Combining RGB and grayscale colors - php

Question

I'm looking for a way to combine a RGB color with a grayscale color.
I am looking into this for a gradient generator that uses stored colors and a template (pre-made style properties for most venders) to create a CSS3 gradient.
I am sure their is a simple solution, but I can not seem to find it. I am not looking for anyone to make me a custom function, I just need to know how to make the function.

Answer 1

Your general approach will probably consist of three parts:
Convert the RGB color to HSL.
Keep the hue and saturation, but apply the luminosity of the desired greyscale color.
Convert this HSL triple back to RGB.
Wikipedia has a great page on the HSL colorspace, including conversion formulas. This information was used to create some JavaScript conversion functions by Michael Jackson (yes, I'm serious):
rgbToHsl(r,g,b)
/**
* Converts an RGB color value to HSL. Conversion formula
* adapted from http://en.wikipedia.org/wiki/HSL_color_space.
* Assumes r, g, and b are contained in the set [0, 255] and
* returns h, s, and l in the set [0, 1].
*
* #param Number r The red color value
* #param Number g The green color value
* #param Number b The blue color value
* #return Array The HSL representation
*/
function rgbToHsl(r, g, b){
r /= 255, g /= 255, b /= 255;
var max = Math.max(r, g, b), min = Math.min(r, g, b);
var h, s, l = (max + min) / 2;
if(max == min){
h = s = 0; // achromatic
}else{
var d = max - min;
s = l > 0.5 ? d / (2 - max - min) : d / (max + min);
switch(max){
case r: h = (g - b) / d + (g < b ? 6 : 0); break;
case g: h = (b - r) / d + 2; break;
case b: h = (r - g) / d + 4; break;
}
h /= 6;
}
return [h, s, l];
}
hslToRgb(h,s,l)
/**
* Converts an HSL color value to RGB. Conversion formula
* adapted from http://en.wikipedia.org/wiki/HSL_color_space.
* Assumes h, s, and l are contained in the set [0, 1] and
* returns r, g, and b in the set [0, 255].
*
* #param Number h The hue
* #param Number s The saturation
* #param Number l The lightness
* #return Array The RGB representation
*/
function hslToRgb(h, s, l){
var r, g, b;
if(s == 0){
r = g = b = l; // achromatic
}else{
function hue2rgb(p, q, t){
if(t < 0) t += 1;
if(t > 1) t -= 1;
if(t < 1/6) return p + (q - p) * 6 * t;
if(t < 1/2) return q;
if(t < 2/3) return p + (q - p) * (2/3 - t) * 6;
return p;
}
var q = l < 0.5 ? l * (1 + s) : l + s - l * s;
var p = 2 * l - q;
r = hue2rgb(p, q, h + 1/3);
g = hue2rgb(p, q, h);
b = hue2rgb(p, q, h - 1/3);
}
return [r * 255, g * 255, b * 255];
}
Since you said you're not looking for anyone to make you a custom function, I trust you'll find little trouble adapting these functions to PHP and leveraging them for your needs. ;)

Answer 2

You can subtract the RGB channels with the brightness of the greyscale colour.
In javascript:
var grey = {r:123,g:123,b:123};
var color = {r:216,g:205,b:120};
var clamp = function(a,b,x){return Math.min(Math.max(a,x),b);}
var mixed = {
r:clamp(0,255,color.r-(255-grey.r)),
g:clamp(0,255,color.g-(255-grey.g)),
b:clamp(0,255,color.b-(255-grey.b))
};// the final combined color.
Hope this can help you.