#version 330 core

in vec2 TexCoords;
uniform sampler2D TexId;
out vec4 FragColor;

vec4 filter_color_invert(vec4 color)
{
  vec4 res = vec4(vec3(1.0) - vec3(color), 1.0);
  return res;
}

vec4 filter_color_grayscale(vec4 color)
{
  // we will need to average the colors
  // float average = (color.x + color.y + color.z) / 3.0f;
  // in reality, human our most sensitive towards green and least to blue, so will need to weight those
  float average = 0.2126 * color.r + 0.7152 * color.g + 0.0722 * color.b;
  vec4 res = vec4(vec3(average), 1.0);

  return res;
}

// @note: different kernels for experimentation
const float kernel_sharpen[9] = float[](
    -1, -1, -1,
    -1,  9, -1,
    -1, -1, -1
);

const float kernel_blur[9] = float[](
    1.0/16.0, 2.0/16.0, 1.0/16.0,
    2.0/16.0, 4.0/16.0, 2.0/16.0,
    1.0/16.0, 2.0/16.0, 1.0/16.0
);

const float kernel_edge_detection[9] = float[](
    1,  1,  1,
    1, -8,  1,
    1,  1,  1
);

vec4 filter_kernal_effects() 
{
  const float offset = 1.0/300.0;
  vec2 offsets[9] = vec2[](
    vec2(-offset,  offset),    // top left
    vec2(      0,  offset),    // top center
    vec2( offset,  offset),    // top right
    vec2(-offset,       0),    // center left
    vec2(      0,       0),    // center center
    vec2( offset,       0),    // center right
    vec2(-offset, -offset),    // bot left
    vec2(      0, -offset),    // bot center
    vec2( offset, -offset)    // bot right
  );

  float kernal[9] = kernel_edge_detection;
  vec3 kernalValue = vec3(0.0);
  vec3 sampleTex[9];
  for (int i=0; i<9; i++) {
    sampleTex[i] = vec3(texture(TexId, TexCoords + offsets[i]));
    kernalValue += (kernal[i] * sampleTex[i]);
  }

  vec4 res = vec4(kernalValue, 1.0);
  return res;
}

void main() {
    FragColor = texture(TexId, TexCoords);
}