# Computer Graphics Lecture 11

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1 1 / 14 Computer Graphics Lecture 11 Dr. Marc Eduard Frîncu West University of Timisoara May 15th 2012

2 2 / 14 Outline 1 Introduction 2 Transparency 3 Reflection 4 Recap

3 3 / 14 Introduction light = local + global components: L = ambient + diffuse + specular + emission+transparency + reflection To compute reflections/transparency we need to add global components as well: transparency (I T ), reflection (I R ) I T is transmitted along direction t & portion of it is bent along dir for refraction: t = ( η L η T dir m 1 η2 L η 2 T ( 1 (dir m) 2 )) m η L η T dir I R is transmitted along mirror direction from eye ( r): r = dir 2(dir m)m Hill, F.S., Kelley, S.M., Computer Graphics Using OpenGL, Third Edition, 2007, Prentice Hall, ISBN:

4 4 / 14 Transparency Simplified model semitransparent objects are filters/ attenuators of occluded objects refraction and object thickness are neglected

5 4 / 14 Transparency Simplified model semitransparent objects are filters/ attenuators of occluded objects refraction and object thickness are neglected Algorithms based on: stipple patterns: pattern of opaque & transparent fragments limited number of fill patterns limited number of transparency levels produces aliasing effects color blending: combine fragment color with framebuffer content C new = α fragment C fragment + α framebuffer C framebuffer for transparency: C new = α fragment C fragment + (1 α fragment )C framebuffer α fragment = 0 C framebuffer unchanged α fragment = 1 C framebuffer replaced by C fragment α fragment (0, 1) C framebuffer replaced by a mix of colors only the α of the incoming fragment matters drawing order matters: back to front blend the color of objects along the same ray

6 5 / 14 Transparency Figure: course_notes/graphics_08_ transparencyreflection.pdf

7 6 / 14 Transparency Order matters polygons/fragments need to be rendered in sorted depth order but hardware usually renders in object order. Depth algorithms?

8 6 / 14 Transparency Order matters polygons/fragments need to be rendered in sorted depth order but hardware usually renders in object order. Depth algorithms?

9 6 / 14 Transparency Order matters polygons/fragments need to be rendered in sorted depth order but hardware usually renders in object order. Depth algorithms? no! they return only the closest fragment per pixel do not sort Dynamic scenes require re-sorting

10 6 / 14 Transparency Order matters polygons/fragments need to be rendered in sorted depth order but hardware usually renders in object order. Depth algorithms? no! they return only the closest fragment per pixel do not sort Dynamic scenes require re-sorting Algorithms convex objects: two depth layers for arbitrarily viewing directions render back faces in a first pass blend with front faces in a second pass arbitrarily objects: 1 object space: use binary space partition (good for static geometry) 2 pixel space: use the functionality of the rendering pipeline (good for dynamic geometry) several rendering passes compute the depth layers (depth peeling) the final pass renders the depth layers

11 7 / 14 Transparency Binary space partition a viewer is on the far side of a plane a polygon on the far side of this plane cannot occlude the plane or any polygon on the near side back-to-front rendering: render far branch of the viewport render root polygon render near branch of the viewport continue recursively Figure: http: //cg.informatik.uni-freiburg.de/ course_notes/graphics_08_ transparencyreflection.pdf

12 Transparency Binary space partition a viewer is on the far side of a plane a polygon on the far side of this plane cannot occlude the plane or any polygon on the near side back-to-front rendering: render far branch of the viewport render root polygon render near branch of the viewport continue recursively Discussion depth sorts all primitives per pixel position requires extra data structures can be precomputed for static scenes requires polygon splits dynamic scenes require recomputation of the data structure Figure: http: //cg.informatik.uni-freiburg.de/ course_notes/graphics_08_ transparencyreflection.pdf 7 / 14

13 8 / 14 Transparency Depth peeling object rendered once for each depth layer depth tests per fragment: fragment must be farther than the one in the previous layer fragment must be nearest of all remaining fragments Figure: course_notes/graphics_08_ transparencyreflection.pdf Figure: transparencyreflection.pdf

14 9 / 14 Reflection Environment mapping technique for providing reflections on the object surface simplification of real reflections: environment is simplified by a texture instead of reflecting the environment we take its pictures and reflect them: sphere mapping cube mapping

15 10 / 14 Reflection Sphere mapping map is obtained by orthographically projecting an image of a mirrored sphere: synthetic scene: ray tracing or warping of a generated cube map real scene: photographing an actual mirrored sphere map stores colors as seen by the reflected rays pros: no interpolation across map faster than cube mapping cons: hard to create on the fly non-linear/non-uniform sampling sampling is view dependent for convex objects only (no self reflection possible) need a reflection map for every object

16 11 / 14 Reflection

17 12 / 14 Reflection Cube mapping map constructed by placing the camera at the center of the object and taking pictures in 6 directions (fov = 90 o, ratio = 1) pros: view independent (unlike sphere mapping) uses a single texture unit entire resolution is sampled improved environment sampling cons: cube map must be rendered again when new lights or moving objects are introduced

18 12 / 14 Reflection Cube mapping map constructed by placing the camera at the center of the object and taking pictures in 6 directions (fov = 90 o, ratio = 1) pros: view independent (unlike sphere mapping) uses a single texture unit entire resolution is sampled improved environment sampling cons: cube map must be rendered again when new lights or moving objects are introduced Applications stable specular highlights, e.g., for conveying a sense of curvature to CAD objects pre-rendered panoramic sky images located at infinite distance from the view port skylight illumination, e.g., the scattering of light in the atmosphere causes the entire sky to act as a light source dynamic reflection, i.e., dynamic moving environment (e.g., moving trees) projection textures, i.e., project images of the environment onto surrounding scene

19 13 / 14 Environment Mapping in OpenGL The next lecture on textures will clear the questions related to texture generation Cube mapping 1: {Load 6 textures for each side of the cube} 2: gl.glbindtexture(gl.gl TEXTURE CUBE MAP POSITIVE X, cubemap[0]); 3: gl.glbindtexture(gl.gl TEXTURE CUBE MAP NEGATIVE X, cubemap[1]); 4: gl.glbindtexture(gl.gl TEXTURE CUBE MAP POSITIVE Y, cubemap[2]); Sphere mapping 5: gl.glbindtexture(gl.gl TEXTURE CUBE MAP NEGATIVE Y, 1: {Texture use for automatic coordinate generation} 2: gl.glbindtexture(gl.gl TEXTURE 2D, texture); 3: gl.glenable(gl.gl TEXTURE 2D); 4: {Sphere mapping and enable s & t texture generation (see next lecture)} cubemap[3]); 6: gl.glbindtexture(gl.gl TEXTURE CUBE MAP POSITIVE Z, cubemap[4]); 7: gl.glbindtexture(gl.gl TEXTURE CUBE MAP NEGATIVE Z, cubemap[5]); 5: gl.gltexgeni(gl.gl S, GL.GL TEXTURE GEN MODE, 8: {Cube mapping} GL.GL SPHERE MAP); 9: gl.glenable(gl.gl TEXTURE CUBE MAP); 6: gl.gltexgeni(gl.gl T, GL.GL TEXTURE GEN MODE, 10: {GL REFLECTION MAP for s,t & r texture coordinates GL.GL SPHERE MAP); (see next lecture)} 7: gl.glenable(gl.gl TEXTURE GEN S); 11: gl.gltexgeni(gl.gl S, GL.GL TEXTURE GEN MODE, 8: gl.glenable(gl.gl TEXTURE GEN T); GL.GL REFLECTION MAP); 9: {Draw the shapes to apply the texture} 12: gl.gltexgeni(gl.gl T, GL.GL TEXTURE GEN MODE, 10: {... } GL.GL REFLECTION MAP); 11: gl.gldisable(gl.gl TEXTURE GEN S); 13: gl.gltexgeni(gl.gl R, GL.GL TEXTURE GEN MODE, 12: gl.gldisable(gl.gl TEXTURE GEN T); 13: gl.gldisable(gl.gl TEXTURE 2D); GL.GL REFLECTION MAP); 14: gl.glenable(gl.gl TEXTURE GEN S); 15: gl.glenable(gl.gl TEXTURE GEN T); 16: gl.glenable(gl.gl TEXTURE GEN R); 17: {Draw the shapes to apply the texture} 18: {... } 19: gl.gldisable(gl.gl TEXTURE GEN S); 20: gl.gldisable(gl.gl TEXTURE GEN T); 21: gl.gldisable(gl.gl TEXTURE GEN R); 22: gl.gldisable(gl.gl TEXTURE CUBE MAP);

20 14 / 14 Lecture Recap transparency: stipple patterns color blending: order matters binary space partitions depth peeling reflection: sphere mapping cube mapping

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