godot dither quantization

shader_type canvas_item;

const float PaletteRGBSize = 4.;
const float ResolutionDivisor = 2.;

float quantize(float inp, float period)
{
    return floor((inp+period/2.)/period)*period;
}

float bayer8x8(vec2 uvScreenSpace, sampler2D image_texture)
{
    return texture(image_texture, uvScreenSpace/(ResolutionDivisor*8.)).r;
}

vec3 getSceneColor(vec2 uv, sampler2D screen_texture)
{
    return texture(screen_texture, uv).rgb;
}

void fragment()
{
    vec3 quantizationPeriod = vec3(1./(PaletteRGBSize-1.));

    vec2 uvPixellated = floor(FRAGCOORD.xy / ResolutionDivisor)*ResolutionDivisor;

    vec3 dc = getSceneColor(uvPixellated / SCREEN_PIXEL_SIZE.xy, SCREEN_TEXTURE);

    dc += (bayer8x8(FRAGCOORD.xy, TEXTURE)-.5)*(quantizationPeriod);

    dc = vec3(
        quantize(dc.r, quantizationPeriod.r),
        quantize(dc.g, quantizationPeriod.g),
        quantize(dc.b, quantizationPeriod.b)
    );
    COLOR = vec4(dc, 1.);
}