) • 2D Polygons • 2D Pixels (I.e., Output Images) Transform (& Lighting) Rasterization

  17. Global Effects shadow multiple reflection translucent surface

  18. Local vs. Global

  19. How Does GPU Draw This?

  20. Quiz • Q1: A straightforward GPU pipeline give us local illumination only. Why? • Q2: What typical effects are missing? Hint: How is an object drawn? Do they consider the relationship with other objects? Shadow, reflection, and refraction…

  21. With Shadows Without Shadows • Wait but I’ve seen shadow and reflection in games before…

  22. Faked Global Illumination • Shadow, Reflection, BRDF…etc. • In theory, real global illumination is not possible in current graphics pipeline: • Conceptually a loop of individual polygons. • No interaction between polygons. • Can this be changed by multi-pass rendering?

  23. Case Study: Shadow Map • Using two textures: color and depth • Relatively straightforward design using pixel (fragment) shaders on GPUs.

  24. Textures Adding “Memory” to the GPU Computation • Modern GPUs allow: • The usage of multiple textures. • Rendering algorithms that use multiple passes. Transform (& Lighting) Rasterization

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