Question

I have been following an opengl tutorial, but for some reason when i added the reshape function(for window resize to fix ratio). the opengl window doesnt show the object(shape) at all, just a black screen.
i dont know if i made a misspelling or something.

Is the reshape function messing things up?

#include <GL/gl.h>
#include <GL/glut.h>

void renderScene(void);
void changeSize(int w, int h);

int main(int argc, char **argv){

// init GLUT and create window
glutInit(&argc, argv);
glutInitDisplayMode(GLUT_DEPTH | GLUT_DOUBLE | GLUT_RGBA);
glutInitWindowPosition(100,100);
glutInitWindowSize(320,320);
glutCreateWindow("Testing");

// register callbacks
glutDisplayFunc(renderScene);

// animation in reshaping
glutReshapeFunc(changeSize);

// enter GLUT event processing cycle
glutMainLoop();

return 1;
}

void renderScene(void){

glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);

glBegin(GL_TRIANGLES);
    glVertex3f(-0.5,-0.5,0.0);
    glVertex3f(0.5,0.0,0.0);
    glVertex3f(0.0,0.5,0.0);
glEnd();

glutSwapBuffers();
}

void changeSize(int w, int h){

if (h==0)
    h = 1;
float ratio = 1.0*w/h;

// use the projection matrix
glMatrixMode(GL_PROJECTION);

//reset matrix
glLoadIdentity();

// set the viewpoint to be the entire window
glViewport(0,0,w,h);

// set the correct perspective
gluPerspective(45, ratio, 1, 1000);

// get back to the modelview
glMatrixMode(GL_MODELVIEW);
}

Thanks for any help!
This is from the Lighthouse3d tutorials.

PS: if i take out the reshape function, the triangle shows up(works fine).

Answer 1

Your triangle, as is, is not visible from that projection matrix you have defined with gluPerspective.

That matrix defines a view cone, pointing in the -z direction, with a near plane at z=-1 and far plane at z=-1000.

Your triangle lays on the z=0 plane, and as such is not visible.

You can either:

1. Move the triangle further back on the z direction (glTranslate(0,0,-2), or move the vertices manually).

2. Use a view matrix that moves the camera away from (0,0,0), such as with gluLookAt

3. Use an orthographic projection matrix that includes the z=0 plane, such as glOrtho(-1, 1, -1, 1, -1, 1)