Introduction to OpenGL

The purpose of this chapter is to introduce OpenGL

OpenGL is a Graphics API
- Can interact with hardware (GPU)
- Or software (mesa)
- It is cross platform
- Open Standard
- Multiple language bindings
- The standard has been through a number of revisions
  - Current major number 4, minor number 5, Aug 11, 2014

Let's take a look at square.cpp

Start with main
- This introduces us to the glut (See section 2.11, page 62)
- argc, argv
  - int argc, char* argv[]
  - This is standard C
  - ```
  for (int i=0; i < argc; i++) {
      cout << "argv[" << i <<"] = " << argv[i] << endl;
  }
  		   
```
- glutInit(&argc, argv)
  - Mostly X based arguments.
- glutInitContextVersion(major, minor)
  - This requests a "version" of OpenGL be used corresponding to the major and minor number.
  - If you request a number your hardware/software doesn't support, this will fail.
- glutInitContextProfiile(profile)
  - Controls access to deprecated features.
  - GLUT_CORE_PROFILE :
  - GLUT_COMPATIBILITY_PROFILE
  - It looks like you might be able to say "don't use deprecated features" with this command.
  - Very poorly documented
- glutInitDisplayMode
  - This controls the type of display we will be using.
  - In this case, we are asking for a single frame buffer
  - And rgba colors
- glutInitWindow*
  - Size and placement of the window.
- glut*Func
  - Mouse and Keyboard as discussed
  - DisplayFunc
    - When Glut detects that it needs to redraw the screen, this is what gets called.
    - This is normally the main drawing function.
    - This is the only function where drawing should be done.
    - You should not call this yourself.
    - A call can be forced with glutPostRedisplay()
  - glutIdleFunc()
    - This gets called when no events have occurred
    - It is useful for animation
    - set to NULL nothing is done.
    - Computation performed here, and in all callback functions should be minimized.
  - ReshapeFunc()
    - If the window changes shape, this gets called
  - glutMotionFunc(), glutPassiveMotionFunc()

Global Variables

As you can see, there is no way to pass generic variables to and from the callbacks.
This is a common problem with older APIs.
Some give you a single generic pointer, but this technique has problems as well.
So we need so come up with a way to get information back from the callbacks.
A singleton might be appropriate here, but it might be overkill as well.
So I give you permission to use global variables under the following conditions.
- They are clearly identified at the top of the file or in a header file.
- They are well documented
- They are only used to pass information to/from callbacks
I have added a variable to control color.
I generally set these in the setup() routine
And really set them in the reset() routine when appropriate.
DON'T go overboard on the globals.
- Only use them when you need them.