Diffuse lighting error on parallel surfaces - glsl

Question

As a test, I created a simple quad. Here are its attributes:
Vertex vertices[] =
{
// Positions Normals
{vec3(-1,-1, 0), vec3(-1,-1, 1)}, // v0
{vec3( 1,-1, 0), vec3( 1,-1, 1)}, // v1
{vec3(-1, 1, 0), vec3(-1, 1, 1)}, // v2
{vec3( 1, 1, 0), vec3( 1, 1, 1)}, // v3
};
And I put it in my world space at (0.0, 0.0, -9.5). Then I put my point light position at (0.0, 0.0, -8.0). My camera is at the origin (0.0, 0.0, 0.0). When I run my program, this works as expected:
But then, when I replace this quad with 9 scaled down quads, put them all at -9.5 on Z (in other word, they are all parallel to each other on Z), my diffuse lighting gets a little weird
It looks like the corners are showing too much lighting, breaking the nice diffuse circle that we see on a regular quad.
Here is my shader program:
precision mediump int;
precision mediump float;
varying vec3 v_position;
varying vec3 v_normal;
#if defined(VERTEX)
uniform mat4 u_mvpMatrix;
uniform mat4 u_mvMatrix;
uniform mat3 u_normalMatrix;
attribute vec4 a_position;
attribute vec3 a_normal;
void main()
{
vec4 position = u_mvMatrix * a_position;
v_position = position.xyz / position.w;
v_normal = normalize(u_normalMatrix * a_normal);
gl_Position = u_mvpMatrix * a_position;
}
#endif // VERTEX
#if defined(FRAGMENT)
uniform vec3 u_pointLightPosition;
void main()"
{
vec3 viewDir = normalize(-v_position);
vec3 normal = normalize(v_normal);
vec3 lightPosition = u_pointLightPosition - v_position;
vec3 pointLightDir = normalize(lightPosition);
float distance = length(lightPosition);
float pointLightAttenuation = 1.0 / (1.0 + (0.25 * distance * distance));
float diffuseTerm = max(dot(pointLightDir, normal), 0.15);
gl_FragColor = vec4(diffuseTerm * pointLightAttenuation);
}
#endif // FRAGMENT
My uniforms are uploaded as followed (I'm using GLM):
const mat4 &view_matrix = getViewMatrix();
mat4 mv_matrix = view * getModelMatrix();
mat4 mvp_matrix = getProjectionMatrix() * mv_matrix;
mat3 normal_matrix = inverseTranspose(mat3(mv_matrix));
vec3 pointLightPos = vec3(view_matrix * vec4(getPointLightPos(), 1.0f));
glUniformMatrix4fv( mvpMatrixUniformID, 1, GL_FALSE, (GLfloat*)&mvp_matrix);
glUniformMatrix4fv( vpMatrixUniformID, 1, GL_FALSE, (GLfloat*)&mv_matrix);
glUniformMatrix3fv(normalMatrixUniformID, 1, GL_FALSE, (GLfloat*)&normal_matrix);
glUniform3f(pointLightPosUniformID, pointLightPos.x, pointLightPos.y, pointLightPos.z);
Am I doing anything wrong?
Thanks!

Answer 1

Without going too much into your code, I think everything is working just fine. I see a very similar result with a quick blender setup:
The issue is the interpolation of the normal doesn't produce a spherical bump.
It ends up being a patch like this (I simply subdivided a smooth shaded cube)...
If you want a more spherical bump, you could generate the normals implicitly in a fragment shader (for example as is done here (bottom image)), use a normal map, or use more tessellated geometry such as an actual sphere.