Hello all,

I have the following function within a vertex shader that I got from “Egon Rath’s Notes” on www.matrix44.com. Unfortunately, I can’t seem to get it to work.

```
uniform mat4 mProjection;
uniform mat4 mView;
uniform mat4 mModel;
uniform vec4 EyePosition;
uniform vec4 LightPosition;
uniform vec4 LightColor;
// The Colors of the material
uniform vec4 AmbientMaterial;
uniform vec4 DiffuseMaterial;
uniform vec4 SpecularMaterial;
uniform vec3 Drawing_Color;
varying vec3 lighted_color;
vec4 GetSpecularColor()
{
// Transform the Vertex and corresponding Normal into Model space
vec4 trans_normal = mModel * vec4( gl_Normal, 1 );
vec4 trans_vertex = mModel * gl_Vertex;
// Get the directional vector to the light and to the camera
// originating from the vertex position
vec4 light_direction = normalize( LightPosition - trans_vertex );
vec4 camera_direction = normalize( EyePosition - trans_vertex );
// Calculate the reflection vector between the incoming light and the
// normal (incoming angle = outgoing angle)
// We have to use the invert of the light direction because "reflect"
// expects the incident vector as its first parameter
vec4 reflection = reflect( -light_direction, trans_normal );
// Calculate specular component
// Based on the dot product between the reflection vector and the camera
// direction.
//
// hint: The Dot Product corresponds to the angle between the two vectors
// hint: if the angle is out of range (0 ... 180 degrees) we use 0.0
float spec = pow( max( 0.0, dot( camera_direction, reflection )), 32 );
return vec4( SpecularMaterial.r * spec, SpecularMaterial.g * spec, SpecularMaterial.b * spec, 1.0 );
}
```

I set the uniforms like this:

```
MyViewMatrix.Get_Current_Matrix(viewMatrix);
MyModelMatrix.Get_Current_Matrix(modelMatrix);
pglUniformMatrix4fv(view_matrix_location, 1, false, &viewMatrix[0]);
pglUniformMatrix4fv(model_matrix_location, 1, false, &modelMatrix[0]);
camera_position[0] = 0.0f;
camera_position[1] = 0.0f;
camera_position[2] = 10.0f;
camera_position[3] = 0.0f;
pglUniform4fv(camera_position_location,1,camera_position);
pglUniform3fv(draw_color_location,1,m_current_color);
pglUniform4fv(light_position_location,1,light_position);
pglUniform4fv(light_color_location,1,light_color);
pglUniform4fv(ambient_material_location,1,ambient_material_color);
pglUniform4fv(diffuse_material_location,1,diffuse_material_color);
pglUniform4fv(specular_material_location,1,specular_material_color);
```

. . . and do the actual rendering with:

```
MyModelMatrix.Rotate(30.0f,1.0f,0.0f,0.0f);
MyModelMatrix.Rotate(frame*8,0.0f,1.0f,0.0f);
glBegin(GL_POLYGON);
glNormal3f( 0.0f, 0.0f, 1.0f);
glVertex3f( 1.5, -1.5, 1.5 );
glVertex3f( 1.5, 1.5, 1.5 );
glVertex3f( -1.5, 1.5, 1.5 );
glVertex3f( -1.5, -1.5, 1.5 );
glEnd();
```

Unfortunately, the light appears at weird angles. I’ve fiddled with it for almost two days and can’t make heads or tails of where the problem is. To make progress in debugging, my first two questions are:

- Is there anything obviously wrong with the vertex shader function?
- Is there anything wrong with the way I set the data for the light position and camera position? (I suspect these two aspects of the process the most.)

Let me know if there is any other information I can give. I’ve attached an image.