功能：指定用於深度緩衝比較值。
參數說明：func指定深度比較函式，GL_NEVER,GL_LESS,GL_EQUAL,GL_LEQUAL,GL_GREATER,GL_NOTE_QUAL,GL_GEQUAL,GL_ALWAYS,預設值GL_LESS，
GL_NEVER,不通過（輸入的深度值不取代參考值）
GL_LESS,如果輸入的深度值小於參考值，則通過
GL_EQUAL,如果輸入的深度值等於參考值，則通過
GL_LEQUAL,如果輸入的深度值小於或等於參考值，則通過
GL_GREATER,如果輸入的深度值大於參考值，則通過
GL_NOTE_QUAL,如果輸入的深度值不等於參考值，則通過
GL_GEQUAL,如果輸入的深度值大於或等於參考值，則通過
GL_ALWAYS,總是通過（輸入的深度值取代參考值）
NAME
glDepthFunc - specify the value used for depth buffer comparisons
C SPECIFICATION
void glDepthFunc( GLenum func )
PARAMETERS
func ? Specifies the depth comparison function. Symbolic constants GL_NEVER, GL_LESS, GL_EQUAL, GL_LEQUAL, GL_GREATER, GL_NOTEQUAL, GL_GEQUAL, and GL_ALWAYS are accepted. The default value is GL_LESS.
DESCRIPTION
glDepthFunc specifies the function used to compare each incoming pixel z value with the z value present in the depth buffer. The comparison is performed only if depth testing is enabled. (See "glEnable" and glDisable of GL_DEPTH_TEST.)
func specifies the conditions under which the pixel will be drawn. The comparison functions are as follows:
GL_NEVER ? Never passes.
GL_LESS ? Passes if the incoming z value is less than the stored z value.
GL_EQUAL ? Passes if the incoming z value is equal to the stored z value.
GL_LEQUAL ? Passes if the incoming z value is less than or equal to the stored z value.
GL_GREATER ?
Passes if the incoming z value is greater than the stored z value.
GL_NOTEQUAL ?
Passes if the incoming z value is not equal to the stored z value.
GL_GEQUAL ? Passes if the incoming z value is greater than or equal to the stored z value.
GL_ALWAYS ? Always passes.
The default value of func is GL_LESS. initially, depth testing is disabled.
ERRORS
GL_INVALID_ENUM is generated if func is not an accepted value.
GL_INVALID_OPERATION is generated if glDepthFunc is called between a call to glBegin and the corresponding call to glEnd.
ASSOCIATED GETS
glGet with argument GL_DEPTH_FUNC
glIsEnabled with argument GL_DEPTH_TEST
SEE ALSO
"glDepthRange" , "glEnable"
