Here's one I did in both Java and Javascript. It's loosely based off this one in javascript. I took the Luminance formula from here. The sweet-spot of the threshold from my eye was about 140.
Java version:
public class Color {
private float CalculateLuminance(ArrayList<Integer> rgb){
return (float) (0.2126*rgb.get(0) + 0.7152*rgb.get(1) + 0.0722*rgb.get(2));
}
private ArrayList<Integer> HexToRBG(String colorStr) {
ArrayList<Integer> rbg = new ArrayList<Integer>();
rbg.add(Integer.valueOf( colorStr.substring( 1, 3 ), 16 ));
rbg.add(Integer.valueOf( colorStr.substring( 3, 5 ), 16 ));
rbg.add(Integer.valueOf( colorStr.substring( 5, 7 ), 16 ));
return rbg;
}
public String getInverseBW(String hex_color) {
float luminance = this.CalculateLuminance(this.HexToRBG(hex_color));
String inverse = (luminance < 140) ? "#fff" : "#000";
return inverse;
}
}
Javascript version:
Here's the same thing in javascript for your front-end things. RGB conversion taken from here:
hex_to_rgb: function(hex) {
let result = /^#?([a-f\d]{2})([a-f\d]{2})([a-f\d]{2})$/i.exec(hex);
return result ? {
r: parseInt(result[1], 16),
g: parseInt(result[2], 16),
b: parseInt(result[3], 16)
} : null;
},
hex_inverse_bw: function(hex) {
let rgb = this.hex_to_rgb(hex);
let luminance = (0.2126*rgb["r"] + 0.7152*rgb["g"] + 0.0722*rgb["b"]);
return (luminance < 140) ? "#ffffff": "#000000";
}