Convert List of Floats to Colors (Color Map)

Question

So, I have an arbitrarily long list of arbitrary float values. I need to write a function that can take in this list of floats return colors for each float such that the maximum float is pure red, the minimum float is pure blue, and the middle of the range is pure white. i.e.

 Float value =>        max         (max - min)/2           min
 RGB value =>    (1, 0, 0, 100) -> (1, 1, 1, 100) -> (0, 0, 1, 100)
 Color name =>         red             white               blue
  Hex Value =>        FF0000           FFFFFF              0000FF

I have this equation to scale a range [min, max] to a new range [a, b]:

          (b - a)(x - min)  
  f(x) =  ----------------  + a
             max - min

But I'm feeling stuck as to what an appropriate range is to scale to. Do I need to just scale it twice, once for the bottom half of values and once for the top half of values?

Answer 1

Answer by Bunny83 · Sep 11, 2018 at 08:45 PM

It's not clear what you mean by max and min float values. Are they arbitrary limits you set? Because floats actually have a huge dynamic range otherwise.

Assuming min and max are some constants you have to do something like this:

 public static Color InterpolateColor(float aInput)
 {
     float range = max - min;
     float f = 2f * (aInput - min) / range; // float in the range of 0 - 2
     if (f > 1)
         return Color.Lerp(Color.white, Color.red, f - 1f); // top half
     return Color.Lerp(Color.blue, Color.white, f); // bottom half
 }

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elenzil · Sep 12, 2018 at 06:35 PM 0

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i think red and blue may be swapped in this, if red should correspond to the maximum float. the OP's diagram is a bit confusing because they wrote "max, middle, min".

Bunny83 elenzil · Sep 12, 2018 at 11:25 PM 0

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Yes you're right. I somehow assumed that the min value was the first one and the max value was the last one. However he has it the other way round. Though currently I'm not home and i'm writing on a panasonic TV so the editing capabilities are quite poor ^^

Answer 2

So this ended up being my solution, thank you anyways Bunny83
btw, what I meant in my case is that I get a text file of ASCII format data that contains on each line a float value that represents the "magnitude" of one data point. I have a script that reads these values into a list of floats, since I don't necessarily even know the length of the file. I don't know any of the values, but they adhere to the range that a float can be. I need to convert this list of floats to a list of colors. In this case the data is temperature values, hence red->white->blue, but it doesn't need to be.

So this ended up essentially being my code:

     public List<Color> ColorMap_RedWhiteBlue(List<float> colorFloatList)
     {
         // Uses this function to scale a range [min, max] to a new range [a, b]
         //          (b - a)(x - min)  
         //  f(x) =  ----------------  + a
         //             max - min

         //Find max, min, and mid point
         float max = colorFloatList.Max();
         float min = colorFloatList.Min();
         float mid = (max - min) / 2f;
         float scaledValue;
         List<Color> colorList = new List<Color>();
 
         //Produce color for every point in the list
         foreach(float point in colorFloatList)
         {
             //Split the values into two groups: below the middle value and above the middle value
             if((point >= min) && (point <= mid))  //Bottom half of range (mid is                                                                            '                                                   now the maximum for this range)
             {
                 //Scale to range (0, 1). 1 (max) makes white, 0  (min) makes blue
                 scaledValue = (((point - min)) / (mid - min));
                 Color color = new Color(scaledValue, scaledValue, 1);
             }else if((point > mid) && (point <= max)) //Top half of range (mid 
                                                       is now the minimum for this range)
             {
                 //Scale to range (0, 1), flip it with (1 - x). 1 (min) makes white, 0 (max) makes red
                 scaledValue = 1 - (((point - mid)) / (max - mid));
                 Color color = new Color(1, scaledValue, scaledValue);
             } 
             colorList.Add(color);
        }
        return colorList;
    }

Convert List of Floats to Colors (Color Map)

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