#version 330 core

out vec4 frag_color;

in vec2 tex_coords;

uniform sampler2D texture1;

vec4
inversion()
{
    vec4 result = vec4(1.0f-vec3(texture(texture1, tex_coords)), 1.0f);
    return(result);
}

vec4
grayscale_average()
{
    vec4 tex_color = texture(texture1, tex_coords);
    float average = (tex_color.r+tex_color.g+tex_color.b)/3.0f;
    vec4 result = vec4(average, average, average, 1.0f);
    return(result);
}

vec4
grayscale_weights()
{
    vec4 tex_color = texture(texture1, tex_coords);
    float average = 0.2126*tex_color.r+0.7152*tex_color.g+0.0722f*tex_color.b;
    vec4 result = vec4(average, average, average, 1.0f);
    return(result);
}

const float offset = 1.0f/300.0f;

vec4
kernel_effect(float[9] kernel)
{
    vec2 offsets[9] = vec2[](
        vec2(-offset, offset), /* top-left */
        vec2(0.0f, offset), /* top-center */
        vec2(offset, offset), /* top-right */
        vec2(-offset, 0.0f), /* center-left */
        vec2(0.0f, 0.0f), /* center-center */
        vec2(offset, 0.0f), /* center-right */
        vec2(-offset, -offset), /* bottom-left */
        vec2(0.0f, -offset), /* bottom-center */
        vec2(offset, -offset) /* bottom-right */
    );

    vec3 sample_texture[9];
    for (int offset_index = 0;
         offset_index < 9;
         ++offset_index)
    {
        sample_texture[offset_index] =
            vec3(texture(texture1, tex_coords+offsets[offset_index]));
    }

    vec3 color = vec3(0.0f);
    for (int i = 0; i < 9; ++i)
        color += sample_texture[i]*kernel[i];

    return(vec4(color, 1.0f));
}

void
main(void)
{
    float strange_kernel[9] = float[](
        -1, -1, -1,
        -1,  9, -1,
        -1, -1, -1
    );

    float gaussian_blur_kernel[9] = float[](
        1.0f/16.0f, 2.0f/16.0f, 1.0f/16.0f,
        2.0f/16.0f, 4.0f/16.0f, 2.0f/16.0f,
        1.0f/16.0f, 2.0f/16.0f, 1.0f/16.0f
    );

    float box_blur_kernel[9] = float[](
        1.0f/9.0f, 1.0f/9.0f, 1.0f/9.0f,
        1.0f/9.0f, 1.0f/9.0f, 1.0f/9.0f,
        1.0f/9.0f, 1.0f/9.0f, 1.0f/9.0f
    );

    float edge_detection_kernel[9] = float[](
        1.0f,  1.0f,  1.0f,
        1.0f, -8.0f,  1.0f,
        1.0f,  1.0f,  1.0f
    );

    float sharpening_kernel[9] = float[](
         0.0f, -1.0f,  0.0f,
        -1.0f,  5.0f, -1.0f,
         0.0f, -1.0f,  0.0f
    );

    float sobel_kernel_vertical[9] = float[](
        -1.0f, 0.0f, 1.0f,
        -2.0f, 0.0f, 2.0f,
        -1.0f, 0.0f, 0.0f
    );
    float sobel_kernel_horizontal[9] = float[](
        -1.0f, -2.0f, -1.0f,
         0.0f,  0.0f,  0.0f,
         1.0f,  2.0f,  1.0f
    );

    float funny_kernel[9] = float[](
        1.0f,  1.0f,  1.0f,
        1.0f, -4.0f,  1.0f,
        1.0f,  1.0f,  1.0f
    );

    /*
    frag_color = kernel_effect(funny_kernel);
    frag_color *= vec4(0.76f, 0.47f, 0.84f, 1.0f);
    */
    frag_color = grayscale_weights();
}