TAILIEUCHUNG - 3D Graphics with OpenGL ES and M3G- P12

3D Graphics with OpenGL ES and M3G- P12:Mobile phones are the new vehicle for bringing interactive graphics technologies to consumers. Graphics that in the 1980s was only seen in industrial flight simulators and at the turn of the millennium in desktop PCs and game consoles is now in the hands of billions of people. This book is about the technology underpinnings of mobile threedimensional graphics, the newest and most rapidly advancing area of computer graphics. | 94 LOW-LEVEL RENDERING CHAPTER 3 both depth fail and pass . A very advanced use case for stenciling is volumetric shadow casting Hei91 . Depth test Depth testing is used for hidden surface removal the depth value of the incoming fragment is compared against the one already stored at the pixel and if the comparison fails the fragment is discarded. If the comparison function is LESS only fragments with smaller depth value than already in the depth buffer pass other fragments are discarded. This can be seen in Figure where the translucent object is clipped to the depth values written by the opaque object. The passed fragments continue along the pipeline and are eventually committed to the frame buffer. There are other ways of determining the visibility. Conceptually the simplest approach is the painter s algorithm which sorts the objects into a back-to-front order from the camera and renders them so that a closer object always draws over the previous farther objects. There are several drawbacks to this. The sorting may require significant extra time and space particularly if there are a lot of objects in the scene. Moreover sorting the primitives simply does not work when the primitives interpenetrate that is a triangle pokes through another. If you instead sort on a per-pixel basis using the depth buffer visibility is always resolved correctly the storage requirements are fixed and the running time is proportional to the screen resolution rather than the number of objects. With depth buffering it may make sense to have at least a partial front-to-back rendering order the opposite that is needed without a depth buffer. This way most fragments that are behind other objects will be discarded by the depth test avoiding a lot of useless frame buffer updates. At least blending and writing to the frame buffer can be avoided but some engines even perform texture mapping and fogging only after they detect that the fragment survives the depth test. Depth offset As already

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