RGB to HSV in PHP

Question

In PHP, what is the most straightforward way to convert a RGB triplet to HSV values?

Answer 1

Here's my spin on it, along with a unit test. Since the S and V values are percentages, this code returns them as integers (0, 100) as opposed to (0, 1) - Example, 75 instead of 0.75.

final class MathService
{    
    /**
     * Converts an RGB point into HSV
     *
     * @param int $r
     * @param int $g
     * @param int $b
     * @return array
     */
    public function rgbToHsv(int $r, int $g, int $b): array
    {
        $rPrime = $r / 255;
        $gPrime = $g / 255;
        $bPrime = $b / 255;

        $max = max([$rPrime, $gPrime, $bPrime]);
        $min = min([$rPrime, $gPrime, $bPrime]);

        $delta = $max - $min;

        // Calculate H
        if ($delta == 0) {
            $h = 0;
        } else {
            if ($max === $rPrime) {
                $h = 60 * ((($gPrime - $bPrime) / $delta) % 6);
            }
            if ($max === $gPrime) {
                $h = 60 * ((($bPrime - $rPrime) / $delta) + 2);
            }
            if ($max === $bPrime) {
                $h = 60 * ((($rPrime - $gPrime) / $delta) + 4);
            }
        }

        // Calculate S
        if ($max == 0) {
            $s = 0;
        } else {
            $s = $delta / $max;
        }

        // Calculate V
        $v = $max;

        return [$h, (int)($s * 100), (int)($v * 100)];
    }
}

PHPUnit test case with PHP 7.2

/**
 * @test
 */
public function rgbToHsv_ComputesCorrectValues(): void
{
    $service = new MathService();
    $samples = [
        // [R, G, B, H, S, V]
        [0, 0, 0, 0, 0, 0],
        [255, 255, 255, 0, 0, 100],
        [255, 0, 0, 0, 100, 100],
        [0, 255, 0, 120, 100, 100],
        [0, 0, 255, 240, 100, 100],
        [255, 255, 0, 60, 100, 100],
        [0, 255, 255, 180, 100, 100],
        [255, 0, 255, 300, 100, 100],
        [192, 192, 192, 0, 0, 75],
        [128, 128, 128, 0, 0, 50],
        [128, 0, 0, 0, 100, 50],
        [128, 128, 0, 60, 100, 50],
        [0, 128, 0, 120, 100, 50],
        [128, 0, 128, 300, 100, 50],
        [0, 128, 128, 180, 100, 50],
        [0, 0, 128, 240, 100, 50],
    ];

    foreach ($samples as $sample) {
        list($r, $g, $b) = array_slice($sample, 0, 3);
        $expected = array_slice($sample, 3);
        $hsv = $service->rgbToHsv($r, $g, $b);
        list($h, $s, $v) = $hsv;

        self::assertEquals($expected, $hsv, "Error converting ({$r}, ${g}, ${b}). Got ({$h}, {$s}, {$v})");
    }
}

Answer 2

<?php
function RGB_TO_HSV ($R, $G, $B)  // RGB Values:Number 0-255
{                                 // HSV Results:Number 0-1
   $HSL = array();

   $var_R = ($R / 255);
   $var_G = ($G / 255);
   $var_B = ($B / 255);

   $var_Min = min($var_R, $var_G, $var_B);
   $var_Max = max($var_R, $var_G, $var_B);
   $del_Max = $var_Max - $var_Min;

   $V = $var_Max;

   if ($del_Max == 0)
   {
      $H = 0;
      $S = 0;
   }
   else
   {
      $S = $del_Max / $var_Max;

      $del_R = ( ( ( $var_Max - $var_R ) / 6 ) + ( $del_Max / 2 ) ) / $del_Max;
      $del_G = ( ( ( $var_Max - $var_G ) / 6 ) + ( $del_Max / 2 ) ) / $del_Max;
      $del_B = ( ( ( $var_Max - $var_B ) / 6 ) + ( $del_Max / 2 ) ) / $del_Max;

      if      ($var_R == $var_Max) $H = $del_B - $del_G;
      else if ($var_G == $var_Max) $H = ( 1 / 3 ) + $del_R - $del_B;
      else if ($var_B == $var_Max) $H = ( 2 / 3 ) + $del_G - $del_R;

      if ($H<0) $H++;
      if ($H>1) $H--;
   }

   $HSL['H'] = $H;
   $HSL['S'] = $S;
   $HSL['V'] = $V;

   return $HSL;
}

Answer 3

Here is a simple, straightforward method that returns HSV values as degrees and percentages, which is what Photoshop's color picker uses.

Note that the return values are not rounded, you can do that yourself if required. Keep in mind that H(360) == H(0), so H values of 359.5 and greater should round to 0

Heavily documented for learning purposes.

/**
 * Licensed under the terms of the BSD License.
 * (Basically, this means you can do whatever you like with it,
 *   but if you just copy and paste my code into your app, you
 *   should give me a shout-out/credit :)
 */

<?php

function RGBtoHSV($R, $G, $B)    // RGB values:    0-255, 0-255, 0-255
{                                // HSV values:    0-360, 0-100, 0-100
    // Convert the RGB byte-values to percentages
    $R = ($R / 255);
    $G = ($G / 255);
    $B = ($B / 255);

    // Calculate a few basic values, the maximum value of R,G,B, the
    //   minimum value, and the difference of the two (chroma).
    $maxRGB = max($R, $G, $B);
    $minRGB = min($R, $G, $B);
    $chroma = $maxRGB - $minRGB;

    // Value (also called Brightness) is the easiest component to calculate,
    //   and is simply the highest value among the R,G,B components.
    // We multiply by 100 to turn the decimal into a readable percent value.
    $computedV = 100 * $maxRGB;

    // Special case if hueless (equal parts RGB make black, white, or grays)
    // Note that Hue is technically undefined when chroma is zero, as
    //   attempting to calculate it would cause division by zero (see
    //   below), so most applications simply substitute a Hue of zero.
    // Saturation will always be zero in this case, see below for details.
    if ($chroma == 0)
        return array(0, 0, $computedV);

    // Saturation is also simple to compute, and is simply the chroma
    //   over the Value (or Brightness)
    // Again, multiplied by 100 to get a percentage.
    $computedS = 100 * ($chroma / $maxRGB);

    // Calculate Hue component
    // Hue is calculated on the "chromacity plane", which is represented
    //   as a 2D hexagon, divided into six 60-degree sectors. We calculate
    //   the bisecting angle as a value 0 <= x < 6, that represents which
    //   portion of which sector the line falls on.
    if ($R == $minRGB)
        $h = 3 - (($G - $B) / $chroma);
    elseif ($B == $minRGB)
        $h = 1 - (($R - $G) / $chroma);
    else // $G == $minRGB
        $h = 5 - (($B - $R) / $chroma);

    // After we have the sector position, we multiply it by the size of
    //   each sector's arc (60 degrees) to obtain the angle in degrees.
    $computedH = 60 * $h;

    return array($computedH, $computedS, $computedV);
}

?>