OpenGL ES Light Question

Question

Hello,

I am new to OpenGL ES and are developing in Objective-C for iPhone.

What I am trying to understand is how light is positioned in my frustum. This is how I do it:

I create a square and put it as "floor" in my frustum. I am putting the light, right above, in the middle of the floor and point the direction of the light right down. Now, I expect the light circle on the floor to be in the middle. But it's not, it'r more to the back (futher away). How come?

near = 8.0
far = 28
light position = {0.0, 0.0, -10.0}
light direction = {0.0, -1.0, 0.0}

Floor coordinates = {-0.3153, -0.473, -20.0},
{-0.3153, -0.473, 0.0},
{0.3153, -0.473, -20.0},
{0.3153, -0.473, 0.0},

Shouldn't this together make the circle of light to be in the middle of the floor and not positioned a bit backwards?

Here's my significant code:

Setting up the view:

 -(void)setupView:(TDRoomView*)view
    {


    const GLfloat zNear = 8, zFar = 28, fieldOfView = 5; 

 GLfloat size; 
 glEnable(GL_DEPTH_TEST);
 glMatrixMode(GL_PROJECTION); 
 size = zNear * tanf(DEGREES_TO_RADIANS(fieldOfView) / 2.0); 
 NSLog(@"tanf(DEGREES_TO_RADIANS(fieldOfView)/2.0); %f ", tanf(DEGREES_TO_RADIANS(fieldOfView)/2.0));

 CGRect rect = view.bounds; 
 glFrustumf(-size, size, -size / (rect.size.width / rect.size.height), size / 
      (rect.size.width / rect.size.height), zNear, zFar);  
 glViewport(-size, size, rect.size.width, rect.size.height);  
 glMatrixMode(GL_MODELVIEW);
    glShadeModel(GL_SMOOTH);
 glEnable(GL_COLOR_MATERIAL);
    glEnable(GL_LIGHTING);

    glEnable(GL_LIGHT0);

 static const Color3D light0Ambient[] = {{1.0, 1.0, 1.0, 1.0}};
 glLightfv(GL_LIGHT0, GL_AMBIENT, (const GLfloat *)light0Ambient);

    static const Color3D light0Diffuse[] = {{0.5, 0.5, 0.5, 1.0}};
 glLightfv(GL_LIGHT0, GL_DIFFUSE, (const GLfloat *)light0Diffuse);

    static const Color3D light0Specular[] = {{0.5, 0.5, 0.5, 1.0}};
    glLightfv(GL_LIGHT0, GL_SPECULAR, (const GLfloat *)light0Specular);

    static const Vertex3D light0Position[] = {{0.0, 0.473, -10.0}};
 glLightfv(GL_LIGHT0, GL_POSITION, (const GLfloat *)light0Position); 

 static const Vertex3D lightVector= {0.0, -1.0, 0.0};

    glLightfv(GL_LIGHT0, GL_SPOT_DIRECTION, (GLfloat *)&lightVector);

    glLightf(GL_LIGHT0, GL_SPOT_CUTOFF, 85);

 glLoadIdentity(); 
 glClearColor(0.0f, 0.0f, 1.0f, 1.0f); 

    }

And drawing:

     -(void)drawView:(TDRoomView*)view
    {

        static const Vertex3D vertices[]= {
  {-0.3153, -0.473, -20.0},
  {-0.3153, -0.473, 0.0},
  {0.3153, -0.473, -20.0},
  {0.3153, -0.473, 0.0},
  {-0.1, -0.25, -3.0},
  {0.0, 0.0, -10.0},
  {0.1, -0.25, -3.0}
 };

 static const Color3D colors[] = {
  {0.7, 0.7, 0.7, 1.0},
  {0.7, 0.7, 0.7, 1.0},
  {0.7, 0.7, 0.7, 1.0},
  {0.7, 0.7, 0.7, 1.0},
  {1.0, 0.0, 0.0, 1.0},
  {0.0, 1.0, 0.0, 1.0},
  {0.0, 0.0, 1.0, 1.0}
       };


 static const GLubyte roomFaces[] = {
  0, 1, 2,
  1, 2, 3};


        static const Vector3D normals[] = {
  {0.000000, 1.000000, 0.000000},
  {0.000000, 1.000000, 0.000000},
  {0.000000, 1.000000, 0.000000},
  {0.000000, 1.000000, 0.000000},
  {0.000000, 1.000000, 0.000000},
  {0.000000, 1.000000, 0.000000},
  {0.000000, 1.000000, 0.000000}
 };

    glLoadIdentity();

    glClearColor(0.7, 0.7, 0.7, 1.0);
    glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
    glEnableClientState(GL_VERTEX_ARRAY);
 glEnableClientState(GL_COLOR_ARRAY);
 glEnableClientState(GL_NORMAL_ARRAY);

        glVertexPointer(3, GL_FLOAT, 0, vertices);
 glColorPointer(4, GL_FLOAT, 0, colors);
 glNormalPointer(GL_FLOAT, 0, normals);
 glDrawElements(GL_TRIANGLE_STRIP, 8, GL_UNSIGNED_BYTE, roomFaces);

        glDisableClientState(GL_NORMAL_ARRAY);
 glDisableClientState(GL_VERTEX_ARRAY);
 glDisableClientState(GL_COLOR_ARRAY); }

Also, I recon changing the

static const Vertex3D light0Position[] = {{0.0, 0.473, -10.0}};

to

static const Vertex3D light0Position[] = {{0.0, 0.473, -8.0}}; 

the bord turns dark, no light shines on it. Why, I only moved the light two steps closer...? The same things happen if I move it two steps further away like this:

static const Vertex3D light0Position[] = {{0.0, 0.473, -12.0}};

Any help in order to "shed som light" on my problem is greatly appreciated! (Couldn't help it... ;) )

Thanks in advance!

Answer 1

I suggest you do a larger grid with more vertices, preferably n by n squares. Try not to have too long thin triangles. This will make it easier to see where the light seem to be as the light is calculated for each vertex and not per pixel for the surface.

http://www.opengl.org/resources/features/KilgardTechniques/oglpitfall/

2. Poor Tessellation Hurts Lighting

OpenGL's lighting calculations are done per-vertex. This means that the shading calculations due to light sources interacting with the surface material of a 3D object are only calculated at the object's vertices. Typically, OpenGL just interpolates or smooth shades between vertex colors. OpenGL's per-vertex lighting works pretty well except when a lighting effect such as a specular highlight or a spotlight is lost or blurred because the effect is not sufficiently sampled by an object's vertices. Such under-sampling of lighting effects occurs when objects are coarsely modeled to use a minimal number of vertices.

:

Novice OpenGL programmers are often tempted to enable OpenGL's spotlight functionality and shine a spotlight on a wall modeled as a single huge polygon. Unfortunately, no sharp spotlight pattern will appear as the novice intended; you probably will not see any spotlight affect at all. The problem is that the spotlight's cutoff means that the extreme corners of the wall where the vertices are specified get no contribution from the spotlight and since those are the only vertices the wall has, there will be no spotlight pattern on the wall.

Answer 2

GL_POSITION is specified in homogeneous coordinates, which means that it requires four values, not three. You need to specify w = 1.0 for your light source, but your w component is instead coming from whatever happens to be next to light0Position in memory.

(Part of the reason that GL_POSITION is in homogeneous coordinates is to allow you to define directional lights by setting a position with w = 0.0.)