uniform sampler2D shadow_map;
varying vec2 vert;


vec4 pack(float depth)
{
    const vec4 bitSh = vec4(256.0 * 256.0 * 256.0,
                       256.0 * 256.0,
                       256.0,
                      1.0);
    const vec4 bitMsk = vec4(0,
                         1.0 / 256.0,
                         1.0 / 256.0,
                             1.0 / 256.0);
    vec4 comp = fract(depth * bitSh);

//    fract returns the fractional part of x. This is calculated as x - floor(x).
    comp -= comp.xxyz * bitMsk;
    return comp;
}



float unpack(vec4 packedZValue)
{
    const vec4 bitShifts = vec4(1.0 / (256.0 * 256.0 * 256.0),
                        1.0 / (256.0 * 256.0),
                        1.0 / 256.0,
                        1.0);
    float shadow = dot(packedZValue, bitShifts);

    return shadow;
}

//float Lerp(float a, float b, float x)     { return a + x * (b - a); }

float bilinearInterpolation(float x, float y)
{

    float texdm = 1.0 / 512.0;
    float ix = float(int(x));
    float iy = float(int(y));

    float ix1 = ix;
    float ix2 = ix+texdm;

    float iy1 = iy;
    float iy2 = iy+texdm;


    float cix2 = ix2;
    float ciy2 = iy2;


    float FQ11 = unpack(texture2D( shadow_map, vec2(ix1, iy1)));
    float FQ21 = unpack(texture2D( shadow_map, vec2(cix2, iy1)));
    float FQ12 = unpack(texture2D( shadow_map, vec2(ix1, ciy2)));
    float FQ22 = unpack(texture2D( shadow_map, vec2(cix2, ciy2)));

float denom = ix2 - ix1;

    float fxy1 = ( (ix2 - x) / denom) * FQ11 + ( (x - ix1) / denom ) * FQ21;
    float fxy2 = ( (ix2 - x) / denom) * FQ12 + ( (x - ix1) / denom ) * FQ22;

    float denom2 = (iy2 - iy1);
    if (denom2 == 0.0) return 0.0;

float a = ( (iy2 - y) / denom2) * fxy1;
float b = ( (y - iy1) / denom2) * fxy2;

    return a + b;

    //return 1.0;
}


void main()
{
vec2 texpos = vert*0.5 + 0.5;
float new_val = bilinearInterpolation(texpos.x, texpos.y);
//float new_val = unpack(texture2D( shadow_map, vert*0.5+0.5));
gl_FragColor = pack(new_val);
}