Porting Screen and Buffer Clearing Commands

OpenGL replaces a variety of IRIS GL clear functions (such as zclear, aclear, sclear, and so on) with a single function, glClear. Specify exactly what you want to clear by passing masks to glClear.

Keep the following points in mind when porting screen and buffer commands:

OpenGL maintains clearing colors separately from drawing colors, with calls like glClearColor and glClearIndex. Be sure to set the clear color for each buffer before clearing.
Instead of using one of several differently named clear calls, you now clear several buffers with one call, glClear, by ORing together buffer masks. For example, czclear is replaced by:
```
glClear( GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT ) 
 
```
IRIS GL references the polygon stipple and the color writemask. OpenGL ignores the polygon stipple but references the color writemask. (The czclear function ignores both the polygon stipple and the color writemask.)

The following table lists the various IRIS GL clear functions with their equivalent OpenGL functions.

IRIS GL Call	OpenGL Call	Meaning
acbuf(AC_CLEAR)	glClear(GL_ACCUM_BUFFER_BIT)	Clear the accumulation buffer.
�	glClearColor	Set the RGBA clear color.
�	glClearIndex	Set the clear-color index.
clear	glClear(GL_COLOR_BUFFER_BIT)	Clear the color buffer.
�	glClearDepth	Specify the clear value for the depth buffer.
zclear	glClear(GL_DEPTH_BUFFER_BIT)	Clear the depth buffer.
czclear	glClear(GL_COLOR_BUFFER_BIT \| GL_DEPTH_BUFFER_BIT)	Clear the color buffer and the depth buffer.
�	glClearAccum	Specify clear values for the accumulation buffer.
�	glClearStencil	Specify the clear value for the stencil buffer.
sclear	glClear(GL_STENCIL_BUFFER_BIT)	Clear the stencil buffer.

When your IRIS GL code uses both gclear and sclear, you can combine them into a single glClear call�this can improve your program's performance.