The 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.


Note : 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 7 for more details.

Any buffers that are enabled are automatically and simultaneously cleared by grBufferClear().
For example,

if depth buffering is enabled (with grDepthBufferMode(), described in Chapter 7), the depth buffer will be cleared to depth.

If alpha buffering is enabled (with grAlphaBlendFunction(), described in Chapter 6), the alpha buffer will be cleared to alpha.

And if writes to the display buffer are enabled (with grColorMask(), described in 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.