Home/ Questions/Q 6712733
In Process

The Archive Base Latest Questions

Editorial Team
  • 0
Asked: 2026-05-26T08:17:40+00:00

I’m having troubles with creating a rectangular surface that has a cone-like spotlight pointing

  • 0

I’m having troubles with creating a rectangular surface that has a cone-like spotlight pointing on it. So far, I’ve tried to create a 2D cube surface (glutSolidCube()) and use GL_SPOT_XXX to create the spotlight. But I don’t for what reason, the light just keeps scattering all over the surface instead of gather into a spot.

So here is my code so far, you can just focus on the init_light() and display() functions. The other functions are there for you to reference if you need to 🙂

void rotateCamera()
{
    /* Rotate camera 2 degrees about selected axis */
    theta[axis] += dtheta;
    if (theta[axis] > 360.0 ) theta[axis] -= 360.0;
    glutPostRedisplay();
}

void computeCameraPosition() {
    GLdouble M[16];

    glMatrixMode(GL_MODELVIEW);
    glPushMatrix();
    glLoadIdentity();
    glTranslatef(centroid[0], centroid[1], centroid[2]);
    glRotatef(theta[0], 1.0, 0.0, 0.0);
    glRotatef(theta[1], 0.0, 1.0, 0.0);
    glRotatef(theta[2], 0.0, 0.0, 1.0);
    glTranslatef(-centroid[0], -centroid[1], -centroid[2]);
    glGetDoublev(GL_MODELVIEW_MATRIX, M);

    GLdouble newvec[16];

    glMultMatrixd(init_vec);
    glGetDoublev(GL_MODELVIEW_MATRIX, newvec);
    glPopMatrix();

    /* the 1st and 2nd columns are the eye position and new y-axis direction */
    memcpy(eye, newvec, sizeof(GLdouble)*4);
    memcpy(yaxis, newvec+4, sizeof(GLdouble)*4);
}

void init_light()
{
    GLfloat spot_direction[] = { 1.0, 1.0, 0.0 };
    glLightfv( GL_LIGHT0, GL_SPOT_DIRECTION, spot_direction );
    glLightf( GL_LIGHT0, GL_SPOT_EXPONENT, 2.0 );
    glLightf( GL_LIGHT0, GL_SPOT_CUTOFF, 45.0 );

    glShadeModel(GL_SMOOTH);
    glEnable(GL_LIGHTING);
    glEnable(GL_LIGHT0);
    glEnable(GL_COLOR_MATERIAL);
}


void display() {
    computeCameraPosition();
    glClearColor(0.9, 0.9, 0.9, 1.0);
    glClear (GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
    glMatrixMode(GL_MODELVIEW);
    glLoadIdentity();
    gluLookAt(eye[0], eye[1], eye[2], 
          centroid[0], centroid[1], centroid[2], 
          yaxis[0], yaxis[1], yaxis[2]); 
    glutSolidCube(2.0);
    glFlush();
    glutSwapBuffers();
}

int main(int argc, char **argv) {
    glutInit(&argc, argv);
    glutInitDisplayMode (GLUT_SINGLE | GLUT_RGB | GLUT_DEPTH);
    glutInitWindowSize(800, 800);
    glutInitWindowPosition (200, 100);
    glutCreateWindow("Scene Editor");
    init_light();
    glutDisplayFunc(display);
    glutMainLoop();
}
Share

Leave an answer

You must login to add an answer.

Need An Account,

1 Answer

  1. Editorial Team

    Standard fixed function OpenGL illumination is only evaluated at the vertex positions and the determined colours interpolated over the primitives. So get a spotlight effect, the illumination must be evaluated with high resolution. You can achieve this by either tesselating your plane, or by using a fragment shader that does per fragment lighting.

    EDIT to answer comment

    Tesselation

    Tesselation means, subdividing your plane into a large number of “patches”. Like this:

    +---------------------+
|                     |
|                     |
|                     |
|                     |
|                     |
|                     |
|                     |
|                     |
|                     |
|                     |
+---------------------+

    →

    +-+-+-+-+-+-+-+-+-+-+-+
| | | | | | | | | | | |
+-+-+-+-+-+-+-+-+-+-+-+
| | | | | | | | | | | |
+-+-+-+-+-+-+-+-+-+-+-+
| | | | | | | | | | | |
+-+-+-+-+-+-+-+-+-+-+-+
| | | | | | | | | | | |
+-+-+-+-+-+-+-+-+-+-+-+
| | | | | | | | | | | |
+-+-+-+-+-+-+-+-+-+-+-+
| | | | | | | | | | | |
+-+-+-+-+-+-+-+-+-+-+-+

    Per fragment lighting

    Per fragment lighting means, that the lighting equation is evaluated at each rasterized fragment (a fragment is what’s commonly referred to a pixel). For this a fragment shader is used, i.e. a small program load into the graphics processor (GPU), that’s executed for each fragment/pixel of the rasterized primitive (triangle, line, point).

    Lighthouse3D has a tutorial about it: http://www.lighthouse3d.com/tutorials/glsl-tutorial/spot-light-per-pixel/ I recommend you read the full GLSL tutorial there, to get an idea, what’s going on.

    • 0