Moving and rotating

What do you need to do to make a light move around? Think about how you would make any other object in the world move around. One way is to set the position of the object after you translate or rotate it. You can do the same thing with lights. When you call GL.Light to define the position or direction of a light, the information you specify is in local space. It will be transformed by the current modelview matrix.

Static Lights

For static lights, lights that never move it makes sense to position the light after you set up the camera (after calling LookAt for example) but before applying any other transformations to the modelview matrix. We rendered our reference grid like this. No matter where the objects in the scene or the camera moved, the reference grid is static at origin. Same thing would happen for lights.

Perspective Lights

How about perspective lights? What's a perspective light? It's a light that stays fixed relative to the eye (camera) position. For instance, when you sit in a car, the headlights are perspective lights. They stay in more or less the same position relative to your eyes. In this case you would set the modelview matrix to indentity, then define your light at the origin, and then continue transformations as you normally would


// Position the cars lights at origin, this can be anywhere,
// it doesn't have to be at origin. In the real world it would
// be up a few units on the z axis, and down a few on the y axis
float[] pos = {0,0,0,1}
GL.Light(LightName.Light0, LightParamater.Position, pos);

// Continue rendering as normal
Matrix4 lookAt = Matrix4.LookAt(eyePosition, lookTarget, new Vector3(0.0f, 1.0f, 0.0f));

Dynamic Lights

But what if you had a moving object, like a flash-light. It would need to be a child of a characters arm if it's being held (thik of Mr.Roboto and how we did his feet).


Matrix4 lookAt = Matrix4.LookAt(eyePosition, lookTarget, new Vector3(0.0f, 1.0f, 0.0f));

// Render character
GL.PushMatrix(); // Push Torso
    GL.Translate(10, 0, 0);
    GL.Rotate(10, 0, 1, 0);
    GL.Scale(1, 1, 1);
    GL.PushMatrix(); // Push Arm
        GL.Translate(0, 5, 0);
        GL.Rotate(2, 1, 0, 0);
        GL.Scale(1, 2, 1);

        // The arm is drawn, we need to nest the flashlight here. 
        // But we want to draw it at the bottom of the arm
        // so we need to translate down a bit
        GL.PushMatrix(); // Push Flashlight
            GL.Translate(0, 2, 0);
            // No need to rotate for this example
            GL.Scale(1, 0.5, 1);

            // Notice we set the light position and direction here! 
            // All other factors can be set elsewhere, but these two
            // rely on world position data, we MUST set them here!
            GL.Light(LightName.Light0, LightParamater.Position, lightPosition);
            GL.Light(LightName.Light0, LightParamater.SpotDirection, spotDirection);

        GL.PopMatrix(); // Pop Flashlight

    GL.PopMatrix(); // Pop Arm
    // ... Render rest of character
GL.PopMatrix(); // Pop Torso

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