XXX - incomplete (needs number, glx protocol, enumerant values)

Name

    SGIX_depth_pass_instrument

Name Strings

    GL_SGIX_depth_pass_instrument

Version

    Last Modified Date: 10/21/1998
    $Date: 1998/10/21 19:08:52 $  $Revision: 1.4 $

Number

    205

Dependencies

    SGIX_instruments is required
    SGIX_multisample affects the definition of this extension.

Overview

    This extension defines an instrument that uses the API defined in
    SGIX_instruments.  The instrument specified by this extension is a
    counter of the number of fragments which passed the Z test during
    rasterization.  The maximum value of the counter is an
    implementation-dependent constant.

    Some systems may maintain counters on different parts of the
    system.  For example, a system with a frame buffer distributed
    across multiple chips may maintain a count of the fragments which
    passed the depth test on each individual chip.  In this extension,
    a queriable constant is defined that indicates the number of 
    responses to expect when a measurement is taken.  This 
    mechanism allows GL implementations to be as efficient as possible.

Issues

    *   Should we count fragments drawn when the depth test was
        disabled? 

	A: Doesn't seem to be a strong reason to do this one way or
           the other -- let's treat disabled depth test the same as a
           depth func of ALWAYS.

    *   Should we guarantee that if other counters are enabled when the
        query is issued, the depth pass responses are placed
        successively in the buffer?  This might make interpreting the
        buffer easier but would complicate and slightly slow the
        OpenGL implementation.  

        A: They should be successive.  The GL burden shouldn't be that
           great since the buffer can be reordered during the
           PollInstrumentsSGIX command.  This way we can probably
           avoid moving the contents of the entire buffer around and
           also keep the resource allocation burden in the
           application.

    *   Having each counter response appear as an independent
        instrument is awkward and wastes space in the buffer.  Could
        we have GL combine them??

        A: No.  It will be natural for some hardware implementations
           to have multiple instruments returned in a non-guaranteed
           order using a DMA.  In this case the implementation will
           need a way to differentiate the responses -- ie, a header.
           By putting the header for each instrument in the
           application's buffer, we allow the DMA destination to be
           the user's buffer.  Combining the responses would
           complicate allocation since space would be required for
           them and would be slower since it would require more data
           movement. 

    *   Should the counter value be considered state?
        SGIX_ir_instrument1 doesn't list its counters as state.

        A: Seems like SGIX_ir_instrument1 may be wrong -- this seems
           like it should be state.  Note that histogram and minmax
           contents are state.

    *   In theory should we add something to the GLX protocol?
        SGIX_instruments doesn't establish a framework yet so it's
        kind of a moot point.

New Tokens

    Accepted by the <cap> parameter of Enable, Disable and IsEnabled:

        DEPTH_PASS_INSTRUMENT_SGIX              0x8310

    Accepted by the <pname> parameter of GetBooleanv, GetIntegerv,
    GetFloatv, and GetDoublev:

        DEPTH_PASS_INSTRUMENT_COUNTERS_SGIX     0x8311
        DEPTH_PASS_INSTRUMENT_MAX_SGIX          0x8312

Additions to Chapter 2 of the 1.0 Specification (OpenGL Operation)

    None

Additions to Chapter 3 of the 1.0 Specification (Rasterization)

    None

Additions to Chapter 4 of the 1.0 Specification (Per-Fragment Operations
and the Frame Buffer)

    Added to subsection 4.1.5 (Depth buffer test) at the end of the
    paragraph which begins "If the depth buffer test fails...":

    If DEPTH_PASS_INSTRUMENT_SGIX is enabled and instruments have been
    started via a call to StartInstrumentSGIX, a counter or counters
    of the number of fragments which have passed the depth test or
    which have been drawn when the depth test is disabled is
    maintained.  The number of counters can be queried using the token
    DEPTH_PASS_INSTRUMENT_COUNTERS_SGIX.  For each fragment which
    passes the depth test or is drawn with depth testing disabled, one
    counter is incremented by one.  If MULTISAMPLE_SGIS is enabled,
    the counter is incremented by one for each fragment containing at
    least one sample for which the depth test passed.  If the
    increment would have caused the counter to go beyond is maximum
    representable value, the count is clamped to the maximum.  The
    maximum value of the counters may be queried using
    DEPTH_PASS_INSTRUMENT_MAX_SGIX.  The counter values are unsigned,
    so DEPTH_PASS_INSTRUMENT_MAX_SGIX may be as high as the maximum
    value of an unsigned integer.

Additions to Chapter 5 of the 1.0 Specification (Special Functions)

    Add to the end of section 5.X entitled Instruments:

    The instrument DEPTH_PASS_INSTRUMENT_SGIX returns for each
    measurement a system-dependent constant number of responses to the
    buffer.  The constant may be queried using the Get with an
    argument of DEPTH_PASS_INSTRUMENT_COUNTERS_SGIX.  Each response is
    formatted as though it came from an independent instrument.  If
    more than one instrument is enabled, the responses from the depth
    pass instrument will be placed successively (as opposed to
    possibly being interrupted by responses from other instruments).
    
    DEPTH_PASS_INSTRUMENT_SGIX responses are formatted as follows
    (starting at the first word):

	DEPTH_PASS_INSTRUMENT_SGIX
	Number of int's in the response (4)
	Counter value
	Marker value

    The counter value is padded to the size of an unsigned int if
    necessary by zero-filling the most significant bits.

    Assuming that none of the counters overflowed, the total number of
    fragments that passed the depth test while the instrument was
    enabled is equal to the sum of all the counter values.

    Although different responses come from different hardware
    counters, they should be considered identical by the software and
    no guarantee is made about how the responses for each counter are
    ordered within a single query response.  For example, a hardware
    implementation may maintain a counter for each quadrant of the
    screen, but which response in the buffer came from which counter
    is not exposed to the application.

Additions to Chapter 6 of the 1.0 Specification (State and State Requests)

    None    

Additions to the GLX Specification

    XXX

Errors

    None

New State

    Get Value				Get Command		Type	Initial Value		Attribute
    ---------				-----------		----	-------------		---------
    DEPTH_PASS_INSTRUMENT_SGIX		ReadInstrumentsSGIX	1*xZ+	0			-

New Implementation Dependent State

    Get Value				Get Command		Type	Minimum Value
    ---------				-----------		----	-------------
    DEPTH_PASS_INSTRUMENT_COUNTERS_SGIX	GetIntegerv		Z+      1