# problem with spherical point sprite shading

Hi everyone,

I’m rendering spheres as points (I’m computing normals in fragment shader)
I have a scene with a sphere, and a directional light. I can rotate and move camera around scene.
The problem I have is that when I rotate camera the sphere shading changes - I mean the sphere diffuse intensity changes becouse the angle between lightDirection vector and sphere normal changes.
I can’t just multiply normals by camera rotation matrix, as I do with light direction vector becouse this will cause the shading of sphere to be constant regardless of the camera position.
I want the shading of sphere to change accordingly to camera position and not to change when camera is rotated.
So my question is how should I transform normals to achieve that?

that is how I do it right now:

``````
uniform mat4 projectionMatrix;
uniform mat4 modelViewMatrix;

in vec4 vertex;

void main()
{
gl_Position = projectionMatrix * modelViewMatrix * v;
}

``````

``````
uniform vec3 lightDirection; // this is already multiplied by camera rotation matrix
out vec4 fragColor;

void main()
{
vec3 normal;
normal.xy = gl_PointCoord.xy * vec2(2.0, -2.0) + vec(-1.0, 1.0);
float mag = dot(normal.xy, normal.xy);

if (mag > 1.0f)

normal.z = sqrt(1.0f - mag);
normal = normalize(normal);

float diffuseIntensity = max(0.0f, dot(normal, lightDir));

fragColor = vec4(0.1f, 0.1f, 0.1f, 1.0f) * diffuseIntensity * vec4(1.0f, 0.0f, 0.0f, 1.0f);
}

``````

Change this

``````normal.xy = gl_PointCoord.xy * vec2(2.0, -2.0) + vec(-1.0, 1.0);

``````

to this

``````
normal.xy = (gl_PointCoord.xy * 2.0) - vec2(1.0);

``````

and also remove the normalization

``````normal = normalize(normal);
``````

Ok, thanks - it has slightly better performance now, but operations you suggested don’t remove my problem.

here are two screenshots from my application, ligt direction vector is (-1, -1, -1), and on both camera is in the same position but it’s rotation is different:  I don’t want shading to change when camera rotates.

I don’t want shading to change when camera rotates

Then don’t multiply the light dir with the camera rotation matrix.

but then shading won’t change if I’ll translate camera

but then shading won’t change if I’ll translate camera

It’s a directional light; translation cannot affect it.

I know, but I need light direction vector to be in camera space (I’m only multiplying it by rotation part of camera matrix).

I’ll try to explain my problem more:

I’m using frame (forward and up vectors and position) to represent camera.
I generate normals so that the central pixel of point (sphere) has normal (0.0, 0.0, 1.0) in camera space.
Normals I generate in fragment shader are already in camera space so I need only to transform light direction to camera space.
Everything is fine until forward vector points directly to the center of the sphere.
But if I rotate camera, light direction coordinates in camera space changes and normal coordinates don’t (so central pixel still has (0, 0, 1)). Thus
angle between normal and light changes.

here is a picture that illustrates this: simply not transforming light direction into camera space is not a solution for me, becouse as I said, when I change camera position and look at sphere, I want the shading to change.