TAILIEUCHUNG - 3D Graphics with OpenGL ES and M3G- P25

3D Graphics with OpenGL ES and M3G- P25: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. | 224 MISCELLANEOUS OPENGL ES FEATURES CHAPTER 10 is used for clearing the various buffers all at the same time. The input parameter mask is a bitwise OR of symbolic constants that indicate which buffers should be cleared. Supported constants are GL_COLOR_BUFFER_BIT GL_DEPTH_BUFFER_BIT and GL_STENCIL_BUFFER_BIT. The value that the buffer is filled with during the clearing is set with the following functions void glClearColor fx T red T green T blue T alpha void glClearDepth fx T depth void glClearStencil GLint s The color or depth argument values are clamped to the range 0 1 whereas the stencil value is ANDed with 2s - 1 where s is the number of stencil bits in the stencil buffer. Their initial values respectively are 0 0 0 0 and 0. Here is an example of how these calls are used glClearColorf glClearDepthf glClear GL_COLOR_BUFFER_BIT GL_DEPTH_BUFFER_BIT READING BACK THE COLOR BUFFER Of the different frame buffer channels only the color buffer can be read back. OpenGL ES does not provide any mechanism for reading the contents of the depth or stencil buffers. One should note that even reading back the color buffer can be a very timeconsuming operation. This is because the call has to flush the rendering pipeline as all buffered rendering commands need to be executed before the function returns. Modern graphics hardware have very deep pipelines often buffering hundreds or thousands of triangles in some architectures even from different frames. Also if the rendering server is implemented on an external accelerator chip the bus connecting it back to the client can be very slow. void glReadPixels GLint x GLint y GLsizei width GLsizei height GLenum format GLenum type GLvoid pixels is used for reading back the contents of the color buffer. The parameters x y width and height are all expressed in pixels and define a rectangular portion of the screen that is copied. The only supported format-type pair copied from desktop OpenGL is .

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