CG_Course5计算机图像学.pptVIP

第五课 Ray Tracing Overview of the Section Why Ray Tracing? What is Ray Tracing? How to tracing rays? How to accelerating ray-tracing? Object-ordering rendering Each polygon or triangle is processed in turn and its visibility is determined using algorithms such as Z-buffer After determining its visibility, the polygon or triangle is projected onto the viewing plane Polygon or triangle is shaded using Phong Model\ OpenGL, rasterization (光栅化) Object-ordering rendering Advantage Fast rendering can be implemented on hardware Disadvantage Difficult to model lighting effects such as reflection or transparency Difficult to model lighting effects are caused by the interaction of objects, i.e. shadows Assumption Light travels from visible objects towards the eye Ray tracing Question Do light proceed from the eye to the light source or from light source to the eye? To answer the question, it’s important to understand Light rays do travel in straight lines Light rays do not interfere if they cross Light rays travel from the light source to the eye but the physics are invariant under path reversal Ray tracing Image order rendering Start from pixel, what do you see through this pixel Allows to add more realism to the rendered scene by allowing effects such as Shadows Transparency Reflections How ray-tracing works Looks through each pixel (e.g. 640 x 480) Determines what eye sees through pixel Trace light ray: eye - pixel (image plane) - scene If a ray intersect any scene object? Yes, render pixel using object color No, it uses the background color Automatically solves hidden surface removal problem Ray misses all objects Ray hits an object Ray hits an object Ray hits object: Check if hit point is in shadow, build secondary ray towards light sources Ray hits an object If shadow ray hits another object before light source: first intersection point is in shadow of the second object. Otherwise, collect light contributions Reflected Ray When a ray hits an reflective object, a reflectio