Name
MESA_program_debug
Name Strings
GL_MESA_program_debug
Contact
Brian Paul (brian.paul 'at' tungstengraphics.com)
Status
XXX - Not complete yet!!!
Version
Last Modified Date: July 20, 2003
Author Revision: 1.0
Number
TBD
Dependencies
OpenGL 1.4 is required
The extension is written against the OpenGL 1.4 specification.
ARB_vertex_program or ARB_fragment_program or NV_vertex_program
or NV_fragment_program is required.
Overview
The extension provides facilities for implementing debuggers for
vertex and fragment programs.
The concept is that vertex and fragment program debuggers will be
implemented outside of the GL as a utility package. This extension
only provides the minimal hooks required to implement a debugger.
There are facilities to do the following:
1. Have the GL call a user-specified function prior to executing
each vertex or fragment instruction.
2. Query the current program string's execution position.
3. Query the current values of intermediate program values.
The main feature is the ProgramCallbackMESA function. It allows the
user to register a callback function with the GL. The callback will
be called prior to executing each vertex or fragment program instruction.
From within the callback, the user may issue Get* commands to
query current GL state. The GetProgramRegisterfvMESA function allows
current program values to be queried (such as temporaries, input
attributes, and result registers).
There are flags for enabling/disabling the program callbacks.
The current execution position (as an offset from the start of the
program string) can be queried with
GetIntegerv(GL_FRAGMENT_PROGRAM_POSITION_MESA, &pos) or
GetIntegerv(GL_VERTEX_PROGRAM_POSITION_MESA, &pos).
IP Status
None
Issues
1. Is this the right model for a debugger?
It seems prudent to minimize the scope of this extension and leave
it up to the developer (or developer community) to write debuggers
that layer on top of this extension.
If the debugger were fully implemented within the GL it's not
clear how terminal and GUI-based interfaces would work, for
example.
2. There aren't any other extensions that register callbacks with
the GL. Isn't there another solution?
If we want to be able to single-step through vertex/fragment
programs I don't see another way to do it.
3. How do we prevent the user from doing something crazy in the
callback function, like trying to call glBegin (leading to
recursion)?
The rule is that the callback function can only issue glGet*()
functions and no other GL commands. It could be difficult to
enforce this, however. Therefore, calling any non-get GL
command from within the callback will result in undefined
results.
4. Is this extension amenable to hardware implementation?
Hopefully, but if not, the GL implementation will have to fall
back to a software path when debugging. This may be acceptable
for debugging.
5. What's the <data> parameter to ProgramCallbackMESA for?
It's a common programming practice to associate a user-supplied
value with callback functions.
6. Debuggers often allow one to modify intermediate program values,
then continue. Does this extension support that?
No.
New Procedures and Functions (and datatypes)
typedef void (*programcallbackMESA)(enum target, void *data)
void ProgramCallbackMESA(enum target, programcallbackMESA callback,
void *data)
void GetProgramRegisterfvMESA(enum target, sizei len,
const ubyte *registerName, float *v)
New Tokens
Accepted by the <cap> parameter of Enable, Disable, IsEnabled,
GetBooleanv, GetDoublev, GetFloatv and GetIntegerv:
FRAGMENT_PROGRAM_CALLBACK_MESA 0x8bb1
VERTEX_PROGRAM_CALLBACK_MESA 0x8bb4
Accepted by the <pname> parameter GetBooleanv, GetDoublev,
GetFloatv and GetIntegerv:
FRAGMENT_PROGRAM_POSITION_MESA 0x8bb0
VERTEX_PROGRAM_POSITION_MESA 0x8bb4
Accepted by the <pname> parameter of GetPointerv:
FRAGMENT_PROGRAM_CALLBACK_FUNC_MESA 0x8bb2
FRAGMENT_PROGRAM_CALLBACK_DATA_MESA 0x8bb3
VERTEX_PROGRAM_CALLBACK_FUNC_MESA 0x8bb6
VERTEX_PROGRAM_CALLBACK_DATA_MESA 0x8bb7
Additions to Chapter 2 of the OpenGL 1.4 Specification (OpenGL Operation)
None.
Additions to Chapter 3 of the OpenGL 1.4 Specification (Rasterization)
None.
Additions to Chapter 4 of the OpenGL 1.4 Specification (Per-Fragment
Operations and the Frame Buffer)
None.
Additions to Chapter 5 of the OpenGL 1.4 Specification (Special Functions)
In section 5.4 "Display Lists", page 202, add the following command
to the list of those that are not compiled into display lists:
ProgramCallbackMESA.
Add a new section 5.7 "Callback Functions"
The function
void ProgramCallbackMESA(enum target, programcallbackMESA callback,
void *data)
registers a user-defined callback function with the GL. <target>
may be FRAGMENT_PROGRAM_ARB or VERTEX_PROGRAM_ARB. The enabled
callback functions registered with these targets will be called
prior to executing each instruction in the current fragment or
vertex program, respectively. The callbacks are enabled and
disabled by calling Enable or Disable with <cap>
FRAGMENT_PROGRAM_ARB or VERTEX_PROGRAM_ARB.
The callback function's signature must match the typedef
typedef void (*programcallbackMESA)(enum target, void *data)
When the callback function is called, <target> will either be
FRAGMENT_PROGRAM_ARB or VERTEX_PROGRAM_ARB to indicate which
program is currently executing and <data> will be the value
specified when ProgramCallbackMESA was called.
From within the callback function, only the following GL commands
may be called:
GetBooleanv
GetDoublev
GetFloatv
GetIntegerv
GetProgramLocalParameter
GetProgramEnvParameter
GetProgramRegisterfvMESA
GetProgramivARB
GetProgramStringARB
GetError
Calling any other command from within the callback results in
undefined behaviour.
Additions to Chapter 6 of the OpenGL 1.4 Specification (State and
State Requests)
Add a new section 6.1.3 "Program Value Queries":
The command
void GetProgramRegisterfvMESA(enum target, sizei len,
const ubyte *registerName,
float *v)
Is used to query the value of program variables and registers
during program execution. GetProgramRegisterfvMESA may only be
called from within a callback function registered with
ProgramCallbackMESA.
<registerName> and <len> specify the name a variable, input
attribute, temporary, or result register in the program string.
The current value of the named variable is returned as four
values in <v>. If <name> doesn't exist in the program string,
the error INVALID_OPERATION is generated.
Additions to Appendix A of the OpenGL 1.4 Specification (Invariance)
None.
Additions to the AGL/GLX/WGL Specifications
None.
GLX Protocol
XXX TBD
Dependencies on NV_vertex_program and NV_fragment_program
If NV_vertex_program and/or NV_fragment_program are supported,
vertex and/or fragment programs defined by those extensions may
be debugged as well. Register queries will use the syntax used
by those extensions (i.e. "v[X]" to query vertex attributes,
"o[X]" for vertex outputs, etc.)
Errors
INVALID_OPERATION is generated if ProgramCallbackMESA is called
between Begin and End.
INVALID_ENUM is generated by ProgramCallbackMESA if <target> is not
a supported vertex or fragment program type.
Note: INVALID_OPERAION IS NOT generated by GetProgramRegisterfvMESA,
GetBooleanv, GetDoublev, GetFloatv, or GetIntegerv if called between
Begin and End when a vertex or fragment program is currently executing.
INVALID_ENUM is generated by ProgramCallbackMESA,
GetProgramRegisterfvMESA if <target> is not a program target supported
by ARB_vertex_program, ARB_fragment_program (or NV_vertex_program or
NV_fragment_program).
INVALID_VALUE is generated by GetProgramRegisterfvMESA if <registerName>
does not name a known program register or variable.
INVALID_OPERATION is generated by GetProgramRegisterfvMESA when a
register query is attempted for a program target that's not currently
being executed.
New State
XXX finish
(table 6.N, p. ###)
Initial
Get Value Type Get Command Value Description Sec. Attribute
--------- ---- ----------- ----- ----------- ---- ---------
FRAGMENT_PROGRAM_CALLBACK_MESA B IsEnabled FALSE XXX XXX enable
VERTEX_PROGRAM_CALLBACK_MESA B IsEnabled FALSE XXX XXX enable
FRAGMENT_PROGRAM_POSITION_MESA Z+ GetIntegerv -1 XXX XXX -
VERTEX_PROGRAM_POSITION_MESA Z+ GetIntegerv -1 XXX XXX -
FRAGMENT_PROGRAM_CALLBACK_FUNC_MESA P GetPointerv NULL XXX XXX -
VERTEX_PROGRAM_CALLBACK_FUNC_MESA P GetPointerv NULL XXX XXX -
FRAGMENT_PROGRAM_CALLBACK_DATA_MESA P GetPointerv NULL XXX XXX -
VERTEX_PROGRAM_CALLBACK_DATA_MESA P GetPointerv NULL XXX XXX -
XXX more?
New Implementation Dependent State
None.
Revision History
8 July 2003
Initial draft. (Brian Paul)
11 July 2003
Second draft. (Brian Paul)
20 July 2003
Third draft. Lots of fundamental changes. (Brian Paul)
23 July 2003
Added chapter 5 and 6 spec language. (Brian Paul)
Example Usage
The following is a very simple example of how this extension may
be used to print the values of R0, R1, R2 and R3 while executing
vertex programs.
/* This is called by the GL when the vertex program is executing.
* We can only make glGet* calls from within this function!
*/
void DebugCallback(GLenum target, GLvoid *data)
{
GLint pos;
GLuint i;
/* Get PC and current instruction string */
glGetIntegerv(GL_VERTEX_PROGRAM_POSITION_ARB, &pos);
printf("Current position: %d\n", pos);
printf("Current temporary registers:\n");
for (i = 0; i < 4; i++) {
GLfloat v[4];
char s[10];
sprintf(s, "R%d", i);
glGetProgramRegisterfvMESA(GL_VERTEX_PROGRAM_ARB, strlen(s), s, v);
printf("R%d = %g, %g, %g, %g\n", i, v[0], v[1], v[2], v[3]);
}
}
/*
* elsewhere...
*/
/* Register our debugger callback function */
glProgramCallbackMESA(GL_VERTEX_PROGRAM_ARB, DebugCallback, NULL);
glEnable(GL_VERTEX_PROGRAM_CALLBACK_MESA);
/* define/bind a vertex program */
glEnable(GL_VERTEX_PROGRAM);
/* render something */
glBegin(GL_POINTS);
glVertex2f(0, 0);
glEnd();