OpenGL Lighting15-462 Computer GraphicsSpring 2009Frank Palermo“OpenGL is just a bunch of hacks.” -Adrien TreuilleWhat Adrien Means...●What Adrien means is that OpenGL was designed to produce reasonable-looking 3D images quickly & simply.●A lot of its design is not based on the way light actually behaves in the real world, or on the way we perceive a scene.●Keep that in mind as we discuss the OpenGL pipeline and lighting model.The OpenGL Pipeline* Featuring slides shamelessly stolen from a previous teaching of 15-462!The OpenGL Pipeline* Featuring slides shamelessly stolen from a previous teaching of 15-462!The OpenGL Pipeline* Featuring slides shamelessly stolen from a previous teaching of 15-462!The OpenGL Pipeline* Featuring slides shamelessly stolen from a previous teaching of 15-462!The OpenGL Pipeline* Featuring slides shamelessly stolen from a previous teaching of 15-462!The OpenGL Pipeline* Featuring slides shamelessly stolen from a previous teaching of 15-462!OpenGL Lighting●By default, OpenGL's fixed-function pipeline implements the Blinn-Phong Shading Model.–Originally from Bui Tuong Phong's Ph.D. work at the University of Utah (1973).–Modified by James F. Blinn (1977).●Light is modeled in three categories: ambient, diffuse, and specular.Ambient Lighting●Approximates the low level of light that is normally present everywhere in a scene (scattered by many objects before reaching the eye).●Constant term; applies equally to all points on the object.Image: Brad SmithDiffuse Lighting●Approximates light scattered by objects with rough surfaces.●Its intensity depends on the angle between the light source and the surface normal (not the direction to the viewer).Image: Brad SmithSpecular Lighting●Approximates light reflected by “shiny” objects with smooth surfaces.●Its intensity depends on the angle between the viewer and the direction of a ray reflected from the light source.Image: Brad SmithThe Equation●Ka, Kd, Ks = Ambient, Diffuse, and Specular Color (Set for Materials , i.e. Objects)●Ia, Id, Is = Ambient, Diffuse, and Specular Intensity (Set for Lights)●Dot Products: Provide the dependence on the light-surface and reflection-viewer angles discussed earlier.The Blinn-Phong Model In OpenGL●Phong: The calculation is done over the entire surface.●Blinn-Phong: The calculation is done only at vertices; the remainder of the surface is interpolated from surrounding vertices.Image: Brad SmithEmissive Lighting●OpenGL adds emissive lighting, which allows an object to “glow” with its own light.●Is this everything we need to model all possible lighting phenomena?The Blinn-Phong Model In OpenGL●OpenGL evaluates the equation for you. You just need to set the material parameters and light intensities (and provide surface normals!).●Chapter 5 of the OpenGL Programming Guide describes the commands you will need and gives examples of their use.Some OpenGL Commands●Setting light intensities:glLightfv(GL_LIGHT0, GL_DIFFUSE, Id);glLightfv(GL_LIGHT0, GL_SPECULAR, Is);●Setting material colors:glMaterialfv(GL_FRONT_AND_BACK, GL_AMBIENT, Ka);glMaterialfv(GL_FRONT_AND_BACK, GL_DIFFUSE, Kd);glMaterialfv(GL_FRONT_AND_BACK, GL_SPECULAR, Ks);●Setting surface normals:glVertex3dv(&vertices[0].x);glNormal3dv(&normals[1].x);Limitations of OpenGL Lighting●The pipeline is essentially one-way (left to right on the illustration). ●Once a polygon has been rendered, it's forgotten and can't be used as part of any other lighting calculations.●No mirrors, glow sticks made with emissive light won't actually light up anything else in the scene, etc.●Can only capture light source → object → viewer (called “direct illumination”).Direct vs. Global Illumination●"Fast Separation of Direct and Global Components of a Scene using High Frequency Illumination" by Nayar et. al. (SIGGRAPH 2006).●OpenGL could render the image at center, but it would miss all of the information in the image at right.Solutions?●Use a completely different algorithm such as raytracing or photon mapping which supports global illumination.–Projects 3 & 4.●Program the pipeline to behave differently using GLSL.–Project 2.●Be happy with Blinn-Phong.–Project 1 (and lecture next
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