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Heya everyone. In
keeping with my promises of the prior month, I thought I’d post a quick
article in my summer series. These will essentially be short blurbs on different
functions in Glut and OpenGL. This article will cover using the mouse functions
built into the Glut system.
Of Mice and Men
Using the mouse
with an OpenGL program, as you’ve probably guessed by now, is essentially just
a matter of registering another callback with the system. Here’s the call to
register the function with Glut:
glutMouseFunc(myMouse);
and the general
prototype for the mouse function is:
void
myMouse(int button, int state, int x, int y){
Every time a mouse
button is used, this function is called. It is passed the button pressed (GLUT_LEFT_BUTTON,
GLUT_RIGHT_BUTTON, GLUT_MIDDLE_BUTTON - note that for systems with only two
buttons, the GLUT_MIDDLE_BUTTON cannot be generated, and I don’t know the
effects of the one button (MAC) systems, as I am a computer bigot :-P), the type
of action (GLUT_DOWN, GLUT_UP), and the (x,y) position in screen coordinates.
For every GLUT_DOWN call, one can assume a GLUT_UP call, provided that the
callback is still registered. If a menu is registered to a button (this will be
covered in a future article), then that button will not call the callback
function. To remove all callbacks, make a call to glutMouseFunc(NULL);
To exemplify the
use of this function ,I’ll just include a generic mouse function from the
first graphics assignment I ever did, and let you work it for yourself. I’ll
integrate it with a program come the return of techtrax. Below is the sample
function:
void
myMouse(int button, int state, int x, int y){
int ry=480-y;
if
(button!=GLUT_LEFT_BUTTON && button!=GLUT_RIGHT_BUTTON)
{
return;
}
if(state==GLUT_DOWN
&& button==GLUT_LEFT_BUTTON){
glBegin(GL_POINTS);
glColor3f(1.0, 0.0, 0.0); //change color, then draw a point
glVertex2i(x,ry);
glEnd();
}
if(state==GLUT_DOWN
&& button==GLUT_RIGHT_BUTTON){
glBegin(GL_POINTS);
glColor3f(0.0, 0.0, 1.0); //change color, then draw a point
glVertex2i(x,ry);
glEnd();
}
glFlush();
}
Note how in order
to convert the screen coordinates I subtract the passed balue from 480 (the
window height for that program). This function only acts when the left or right
buttons are pressed, and only on the down press. Each mouse click creates a red
or blue point on the screen at the location of the click, based on the mouse
button pressed. The middle button is ignored.
One more quick note
- the command glFlush() is used instead of glPostRedisplay() because this
program was written using only a single buffer. In a single-buffered system, the
screen is updated in one fell swoop with a glFlush() call, no glPostRedisplay()/glSwapBuffers()
involved.
And that’s all
for now. As always, if you have any questions, drop me a line at evilstickman@evilstickman.com
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