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33. OpenGL Programming 1

Dated: 03-07-2025

Because OpenGL is an independent platform, we can write code for any operating system using the OpenGL Utility Library (glut). This library handles things like making windows, getting keyboard inputs, run an event handler or message loop in our graphic application.

  • Functions with "gl" prefix are from core OpenGL
  • Functions with "glu" prefix are from glut
int glutCreateWindow(char* name);

This creates a window which is set to current window implicitly. Each window has an OpenGL context associated with it and changes to its state can be done immediately after the window is created. The display state of window is not acted upon unless we have entered the glutMainLoop.

X Implementation Notes

The proper X Inter-Client Communication Conventions Manual (ICCCM) top-level properties are established. The WM_COMMAND property that lists the command line used to invoke the GLUT program is only established for the first window created.

Example Functions

#include <GL/glut.h>
int main () {
    // Set up the OpenGL rendering context
    glutInitDisplayMode(GLUT_RGB | GLUT_DOUBLE | GLUT_DEPTH);

    // Create a window and set its width and height.
    glutCreateWindow("Deform");
    glutReshapeWindow(640, 480);

    // The keyboard function gets called whenever we hit a key.
    glutKeyboardFunc(keyboard);

    // The display function gets called whenever the window requires an update or when we explicitly call
    glutDisplayFunc(display);

    // The reshape function gets called whenever the window changes shape
    glutReshapeFunc(reshape);

    // The idle function gets called when there are no window-events to process.
    glutIdleFunc(idle);

    // Get the ball rolling!
    glutMainLoop();
}

void glutInitDisplayMode(unsigned int mode);

This function sets the initial display mode.

Mode

Display mode, normally the bitwise OR-ing of GLUT display mode bit masks.

GLUT_RGBA

Bit mask to select an RGBA mode window. This is the default if neither GLUT_RGBA nor GLUT_INDEX are specified.

GLUT_RGB

An alias for GLUT_RGBA.

GLUT_INDEX

Bit mask to select a color index mode window. This overrides GLUT_RGBA if it is also specified.

GLUT_SINGLE

Bit mask to select a single buffered window. This is the default if neither GLUT_DOUBLE or GLUT_SINGLE are specified.

GLUT_DOUBLE

Bit mask to select a double buffered window. This overrides GLUT_SINGLE if it is also specified.

GLUT_ACCUM

Bit mask to select a window with an accumulation buffer.

GLUT_ALPHA

Bit mask to select a window with an alpha component to the color buffer(s).

GLUT_DEPTH

Bit mask to select a window with a depth buffer.

GLUT_STENCIL

Bit mask to select a window with a stencil buffer.

GLUT_MULTISAMPLE

Bit mask to select a window with multisampling support. If multisampling is not available, a non-multisampling window will automatically be chosen. Both the OpenGL client-side and server-side implementations must support the GLX_SAMPLE_SGIS extension for multisampling to be available.

GLUT_STEREO

Bit mask to select a stereo window.

GLUT_LUMINANCE

Bit mask to select a window with a luminance color model. This model provides the functionality of OpenGL's RGBA color model, but the green and blue components are not maintained in the frame buffer.1 Instead each pixel2's red component is converted to an index between zero and glutGet(GLUT_WINDOW_COLORMAP_SIZE) - 1 and looked up in a per window color map to determine the color of pixels2 within the window. The initial colormap of GLUT_LUMINANCE windows is initialized to be a linear gray ramp, but can be modified with GLUT's colormap routines.

GLUT_LUMINANCE is not supported on most OpenGL platforms.

void glutReshapeWindow(int width, int height);

Requests a change to the size of the current window. It disables the full screen status of a window if previously enabled.

void glutKeyboardFunc(void (*func)(unsigned char key, int x, int y))

Requires a callback function which takes the ASCII data alongside the mouse coordinates (x and y) when the key is pressed.

glutGetModifiers can be used to determine the state of modifier keys.

void glutDisplayFunc(void (*func)(void));

GLUT determines when the display callback should be triggered based on the window's redisplay state. The redisplay state for a window can be either set explicitly by calling glutPostRedisplay or implicitly as the result of window damage reported by the window system. Multiple posted redisplays for a window are coalesced by GLUT to minimize the number of display callbacks called.

void glutReshapeFunc(void (*func)(int width, int height));

A reshape call back is also triggered alongside the first display call back.

void glutIdleFunc(void (*func)(void));

Sets the global idle callback to be func so a GLUT program can perform background processing tasks or continuous animation when window system events are not being received. If enabled, the idle callback is continuously called when events are not being received.

void glutMainLoop(void);

glutMainLoop enters the GLUT event processing loop. This routine should be called at most once in a GLUT program. Once called, this routine will never return. It will call as necessary any callbacks that have been registered.

void glutSwapBuffers(void);

An implicit glFlush is done by glutSwapBuffers before it returns.

References

  1. Read more about frame buffer. ↩

  2. Read more about pixels. ↩↩