GZDoom Pixelate + Posterize Shader

out vec4 FragColor;
in vec2 TexCoord;
uniform sampler2D Texture;
//uniform float new_w, new_h, palette;

vec4 pixelate(sampler2D tex, vec2 uv)
{
vec2 coord = vec2( ceil(uv.x * 320) /320,
ceil(uv.y * 200) / 200 );
return texture2D(tex, coord);
}

vec4 reduce_palette(vec4 color, float max_colors_per_channel)
{
if(max_colors_per_channel < 0) {
return color;
}

return ceil(color * max_colors_per_channel) / max_colors_per_channel;
}

void main()
{
vec4 color = pixelate(Texture, TexCoord);
FragColor = reduce_palette(color, 20);
}