16 - Other Topics. The Stencil Buffer. Erosion. Fragment Discarding. Multi-Texturing + +

Transcription

1 CSc 155 Advanced Computer Graphics 16- Other Topics The Stencil Buffer An additional per-pixel buffer typically used to direct OpenGL to behave differently depending on what is in the buffer Useful for specifying which portions of the frame to render, and what not to render (like a stencil) Other applications: rendering a region differently (e.g., a rear-view mirror) implementing shadow volumes culling for optimization Fragment Discarding Fragment shaders can be told to stop processing the current fragment : Can control with a discard texture. uniform samplerd discardtexture;... Discard Texture void main(void) { vec discardcolor = textured(discardtexture,varyingtexcoord0.st); } if (discardcolor.rgb == vec(1.0, 0.0, 0.0))... code here to process fragment as normal... Erosion use a noise texture to update a dissolve factor based on a timer uniform samplerd discardtexture; uniform float dissolvefactor; ErosionTexture //limit for displaying pixels... void main(void) { vec discardcolor = textured(discardtexture,varyingtexcoord0.st); if (discardcolor.r < dissolvefactor) }... code here to process fragment as normal... fragment Multi-Texturing Multi-Texture Example: Tarnish Cube Map Texture Environment Mapping + + Tarnish texture TEXTURE0 TEXTURE1 TEXTURE TEXTURE = 5 Image credits: OpenGL SuperBible, 5 th Edition, Richard Wright et. al. 6

2 Procedural Texturing checkerboard texture applied: Example checkerboard texture: Stride = Observations: if (supercol==superrow) color = yellow; 1 Texel Super-row if (even(supercol) && even(superrow)) color = yellow; if (odd(supercol) && odd(superrow)) color = blue; if (even(supercol+superrow)) color = yellow ; else color = blue ; Size = x (texels, columns) Super-column 7 example: Toy Ball Shader Pixar classic Toy Ball: 5 size 9 A point is in the star if and only if it lies on the side of at least half-space lines. A point P lies on the positive side of a line P0-P1 if and only if L 1 L is positive: L P P0 P0 P θ cos( ) L L 1 + P1 L P1 0 1 P 10 D Textures D Texture Images are painted on and sometimes they look like it Desired: make it look like an object is made out of some material D Texture Example: Wood-grain Wood grain is caused by tree rings Top View Concentric growth ring cylinders Object point (x,y,z) Object to be carved from wood Image source: ACM SIGGRAPH Education Slide Set, R. Wolfe, DePaul Univ. 11 Needed: a function f(x,y,z) mapping an object location into the wood space i.e. obtaining the wood color 1

3 wood-grain example (cont.) wood-grain sample results Computing cylindrical location for (x,y,z) object point Y Top View (x,y,z) R z Variations in ring density x X Y Z R x z tan z 1 x H y Z X Variations in Ring Width Image source: ACM SIGGRAPH Education Slide Set, R. Wolfe, DePaul Univ. 1 1 Randomness is important in many D textures: o Natural materials (wood, marble, granite ) o Man-made materials (stucco, asphalt, cement ) o Natural phenomena (clouds, fire, smoke, wind effects ) o Model imperfections (rust, dirt, smudges, dents ) o Pattern and motion imperfections (bumps, wobbles, jitters ) Perlin Noise Ken Perlin (NYU), 195 o Classic Noise (195) Gradient noise using cubic Hermite interpolation o Improved Noise (001) Gradient noise using quintic interpolation and precomputed gradients o Simplex Noise (005) Hardware implementation Computer Graphics V19 No., July 199 AMPAS Oscar, Fractals The Mandelbrot Set The set of all complex numbers z for which the sequence Z(0) = z, Z(n+1) = Z(n)*Z(n) + z, n = 0,1,,... remains bounded Image source: ACM SIGGRAPH Education Slide Set, R. Wolfe, DePaul Univ. 17 1

4 Fractal Surfaces Apply the same technique (recursively) to an area: 19 0 Fractal Terrain & Mountains Fractal Trees Trees are recursively self-similar 1 Fractal Plants and Trees Basic approach: Billboards o Replace a complex D model with a D image o Maintain image orientation toward camera Works well when o Model is sufficiently far away o Model tends to have axial symmetry Image source: Combinatorial Analysis of Ramified Patterns and Computer Imagery of Trees, Viennot et.al., Computer Graphics, July 199

5 Basic steps: Reflector Reflector Objects o Define reflectors o Create virtual reflected versions of objects o Draw virtual objects clipped to reflector o (another approach - use a stencil buffer) Particle Systems A collection of individual particle elements Can be used to create a wide variety of effects: Explosions, fireworks, rain, snow, smoke, liquid flow/spray, clouds, star fields, bird flocks, cloth First developed by Pixar * for Star Trek II: The Wrath of Khan Genesis Device sequence World Object Virtual Reflected Object ( behind reflector) Clipped Object * while it was still the Computer Graphics Division of Industrial Light & Magic (ILM), which was owned by LucasFilm 5 6 Non-Autonomous Particles Particles may be connected to neighbors Forces between connected particles can be modeled by spring/dampers Particle behavior is a combination of external forces and spring/damper forces Cloth Modeled using a network of spring/damper connected particles: Particle A Spring/Damper Particle B 7 cloth example: OpenCL Open standard for parallel programming For harnessing GPU for non-graphics tasks that require huge numbers of computations Current version is.0 Supported by Khronos group (same as OpenGL) Java binding is JOCL Image credit: Prof. Steve RotenBerg, CSE 169, UCSD 9 0