Table of Contents Glide Programming Guide
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- 1Synchronizing with Vertical Retrace
- 8Clearing Buffers
grBufferNumPending() returns the number of queued buffer swap requests. The maximum value returned is 7, even though there may be more buffer swap requests in the queue. To minimize rendering latency in response to interactive input, grBufferNumPending() should be called in a loop once per frame until the returned value is less than some small number such as 1, 2, or 3.
1Synchronizing with Vertical RetraceSynchronization to vertical retrace is supported with the grSstVRetraceOn() and grSstVideoLine() functions. grSstVRetraceOn() returns FXTRUE if the monitor is in vertical retrace and FXFALSE otherwise. FxBool grSstVRetraceOn( void ) grSstVideoLine() returns the current line number of the display beam. This number is 0 during vertical retrace and increases as the display beam progresses down the screen. There are a small number of video lines that are not displayed at the top of the screen: the vertical backporch. Thus, grSstVideoLine() returns a small positive number when the display beam is at the top of the screen; as the beam goes off the bottom of the screen, the line number may exceed the number returned by grSstScreenHeight(). FxU32 grSstVideoLine( void ) The Glide 2.1 release was the first release to include grSstVideoLine(). Earlier versions used grSstVRetraceTicks(), now obsolete. Note that an application does not need to explicitly synchronize to vertical retrace if it only wishes to remove tearing artifacts. grBufferSwap() will automatically synchronize to vertical retrace if desired. 8Clearing BuffersThe ability to clear a display buffer is fundamental to animation, since the remnants of a previously rendered scene must be reset before a new scene can be rendered. The Voodoo Graphics hardware allows the back buffer and alpha or depth buffer to be cleared simultaneously. A buffer clear fills pixels at twice the rate of triangle rendering, therefore the performance cost of clearing the buffer is half the cost of rendering a rectangle. Clearing the buffer is not necessary when the scene paints a background that covers the entire area. Buffers are cleared by calling grBufferClear(). The area within the buffer to be cleared is defined by grClipWindow(), described in the next chapter. The three parameters specify the values that will be used to clear the display buffer (color), the alpha buffer (alpha), and the depth buffer (depth). Although the color, alpha, and depth parameters are always specified, the parameters actually used will depend on the current configuration of the hardware; the irrelevant parameters are ignored. The depth parameter can be one of the constants GR_ZDEPTHVALUE_NEAREST, GR_ZDEPTHVALUE_FARTHEST, GR_WDEPTHVALUE_NEAREST, GR_WDEPTHVALUE_FARTHEST, or a direct representation of a value in the depth buffer. See Chapter Chapter 7. for more details. void grBufferClear( GrColor_t color, GrAlpha_t alpha, FxU16 depth ) Any buffers that are enabled are automatically and simultaneously cleared by grBufferClear(). For example, if depth buffering is enabled (with grDepthBufferMode(), described in Chapter Chapter 7. ), the depth buffer will be cleared to depth. If alpha buffering is enabled (with grAlphaBlendFunction(), described in Chapter Chapter 6. ), the alpha buffer will be cleared to alpha. And if writes to the display buffer are enabled (with grColorMask(), described in Chapter Chapter 5. ), then it will be cleared to color. If an application does not want a buffer to be cleared, it should mask off writes to the buffer using grDepthMask() and grColorMask() as appropriate. Download 6.22 Mb. Share with your friends:
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