CODEWITHSUNDEEP
c++openglglslshader
succulentLover
succulentLover

Reputation: 23

How is Phong Shading implemented in GLSL?

I am implementing a Phong Shader in GLSL for an assignment, but I don't get the same specular reflection as I should. The way I understood it, it's the same as Gouraud Shading, but instead of doing all the calculations in the vertex shader, you do them in the fragment shader, so that you interpolate the normals and then apply the phong model at each pixel. As part of the assignment I had to develop also the Gouraud shader and that works as supposed and I thought you just needed to put the vertex shader code into the fragment shader, but that doesn't seem to be the case.

What I do in the vertex shader is that I simply transform the vertex position into view coordinates and I apply the transpose inverse of the model view matrix to the vertex normal. Then, in the fragment shader I apply just the view transform to the light position and use these coordinates to calculate the vectors needed in the Phong model The lighting is almost correct, but some specular light is missing. All the parameters have been tested, so I would assume it's the light's position that is wrong, however I have no idea why my code wouldn't work when it does so in the other shader. Also, I know this isn't the most efficient way of doing it (pre-computing the normal matrix would be faster), I'm just trying to first get it to work properly.

Upvotes: 2

Views: 1685

Answers (1)

BDL
BDL

Reputation: 22168

The problem is the way how the light source position is calculated. The following code

vec3 l = normalize(mat3(VMatrix)*light);

treats light as a direction (by normalizing it and because the translation part of the view matrix is ignored), but it actually is a position. The correct code should be something like

vec3 l = (VMatrix * vec4(light, 1.0)).xyz

Upvotes: 2

Related Questions