HP c3700 - Workstation, Reference Manual, Page: 480 / 486

V
glVisibilityBufferhp
Chapter 20
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glVisibilityBufferhp
glVisibilityBufferhp
- establish a buffer for Visibility Test results
C Specification
void glVisibilityBufferhp(GLSizei size,
GLboolean *buffer,
GLboolean wait_on_get)
Parameters
size Specifies the size of buffer (in bytes)
buffer Returns the Visibility Test results
wait_on_get Specifies whether a glGet of VISIBILITY_TEST_hp waits for all
Visibility Test data to be returned from the hardware before returning
to the calling program.
Description
glVisibilityBuffer has three arguments: ’buffer’ is a pointer to an array of Boolean, and
’size’ indicates the size of the array. ’buffer’ returns values of GL_TRUE or GL_FALSE
for each Visibility Test issued. A Visibility Test begins when a call is made to
glEnable(GL_VISIBILITY_TEST_hp), and ends when a call is made to either
glDisable(GL_VISIBILITY_TEST_hp), or glNextVisibilityTesthp(). When all Visibility
Tests have been performed, glDisable(GL_VISIBILITY_TEST_hp) should be called,
followed by a call to glGet of GL_VISIBILITY_TEST_hp. Calling glGet causes the
contents of ’buffer’ to be up to date (based on the value of the ’wait_on_get’ parameter,
discussed below). A Visibility Test result of GL_TRUE indicates that some portion of the
primitive(s) rendered during that test were visible. A value of GL_FALSE indicates that
no portion of the primitive(s) rendered during the test was visible.
The third parameter, ’wait_on_get’, is intended to allow for potential optimizations
during Visibility Tests. When Visibility Tests are made, there is often some amount of
latency between the time the request is made and when the answer is available. Once a
glGet of GL_VISIBILITY_TEST_hp is called after
glDisable(GL_VISIBILITY_TEST_hp), there will usually be a short delay (perhaps as
much as 50 microseconds) before the answer is available. If the application would prefer
to wait until the Visibility Test results are available before proceeding, then a value of
GL_TRUE should be specified for the ’wait_on_get’ parameter. If, on the other hand, the
application has some useful work to do during that time, a value of GL_FALSE can be
specified for the ’wait_on_get’ parameter, and glGet will return immediately. However,
’buffer’ is not guaranteed to be defined until some time later. To check to see when the
Visibility Test results are complete, the programmer can look at the data returned by a
glGet of GL_VISIBILITY_TEST_hp. Two Boolean values are returned; The first
indicates whether or not GL_VISIBILITY_TEST_hp is currently enabled. The second
indicates if there are any test results pending. If there are no test results pending, the
programmer may conclude that ’buffer’ is up to date.