Why does my GLSL shader lighting move around the scene with the objects it's shining on?

Question

I'm following a tutorial on OpenGL ES 2.0 and combining it with a tutorial on GLSL lighting that I found, using a handy Utah teapot from developer.apple.com.

After a lot of fiddling and experimentation I have the teapot drawn moderately correctly on the screen, spinning around all three axes with the 'toon shading' from the lighting tutorial working. There's a few glitches in the geometry due to me simply drawing the whole vertex list as triangle strips (if you look in the teapot.h file there are '-1' embedded where I'm supposed to start new triangle strips, but this is only test data and not relevant to my problem).

The bit I am really confused about is how to position a light in the scene. In my Objective-C code I have a float3 vector that contains {0,1,0} and pass that into the shader to then calculate the intensity of the light.

Why does the light appear to move in the scene too? What I mean is the light acts as though it's attached to the teapot by an invisible stick, always pointing at the same side of it no matter what direction the teapot is facing.

This is the vertex shader

attribute vec4 Position;
attribute vec4 SourceColor;
attribute vec3 Normal;

uniform mat4 Projection;
uniform mat4 Modelview;

varying vec3 normal;

void main(void) {
    normal = Normal;
    gl_Position = Projection * Modelview * Position;
}

'Position' is set by the Obj-C code and is the vertices for the object, 'Normal' is the list of normals both from a vertex array (VBO), 'Projection' and 'Modelview' are calculated like this:

(A CC3GLMatrix is from the Cocos3D library, mentioned in the GLES tutorial linked above)

CC3GLMatrix *projection = [CC3GLMatrix matrix];
float h = 4.0f * self.frame.size.height / self.frame.size.width;
[projection populateFromFrustumLeft:-2 andRight:2 andBottom:-h/2 andTop:h/2 andNear:1 andFar:100];
glUniformMatrix4fv(_projectionUniform, 1, 0, projection.glMatrix);

CC3GLMatrix *modelView = [CC3GLMatrix matrix];
[modelView populateFromTranslation:CC3VectorMake(0, 0, -7)];
[modelView scaleBy:CC3VectorMake(30, 30, 30)];

_currentRotation += displayLink.duration * 90;
[modelView rotateBy:CC3VectorMake(_currentRotation, _currentRotation, _currentRotation)];

glUniformMatrix4fv(_modelViewUniform, 1, 0, modelView.glMatrix);

And I set the light in the scene by doing

float lightDir[] = {1,0,1};
glUniform3fv(_lightDirUniform, 1, lightDir);

The fragment shader looks like this

varying lowp vec4 DestinationColor; // 1
varying highp vec3 normal;
uniform highp vec3 LightDir;


void main(void) {
    highp float intensity;
    highp vec4 color;
    intensity = dot(LightDir,normal);
    if (intensity > 0.95)
        color = vec4(1.0,0.5,0.5,1.0);
    else if (intensity > 0.5)
        color = vec4(0.6,0.3,0.3,1.0);
    else if (intensity > 0.25)
        color = vec4(0.4,0.2,0.2,1.0);
    else
        color = vec4(0.2,0.1,0.1,1.0);

    gl_FragColor = color;
}

While trying to work this out I come across code that references the (non-existant in GLES) 'gl_LightSource' and 'gl_NormalMatrix' but don't know what to put into equivalents I have to pass into the shaders from my code. The references to 'eye space' 'camera space' 'world space' and so on are confusing, I know I should probably be converting things between them but don't understand why or how (and where - in code, or in the shader?)

Every frame do I need to modify the light source? The code I have for setting it looks too simplistic. I'm not really moving the teapot around, am I, instead I'm moving the entire scene - light and all around?

Answer 1

First of all some definitions:

• world space: the space your whole world is defined in. By convention it is a static space that never moves.

• view space/camera space/eye space: the space your camera is defined in. it is usually a position and rotation relative to world space

• model space: the space your model is defined in. Like camera space, it is usually a position and rotation relative to world space

• light space: same as model space

In simple examples (and i guess in your's) model space and world space are the same. In addition OpenGL by itself doesn't have a concept of world space, which doesn't mean you cannot use one. It comes in handy when you want to have more than one object moving around independently in your scene.

Now, what you are doing with your object before rendering is creating a matrix that transforms the vertices of a model into viewspace, hence 'modelViewMatrix'.

With light in this case it's a little different. Light calculation in your shader is done in modelspace, so you have to transform your lightposition every frame into modelspace.

This is done by calculating something like:

_lightDirUniform = inverseMatrix(model) * inverseMatrix(light) * lightPosition;

The lightposition is transformed from light into world and then into model space. If you don't have a world space, just leave out the model space transformation and you should be fine.