Why does my OpenGL Phong shader behave like a flat shader?

Question

I\'ve been learning OpenGL for the past couple of weeks and I\'ve run into some trouble implementing a Phong shader. It appears to do no interpolation between vertexes despi

Answer 1

Hmm... You're interpolating the normal as a varying variable, so the fragment shader should receive the correct per-pixel normal.

The only explanation (I can think of) of the fact that you're having the result as on your left image is that every fragment on a given face ultimately receives the same normal. You can confirm it with a fragment shader like:

void main() {
   vFragColor = normalize(vVaryingNormal);
}

If it's the case, the question remains: Why? The vertex shader looks OK.

So maybe there's something wrong in your geometry? What is the data which you send to the shader? Are you sure you have correctly calculated per-vertex normals, not just per-face normals?

The orange lines are normals of the diagonal face, the red lines are normals of the horizontal face.

If your data looks like the above image, then even with a correct shader you'll get flat shading. Make sure that you have correct per-vertex normals like on the lower image. (They are really simple to calculate for a sphere.)

Answer 2

As an addition to this answer, here is a simple geometry shader which will let you visualize your normals. Modify the accompanying vertex shader as needed based on your attribute locations and how you send your matrices.

But first, a picture of a giant bunny head from our friend the Stanford bunny as an example of the result !

Major warning: do note that I get away with transforming the normals with the modelview matrix instead of a proper normal matrix. This won't work correctly if your modelview contains non uniform scaling. Also, the length of your normals won't be correct but that matters little if you just want to check their direction.

Vertex shader:

#version 330

layout(location = 0) in vec4 position;
layout(location = 1) in vec4 normal;
layout(location = 2) in mat4 mv;

out Data
{
    vec4 position;
    vec4 normal;
    vec4 color;
    mat4 mvp;
} vdata;

uniform mat4 projection;

void main()
{
    vdata.mvp = projection * mv;
    vdata.position = position;
    vdata.normal = normal;
}

Geometry shader:

#version 330
layout(triangles) in;
layout(line_strip, max_vertices = 6) out;

in Data
{
    vec4 position;
    vec4 normal;
    vec4 color;
    mat4 mvp;
} vdata[3];

out Data
{
    vec4 color;
} gdata;

void main()
{
    const vec4 green = vec4(0.0f, 1.0f, 0.0f, 1.0f);
    const vec4 blue = vec4(0.0f, 0.0f, 1.0f, 1.0f);

    for (int i = 0; i < 3; i++)
    {
        gl_Position = vdata[i].mvp * vdata[i].position;
        gdata.color = green;
        EmitVertex();

        gl_Position = vdata[i].mvp * (vdata[i].position + vdata[i].normal);
        gdata.color = blue;
        EmitVertex();

        EndPrimitive();
    }
}

Fragment shader:

#version 330

in Data
{
    vec4 color;
} gdata;

out vec4 outputColor;

void main()
{
    outputColor = gdata.color;
}