Simple shader example with of pointLight

beginners
#1

I’m quite confused regarding the behaviour of pointLight. I’m drawing a sphere and I’m applying a simple shader to it.

This is the setup of the scene

sphere.set(100, 100);
ofVec3f lightPos = ofVec3f(130,130,130);
light.setPointLight();
// see light docs http://openframeworks.cc/documentation/gl/ofLight/
// https://forum.openframeworks.cc/t/understanding-oflight/26230/3
light.setup();
light.enable();
light.setPosition(lightPos);

This is my vertex shader:

#version 150

uniform mat4 modelViewProjectionMatrix;
in vec4 position;
in vec4 normal;

out vec4 vPosition;
out vec3 vNormal;

void main() {
    vNormal = normal.xyz;
    vPosition = modelViewProjectionMatrix * position;
    gl_Position = vPosition;
}

and this is my fragment shader:

#version 150
uniform vec3 lightPos;
uniform vec4 materialColor;

in vec4 vPosition;
in vec3 vNormal;

out vec4 vFragColor;

void main() {
    vec4 color = vec4(materialColor);
    vec3 lightDirection = normalize(lightPos - vPosition.xyz);
    float dProd = max(0.3, dot(vNormal, lightDirection));
    vec4 colorWithLight = vec4( vec3( dProd ) * vec3( color ), 1.0 );
    vFragColor = colorWithLight;
}

The code works but as you see the reflection of the light looks wrong, the brightest part of the sphere is not facing the light

If I change the value of vPosition being passed to the fragment shader like this:

void main() {
    vNormal = normal.xyz;
    vPosition = position;
    gl_Position = modelViewProjectionMatrix * position;
}

The light looks correct

But now I’m passing the position of the vertext to the fragment shader without applying the model view projection matrix transformation, that feels weird.

I assume that when calculating the light direction:

vec3 lightDirection = normalize(lightPos - vPosition.xyz);

Both light position and vertex position has to be in the same coordinates space, but I do not know which is the way to go.

#2

that’s correct, for calculating lights you usually use world coordinates or eye coordinates (multiplying by modelViewMatrix) but not by the projection. The only thing you have to take into account is that everything is in the same coordinate space, in your second example both the light position and the vertex/fragment position are in world coordinates so it works while in the first the fragment position is already in clip space coordinates (projected in 2d) and the light in world coordinates

#3

Ok, thank you.