tailieunhanh - Advanced Maya Texturing and Lighting- P13

Advanced Maya Texturing and Lighting- P13: I should stress that I am self-taught. In 1994, I sat down at a spare seat of Alias PowerAnimator and started hacking away. After several years and various trials by fire, 3D became a livelihood, a love, and an obsession. Along the way, I was fortunate enough to work with many talented artists at Buena Vista Visual Effects and Pacific Data Images. In 2000, I switched from PowerAnimator to Maya and have since logged tens of thousands of hours with the subject of this book | Comparing the Scanline and Raytracing Processes Before I describe the raytracing process in more detail the scanline process is worth a closer look. In general the scanline process is as follows The renderer examines the objects in the scene. The objects within the camera frustum are added to a list and their bounding boxes are calculated. The image to be rendered is divided into tiles to optimize memory usage. The complexity of the objects within a tile determines the tile s size. Polygon triangles associated with visible objects are processed in scanline order. Each triangle is projected into screen space and is clipped to the boundaries of each pixel it covers. That is the portions of the triangle outside the pixel boundaries are temporarily discarded. Each pixel is thus given a list of clipped triangle fragments. The fragments are stored in the lists as bit masks which are binary representations of fragment visibility within a pixel. The algorithm responsible for this process is known as A-buffer. The colors of each fragment are derived from the material qualities of the original polygons and the influence of lights in the scene. The final color of the pixel is determined by averaging the fragment colors with emphasis given to those fragments that are the most visible. As part of this process fragments are depth-sorted and additional clipping is applied to those fragments that are occluded. By comparison the raytracing process fires off a virtual ray from the camera eye through each pixel of a view plane see Figure . The number of pixels in the view plane corresponds to the number of pixels required for a particular render resolution. The first surface the ray intersects determines the pixel s color. That is the material qualities of the surface are used in the shading calculation of the pixel. If raytrace shadows are turned on secondary shadow rays are fired from the point of intersection to each shadow-producing light. If a shadow ray intersects another