GLSL part II

• Control Structures
  • If-then, if-then-else as in c.
  • GLSL 1.30 and on switch-case
  • for, while, do-while
  • break, continue, return and in the fragment shader discard
• Functions
  • As C, but
  • Access modifiers: in, const in, out, inout
    • in is the default if nothing is specified.
    • the const checks to see if the variable is modified
      • This seems more like an information to programmer item
• Appendix C provides additional resources for programmers
  • There is a description of the variables, where they are available and their use.
  • A list of built-in functions begins on page 686
• Errors:
  • This is from Appendix H and other sources.
  • The good? news is that OpenGL mostly ignores errors and tries to continue.
  • There are two mechanisms for checking for errors.
  • GLenum glGetError(void).
    • Returns a constant corresponding to an error that has occurred.
    • And resets the error state.
    • There is a possibility that multiple error flags could be set.
    • So this needs to be called in a loop.
    • See the code in CheckForErrors()
  • A newer mechanism is using void glDebugMessageCallback( DEBUGPROC callback, void * userParam);
    • This is much more complex. It requires:
      • Creating a debugging context
      • Setting up a callback function
      • Registering the callback function
      • Enabling debugging
    • And also version 4.3 or an implementation with this capability.
      • It appears that MESA, at least my version, has this capability
    • Step 1, request a debugging context.
      • glutInitContextProfile(GLUT_FORWARD_COMPATIBLE | GLUT_DEBUG);
      • This is probably possibly not portable.
    • Step 2, define a callback function.

      typedef void (APIENTRY *DEBUGPROC)(GLenum source,
                  GLenum type,
                  GLuint id,
                  GLenum severity,
                  GLsizei length,
                  const GLchar *message,
                  void *userParam);
      		

      • But the lab machines did not like this, so I switched to

        void DebugCallbackFunction(unsigned int source,
                       unsigned int type, unsigned int id, unsigned int severity,
                       int length, const GLchar* message, void* userParam);
        

      • The parameters are
        • source: an enumerated that tells where the message came from (See pages 869-870)
        • type: an enumerated that tells the type of error, see page 870-871
        • severity: high, medium, low or notification (871)
        • message: a string generated by OpenGL which is an error message.
        • length: the length of the message.
        • Finally a pointer to a user defined parameter.
      • I have copied this function (mostly) from a tutorial, the link is included in the code.
    • Step 3: enable the callback function
      • Again this is from the tutorial
      • He starts by testing to see if the function is available.

            if(glDebugMessageCallback){
               // other code
            } else {
                cout << "glDebugMessageCallback not available" << endl;
            }
        

      • I think I knew about this once, but have forgotten.
      • Next enable two different debugging levels

        glEnable(GL_DEBUG_OUTPUT_SYNCHRONOUS);
        glEnable(GL_DEBUG_OUTPUT);
        		

      • Next register the callback.

        glDebugMessageCallback(DebugCallbackFunction, NULL);
        

      • Finally, tell OpenGL what type of messages you want.
        • void glDebugMessageControl( GLenum source, GLenum type, GLenum severity, GLsizei count, const GLuint *ids, GLboolean enabled);
        • This is a filtering function.
        • The first three parameters identify the type of message you want to receive.
        • The length should be 0 if we want everything
        • The pointer is a pointer to a list of message ids to enable or disable.
        • The enabled flag either enables or disables this list of parameters.

          GLuint unusedIds = 0;
          glDebugMessageControl(GL_DONT_CARE, GL_DONT_CARE, GL_DONT_CARE,
                      0, &unusedIds, true);
          
          

    • After all this is icomplete, the callback function will be called upon error.
  • Debugging has an additional level, but this will probably do for now.
  • You should select one of these (probably the second) and use it.