人机接口与图形学双语shading.ppt

* Shading Department of CS E, East China University of Science Technology Lighting Review Lighting Models Ambient Normals don’t matter Lambert/Diffuse Angle between surface normal and light Phong/Specular Surface normal, light, and viewpoint Applying Illumination We now have an illumination model for a point on a surface Assuming that our surface is defined as a mesh of polygonal facets, which points should we use? Keep in mind: It’s a fairly expensive calculation Several possible answers, each with different implications for the visual quality of the result Applying Illumination With polygonal/triangular models: Each facet has a constant surface normal If the light is directional, the diffuse reflectance is constant across the facet. Why? Flat Shading The simplest approach, flat shading, calculates illumination at a single point for each polygon: If an object really is faceted, is this accurate? Is flat shading realistic for faceted object? No: For point sources, the direction to light varies across the facet For specular reflectance, direction to eye varies across the facet Flat Shading We can refine it a bit by evaluating the Phong lighting model at each pixel of each polygon, but the result is still clearly faceted: To get smoother-looking surfaceswe introduce vertex normals at eachvertex Usually different from facet normal Used only for shading Think of as a better approximation of the real surface that the polygons approximate Vertex Normals Vertex normals may be Provided with the model Computed from first principles Approximated by averaging the normals of the facets that share the vertex Gouraud Shading This is the most common approach Perform Phong lighting at the vertices Linearly interpolate the resulting colors over faces Along edges Along scanlines C1 C2 C3 c1 + t1(c2-c1) c1 + t2(c3-c1) c1 + t1(c2-c1) + t3(c1 + t2(c3-c1)- c1 - t1(c2-c1)) This is what OpenGL does Does this eliminate the facets? Gouraud Shading Artifacts Often a