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vbuser1338

OpenGL Lighting Problem

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I just got Beginning OpenGl game programming and I just can't seem to get lighting to work. I have been working on creating a what I thought would be a simple spot light for over 2 hours now and can't seem to figure out why its not working. I have a green quad at 0,0,0 and want to shine a spot light on it. So I place my camera 20 units out of the screen and expect it to shine. But the thing is if my cut-off value is less than 20 then you just can't see the shape and even if it is higher it still ends up being really dark black. I think I am setting it up right but I don't know why it won't work.
bool SetupGL(int width,int height) {
	glClearColor(0.0f, 0.0f, 0.0f, 0.5f); // Black Background				// Enable Depth Testing
	glClearDepth(1.0f);		    
	glEnable(GL_DEPTH_TEST);    // Allow depth testing
	glDepthFunc(GL_LEQUAL);	    // Type of depth testing	

	glViewport(0,0,(GLsizei)(width),(GLsizei)(height));	//Set up viewport
	glMatrixMode(GL_PROJECTION);	// Switch to projection to set up view
	glLoadIdentity();	// Reset it
	gluPerspective(60.0f,(GLfloat)width/(GLfloat)height,0.1f,400.0f);   // Apply the perspective
	
	glMatrixMode(GL_MODELVIEW);	//switch to modelview
	glLoadIdentity();	//Reset it
	
	GLfloat pos [] = { 0.0f,0.0f,20.0f,1.0f };   // Set the position
	GLfloat dir [] = { 0.0f,0.0f,-1.0f };        // Direction pointing into screen
	GLfloat color [] = { 0.2f,0.2f,0.2f,1.0f }; // Set the color
	
	// Set up light
	glEnable(GL_LIGHTING);  // Enable Lighting
	glEnable(GL_LIGHT0);    // Enable light0
	glLightfv(GL_LIGHT0,GL_POSITION,pos);    // Set the position
	glLightfv(GL_LIGHT0,GL_AMBIENT,color);  // Set the type of light
	glLightfv(GL_LIGHT0,GL_DIFFUSE,color);
	glLightfv(GL_LIGHT0,GL_SPECULAR,color);
	glLightfv(GL_LIGHT0,GL_SPOT_DIRECTION,dir);  // Set the direction
	glLightf(GL_LIGHT0,GL_SPOT_CUTOFF,20.0f);   // Set the cutoff
	
	return true;
}

That is my init code
void Render() {
    glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);	// Clear the buffers
    glLoadIdentity();   // Reset the view
    
    gluLookAt(0.0f,0.0f,20.0f,0.0f,0.0f,0.0f,0.0f,1.0f,0.0f);    // Set up camera
    
    glBegin(GL_QUADS);
        glColor3f(0.0f,1.0f,0.0f);      // Green
        glNormal3f(0.0f,0.0f,1.0f);     // Point out
        glVertex3f( 5.0f, 5.0f,0.0f);   // Top Right
        glVertex3f(-5.0f, 5.0f,0.0f);   // Top Left
        glVertex3f(-5.0f,-5.0f,0.0f);   // Bottom Left
        glVertex3f( 5.0f,-5.0f,0.0f);   // Bottom Right
    glEnd();

    SDL_GL_SwapBuffers(); // Swap the buffers
}

and That is my rendering code. Can anyone help me try to figure this out. Thanks, vbuser

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GL lighting is vertex-based. That is, it only computes the color for each vertex, and then interpolates it across the primitive. Since all you have is a single large quad, it only computes the lighting in the 4 vertices, not "inside" it. If you want more detailed lighting, you must tesselate the polygon, that is, divide it into NxN smaller quads. The more you tesselate it, the better it will look. There are techniques which override GL lighting and perform lighting per-pixel, but since you're a beginner you'd better off experimenting with traditional GL lighting for now.

Also, one other thing. The light position specified with glLightf(...) is transformed by the current modelview matrix, just like any vertex, and stored in eye space. For that reason, you must update the light position in each frame after the camera setup code- in your case, right after gluLookAt(...)

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