How to alter brightness of a single rgb color simply and easily via php

color-schemecolorshslrgb

A quesion about RGB color and finding the simplest, tiniest, php conversion code for manipulating the lightness/darkness of a given RGB hue.

Imagine a variable $colorA containning a valid six char RGB color, like F7A100 which we want to make a bit lighter and/or darker:

$color  = B1B100;                     // original RGB color manually set.

Then, at any page have that color bit darker/lighter on the fly:

$colorX = someFunction($color, +10);  // original color 10 steps lighter
$colorY = someFunction($color, -25);  // original color 25 steps darker

What would be YOUR way of solving this? Keep the RGB as is or first change it to HSL? Hints and suggestions are welcome. Your sample/code is welcome too.

This really focuses to the TINIES / SIMPLES / SHORTEST possible code to just make the same hue bit darker/lighter.

I deliberately do not suggest my code, as I want to keep possibilities open in here.

Best Answer

The absolutely simplest solution is to add some constant (like 1) to each part of the color representation: [R, G, B]. This is due to the fact that max values of all [R, G, B] represent white, while min values - black. In pseudo-code (assuming 255 is max, sorry, I don't know PHP):

lighter(R, G, B) = [
    min(255, R + 1), 
    min(255, G + 1), 
    min(255, B + 1)
]

You must keep in mind though that this transformation is way too simplistic and the proper implementation would be to convert to HSL/HSB, increase H and transform back to RGB.

Related Solutions

Programmatically Lighten a Color

As Wedge said, you want to multiply to make things brighter, but that only works until one of the colors becomes saturated (i.e. hits 255 or greater). At that point, you can just clamp the values to 255, but you'll be subtly changing the hue as you get lighter. To keep the hue, you want to maintain the ratio of (middle-lowest)/(highest-lowest).

Here are two functions in Python. The first implements the naive approach which just clamps the RGB values to 255 if they go over. The second redistributes the excess values to keep the hue intact.

def clamp_rgb(r, g, b):
    return min(255, int(r)), min(255, int(g)), min(255, int(b))

def redistribute_rgb(r, g, b):
    threshold = 255.999
    m = max(r, g, b)
    if m <= threshold:
        return int(r), int(g), int(b)
    total = r + g + b
    if total >= 3 * threshold:
        return int(threshold), int(threshold), int(threshold)
    x = (3 * threshold - total) / (3 * m - total)
    gray = threshold - x * m
    return int(gray + x * r), int(gray + x * g), int(gray + x * b)

I created a gradient starting with the RGB value (224,128,0) and multiplying it by 1.0, 1.1, 1.2, etc. up to 2.0. The upper half is the result using clamp_rgb and the bottom half is the result with redistribute_rgb. I think it's easy to see that redistributing the overflows gives a much better result, without having to leave the RGB color space.

Lightness gradient with clamping (top) and redistribution (bottom)

For comparison, here's the same gradient in the HLS and HSV color spaces, as implemented by Python's colorsys module. Only the L component was modified, and clamping was performed on the resulting RGB values. The results are similar, but require color space conversions for every pixel.

Lightness gradient with HLS (top) and HSV (bottom)

Formula to determine perceived brightness of RGB color

The method could vary depending on your needs. Here are 3 ways to calculate Luminance:

Luminance (standard for certain colour spaces): (0.2126*R + 0.7152*G + 0.0722*B) source
Luminance (perceived option 1): (0.299*R + 0.587*G + 0.114*B) source
Luminance (perceived option 2, slower to calculate): ~~sqrt( 0.241*R^2 + 0.691*G^2 + 0.068*B^2 )~~ → sqrt( 0.299*R^2 + 0.587*G^2 + 0.114*B^2 ) (thanks to @MatthewHerbst) source

^{_{[Edit: added examples using named css colors sorted with each method.]}}

Best Answer

Related Solutions

Programmatically Lighten a Color

Formula to determine perceived brightness of RGB color

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