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iAnorak

OpenGL GLSL - per pixel lighting

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I've been following this tutorial for creating per-pixel spot lights, and both shading files seem to work fine if the camera is static. However, when I rotate the camera, the shadowing changes, which shouldn't be happening (I think). Camera rotation is done via the mouse (changing cameraRotate). Here's the basic pseudocode of what I am doing: render function:
glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
glLoadIdentity();

glTranslated(0.0, 0.0, -10.0);
glRotated(cameraRotate[0], 1.0, 0.0, 0.0);
glRotated(cameraRotate[1], 0.0, 1.0, 0.0);

glLightfv(GL_LIGHT0, GL_POSITION, lpos);
drawLight();

glutSolidTeapot(1);

glFlush();
glutSwapBuffers();
Here are the vertex and fragment shader functions (copied directly from the tutorial). vertex shader:
varying vec4 diffuse,ambientGlobal, ambient;
varying vec3 normal,lightDir,halfVector;
varying float dist;

void main() {
  vec4 ecPos;
  vec3 aux;
	
  /* first transform the normal into eye space and normalize the result */
  normal = normalize(gl_NormalMatrix * gl_Normal);
	
  /* now normalize the light's direction. Note that according to the
  OpenGL specification, the light is stored in eye space.*/
  ecPos = gl_ModelViewMatrix * gl_Vertex;
  aux = vec3(gl_LightSource[0].position-ecPos);
  lightDir = normalize(aux);
	
  /* compute the distance to the light source to a varying variable*/
  dist = length(aux);

  /* Normalize the halfVector to pass it to the fragment shader */
  halfVector = normalize(gl_LightSource[0].halfVector.xyz);
	
  /* Compute the diffuse, ambient and globalAmbient terms */
  diffuse = gl_FrontMaterial.diffuse * gl_LightSource[0].diffuse;
  ambient = gl_FrontMaterial.ambient * gl_LightSource[0].ambient;
  ambientGlobal = gl_LightModel.ambient * gl_FrontMaterial.ambient;

  gl_Position = ftransform();	
}
fragment shader:
varying vec4 diffuse,ambientGlobal, ambient;
varying vec3 normal,lightDir,halfVector;
varying float dist;

void main()
{
  vec3 n,halfV;
  float NdotL,NdotHV;
  vec4 color = ambientGlobal;
  float att,spotEffect;
	
  /* a fragment shader can't write a verying variable, hence we need
  a new variable to store the normalized interpolated normal */
  n = normalize(normal);
	
  /* compute the dot product between normal and ldir */
  NdotL = max(dot(n,normalize(lightDir)),0.0);

  if (NdotL > 0.0) {
    spotEffect = dot(normalize(gl_LightSource[0].spotDirection), normalize(-lightDir));

    if (spotEffect > gl_LightSource[0].spotCosCutoff) {
      spotEffect = pow(spotEffect, gl_LightSource[0].spotExponent);
      att = spotEffect / (gl_LightSource[0].constantAttenuation + gl_LightSource[0].linearAttenuation * dist + gl_LightSource[0].quadraticAttenuation * dist * dist);
				
      color += att * (diffuse * NdotL + ambient);

      halfV = normalize(halfVector);
      NdotHV = max(dot(n,halfV),0.0);
      color += att * gl_FrontMaterial.specular * gl_LightSource[0].specular * pow(NdotHV,gl_FrontMaterial.shininess);
    }
  }

  gl_FragColor = color;
}
Any help would be greatly appreciated!

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Are you sure it's not caused by the specular color of the material/light? The specular reflection can cause some weird effects in combination with camera motion if you're not careful about the values you use...

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Quote:
Original post by kloffy
Are you sure it's not caused by the specular color of the material/light? The specular reflection can cause some weird effects in combination with camera motion if you're not careful about the values you use...


I think I found out what is causing my confusion. The light is being drawn with respect to the camera's position. So if I zoom the camera out, the light will draw a broader spot light; if I zoom the camera in, the light will draw a more narrow spot light on the objects. Any idea how I can fix this? It seems like a simple problem.

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When you set a light position with glLight, the position is multiplied by the current modelview matrix. So, in order to set it in world coordinates, you must have your modelview set to the identity matrix when you set your light position (with glLight).

glLight man page

-Riku

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Quote:
Original post by Rasmadrak
Put the light position before translations and rotations - unless you want to move the light?


I would like the ability to move the light, and also so that if the camera moves the light will stay in the same spot (relative to the world). This would be done by setting the light after the camera rotations/translations have been done.

I guess what I am really asking how to do is deal with the camera in GLSL. I've been reading that I have to pass in the camera inverse matrix, then base my calculations off of that? Are there any tutorials on how I can do that?

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I had a similar problem a while back, I send in the light position and camera position as uniform's so no matrix transformations affect the light position.

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Quote:
Original post by iAnorak
Quote:
Original post by Rasmadrak
Put the light position before translations and rotations - unless you want to move the light?

I would like the ability to move the light, and also so that if the camera moves the light will stay in the same spot (relative to the world). This would be done by setting the light after the camera rotations/translations have been done.
Nope, it doesn't work quite like that. You need to set the light position (in world coordinates) *before* you apply the camera transform. You can move the light around by changing its position, but if you apply the camera transform first, it ends up in eye-space; i.e. it will move relative to the camera.

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