#version 120

varying vec2 uv;
varying vec4 color;

uniform sampler2D tex;
uniform vec4 blending_color;
uniform vec2 texture_size;

uniform vec2 k;

float kernel(float x)
{
    x = abs(x);
    if(x < k.x) {
        return 1 - x * k.y;
    }
    return 0.0;
}

vec4 filtering(sampler2D texture_sampler, vec2 uv)
{
    // size of one texel (< 1.0)
    float texel_wid = 1.0 / texture_size.x;
    float texel_hei = 1.0 / texture_size.y;

    vec4 sum = vec4(0.0, 0.0, 0.0, 0.0);
    vec4 denom = vec4(0.0, 0.0, 0.0, 0.0);

    // get the decimal part
    float a = fract(uv.x * texture_size.x);
    float b = fract(uv.y * texture_size.y);


    // Sampling
    float f1  = kernel(-a);

    {
        vec4 color = texture2D(texture_sampler, uv);

        float f = f1 * kernel(b);
        vec4 c1c2 = vec4(f, f, f, f);

        sum = sum + (color * c1c2);
        denom = denom + c1c2;


        color = texture2D(texture_sampler, uv + vec2(0, texel_hei));

        f = f1 * kernel(b - 1.0);
        c1c2 = vec4(f, f, f, f);

        sum = sum + (color * c1c2);
        denom = denom + c1c2;
    }

    f1 = kernel(1.0 - a);

    {
        vec4 color = texture2D(texture_sampler, uv + vec2(texel_wid, 0));

        float f = f1 * kernel(b);
        vec4 c1c2 = vec4(f, f, f, f);

        sum = sum + (color * c1c2);
        denom = denom + c1c2;


        color = texture2D(texture_sampler, uv + vec2(texel_wid, texel_hei));

        f = f1 * kernel(b - 1.0);
        c1c2 = vec4(f, f, f, f);

        sum = sum + (color * c1c2);
        denom = denom + c1c2;
    }

    return sum / denom;
}

void main()
{
    gl_FragColor = filtering(tex, uv) * color * blending_color;
}