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I need for a projet to get a color from a value. I explain, I have datas and each data must be represented by a color.

The red color is for the maximum, and the blue for the minimum and the green is for the middle value. A kind of heatmap.

So, I need to have a function returning the right color.

I tried something like this :

    function datatocolor($min, $max, $value)
    {
        $half = (($min + $max) / 2);

        if ($value > $half)
        {
            $r = (255 * ($value+$min-$half)) / $half;
            $g = 255 - $r;
            $b = 0;
        } 
        else {
            $b = (255 * ($half-$value+$min)) / $half;
            $g = 255 - $b;
            $r = 0;         
        }
        $color = array(intval($r), intval($g),  intval($b));
        return $color;
    }

But, I get red and blue, and never green ... I tried a lot of operations, I must be stupid but I don't find the right operation ... Thanks in advance for your help !

Julian T
  • 35
  • 6

1 Answers1

2

I'm not a php expert, but as far as I can tell, the problem isn't with this code block. I tested your algorithm in java just to be sure, and it looks to be correct:

public static void main(String[] args) {
    int min = 0;
    int max = 10;
    int half = (min + max) / 2;

    int r, g, b;

    // Cycling through the values for completeness' sake.
    for (int value = 0; value <= 10; value++) {
        if (value > half) {
        r = (255 * (value + min - half)) / half;
        g = 255 - r;
        b = 0;
    } else {
        b = (255 * (half - value + min)) / half;
        g = 255 - b;
        r = 0;
    }
    System.out.println("Value: " + value + " - " + new Color(r, g, b));
}

The output from this is what you would expect -- pure blue at minimum, pure green at the middle, and pure red at the maximum:

Value: 0 - java.awt.Color[r=0,g=0,b=255]
Value: 1 - java.awt.Color[r=0,g=51,b=204]
Value: 2 - java.awt.Color[r=0,g=102,b=153]
Value: 3 - java.awt.Color[r=0,g=153,b=102]
Value: 4 - java.awt.Color[r=0,g=204,b=51]
Value: 5 - java.awt.Color[r=0,g=255,b=0]
Value: 6 - java.awt.Color[r=51,g=204,b=0]
Value: 7 - java.awt.Color[r=102,g=153,b=0]
Value: 8 - java.awt.Color[r=153,g=102,b=0]
Value: 9 - java.awt.Color[r=204,g=51,b=0]
Value: 10 - java.awt.Color[r=255,g=0,b=0]

Based on what you've provided, the problem seems to be either in the way you're calling the function, or in the way you're using the array it returns.

Argumentum ad Stultitiam
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  • Thanks for your test, it's work yes, but I want to have a HSV scale. That means : Red -> Orange -> Yellow -> Green -> Cyan -> Blue. I continue to seek ... :) – Julian T Jul 22 '11 at 11:22
  • A quick Google search for `php hsv` yields a handful of conversion functions between RGB and HSV color spaces. – Argumentum ad Stultitiam Jul 22 '11 at 14:03
  • Yes I found the functions to convert in HSV. But, I think I need to convert my values to HSV format, then to RGB to have the right gradient... – Julian T Jul 22 '11 at 14:24
  • For a quick solution, you can simply have a set `$s` and `$v`, calculate `$h` between min/blue and max/red, and run the result through a converter function to get your RGB value. But now we're in the messy world of colors and computing, so you may want to create a new question if you're still having trouble. – Argumentum ad Stultitiam Jul 22 '11 at 15:18
  • Thanks for this answer, I will try this method, but I am (of course) in trouble to get the $v and $h values. I will post a new question... – Julian T Jul 25 '11 at 12:55