flat_shading.fs

uniform vec3 light_direction;

varying vec3 vPosition;
varying vec3 vColor;
varying mat4 vInvertMatrix;

void main() {
  vec3 normal = normalize(cross(dFdx(vPosition), dFdy(vPosition)));
  vec3 inv_light = normalize(vInvertMatrix * vec4(light_direction, 1.0)).xyz;
  float diff = clamp(dot(normal, inv_light), 0.0, 1.0);
  gl_FragColor = vec4(vColor * diff, 1.0);
}

flat_shading.vs

attribute vec3 position;
attribute vec3 color;

uniform mat4 modelMatrix;
uniform mat4 modelViewMatrix;
uniform mat4 projectionMatrix;

varying vec3 vPosition;
varying vec3 vColor;
varying mat4 vInvertMatrix;

#pragma glslify: inverse = require(glsl-inverse);

void main() {
  vPosition = position;
  vPosition = color;
  vInvertMatrix = inverse(modelMatrix);
  gl_Position = projectionMatrix * modelViewMatrix * position;
}