Simple GLSL Spotlight Shader - opengl

Question

I need help with a simple spotlight shader.
All vertices inside the cone should be colored yellow, all vertices outside the cone should be colored black.
I just can't get it work. I asume it has something to do with the transformation from world into eye coordinates.
Vertex Shader:
uniform vec4 lightPositionOC; // in object coordinates
uniform vec3 spotDirectionOC; // in object coordinates
uniform float spotCutoff; // in degrees
void main(void)
{
vec3 lightPosition;
vec3 spotDirection;
vec3 lightDirection;
float angle;
gl_Position = gl_ModelViewProjectionMatrix * gl_Vertex;
// Transforms light position and direction into eye coordinates
lightPosition = (lightPositionOC * gl_ModelViewMatrix).xyz;
spotDirection = normalize(spotDirectionOC * gl_NormalMatrix);
// Calculates the light vector (vector from light position to vertex)
vec4 vertex = gl_ModelViewMatrix * gl_Vertex;
lightDirection = normalize(vertex.xyz - lightPosition.xyz);
// Calculates the angle between the spot light direction vector and the light vector
angle = dot( normalize(spotDirection),
-normalize(lightDirection));
angle = max(angle,0);
// Test whether vertex is located in the cone
if(angle > radians(spotCutoff))
gl_FrontColor = vec4(1,1,0,1); // lit (yellow)
else
gl_FrontColor = vec4(0,0,0,1); // unlit(black)
}
Fragment Shader:
void main(void)
{
gl_FragColor = gl_Color;
}
Edit:
Tim is right. This
if(angle > radians(spotCutoff))
should be:
if(acos(angle) < radians(spotCutoff))
New question:
The light seems not to stay at a fixed position in the scene, instead it seems to move relative to my camera as the cone gets smaller or bigger when I move forward or backward.

Answer 1

(Let spotDirection be vector A, and lightDirection be vector B)
You are assigning;
angle = dot(A,B)
Shouldn't the formula be:
cos(angle) = dot(A,B)
or
angle = arccos(dot(A,B))
http://en.wikipedia.org/wiki/Dot_product#Geometric_interpretation

Answer 2

in my old shader I used that code:
float spotEffect = dot(normalize(gl_LightSource[0].spotDirection.xyz),
normalize(-light));
if (spotEffect < gl_LightSource[0].spotCosCutoff)
{
spotEffect = smoothstep(gl_LightSource[0].spotCosCutoff-0.002,
gl_LightSource[0].spotCosCutoff, spotEffect);
}
else spotEffect = 1.0;
instead of sending Angles to the shader it is better to send Cos of those Angles

Answer 3

To answer your new question, here is a link to a similar question: GLSL point light shader moving with camera.
The solution is to remove gl_NormalMatrix and gl_ModelViewMatrix.
spotDirection = normalize(spotDirectionOC * gl_NormalMatrix);
Would become:
spotDirection = normalize(spotDirectionOC);

Answer 4

https://code.google.com/p/jpcsp/source/browse/trunk/src/jpcsp/graphics/shader.vert?r=1639
if (spotEffect >= cos(radians(uLightOuterCone[index])))
and
//vec3 NSpotDir = (uViewMatrix * vec4(uLightDirection[index],0)).xyz; //must do outside or become flashlight follow
vec3 NSpotDir = normalize(uLightDirection[index]);