Important: The standard way to include the OpenGL headers is to include the headers
#include <GL/gl.h>
#include <GL/glu.h>
However, to ensure portability you should not do this; use
the following inclusion instead:
#include <gfx/gl.h>
The primary reason for this is that the Microsoft OpenGL headers do
not work properly unless you have included <windows.h>
first. This libgfx header takes care of the necessary
#ifdefs and keeps your code looking cleaner.
#include <gfx/gltools.h>
Given a pixel coordinate in the OpenGL viewport, it is often necessary to project this to a 3-D point in the world being displayed. You can use the function
int unproject_pixel(int *pixel, double *world, double z=0.0);
to accomplish this.
To simplify the display of 3-D scenes, the function
void camera_lookat(const Vec3& min, const Vec3& max, double aspect);
will set up a standard viewing geometry for displaying an object
bounded by the axis-aligned box [min, max] in a
window whose aspect ratio is aspect.
The camera will be looking at the center of the box, from a position
further along the z axis, with a 60° field of view.
The viewing transform will be multiplied into the current matrix,
using calls to gluPerspective() and gluLookAt().
Errors during OpenGL processing are not reported to the user, they are merely flagged in the current OpenGL state. To check for and report any OpenGL errors, you can call the function:
void check_opengl_errors(const char *msg=NULL);
An optional message can be provided which will be prepended to the error
reported (if any).
Interactive programs frequently need to allow the user to select individual components of the scene being displayed by clicking on the rendered image. Given a pixel location in the window, the application must determine which entity the user clicked on. The standard technique for doing this with OpenGL is with the selection buffer. The libgfx library provides some utility functions to make using the selection buffer somewhat easier. Details on using the selection buffer can be found in the OpenGL Programming Guide.
Before drawing your primitives, call the function
void begin_opengl_pick(int *ctr, double radius, GLuint *buf, int size);
with the location of the user's pointer (ctr) and
the radius of the region to consider. You will also need to allocate
and pass a buffer of object identifiers to hold the candidate objects.
Having set things up, you can draw your primitives as usual, assigning an
integer identifier to each using the glLoadName() function.
After all primitives have been drawn, call the function
GLuint complete_opengl_pick(GLuint *buffer);
The returned value will be the identifier of the object clicked on, or
opengl_pick_nil if the user clicked on the background.