shader_type canvas_item;

uniform sampler2D palette; // The palette texture.
uniform float shift; // The shift amount on the palette texture.


// Rounds a value.
float roundval(float value){
    float frac = value - floor(value);
    float result;

    if(frac  < 0.5) { result = floor(value); }
    if(frac >= 0.5) { result = ceil(value); }

    return result;
}

void fragment()
{
vec4 pixcol;
// The pixel coordinate being operated on.
vec2 pixpos = SCREEN_UV;

float over_x = ceil(UV.x * 16.0);
float over_y = ceil(UV.y * 16.0);

// Get dither pixel
vec2 overlayCoord = vec2(over_x, over_y);
// Get 1 or 0 based on the pixel location.
float overlayPixelColor = mod(overlayCoord.x + overlayCoord.y, 2.0);
// Dither is black and white every other pixel.
vec4 overlayPixel = vec4(overlayPixelColor, overlayPixelColor, overlayPixelColor, 1);

// Get base color.
pixcol = vec4(texture(SCREEN_TEXTURE, SCREEN_UV).rgb, 0);

// Mix dither texture.
pixcol = mix(pixcol, overlayPixel, 0.1);

// Determine the brightness of the pixel in a dumb way.
float gray = (pixcol.r + pixcol.g + pixcol.b) / 3.0;

// Round it to the nearest 0.25.
gray = roundval(gray / 0.25) * 0.25;

// Map the palette to the pixel based on the brightness and shift.
pixcol = texture(palette, vec2(gray, shift));

// Multiply through the gl_Color for final output.
COLOR = pixcol;
}