Grayscale to Red-Green-Blue (MATLAB Jet) color scale

Question

I was given a data set that is essentially an image, however each pixel in the image is represented as a value from -1 to 1 inclusive. I am writing an application that need

Answer 1

I'm not really sure why there are so many complex answers to this simple equation. Based on the MatLab JET Hot-to-Cold color map chart and graph plot posted above in Amro's comment (thank you), the logic is very simple to calculate the RGB values using high-speed/basic math.

I use the following function for live-rendering normalized data to display spectrograms and it's incredibly fast and efficient with no complex math outside double precision multiplication and division, simplified by ternary logic chaining. This code is C# but very easily ported to almost any other language (sorry PHP programmers, you're out of luck thanks to abnormal ternary chain order).

public byte[] GetMatlabRgb(double ordinal)
{
    byte[] triplet = new byte[3];
    triplet[0] = (ordinal < 0.0)  ? (byte)0 : (ordinal >= 0.5)  ? (byte)255 : (byte)(ordinal / 0.5 * 255);
    triplet[1] = (ordinal < -0.5) ? (byte)((ordinal + 1) / 0.5 * 255) : (ordinal > 0.5) ? (byte)(255 - ((ordinal - 0.5) / 0.5 * 255)) : (byte)255;
    triplet[2] = (ordinal > 0.0)  ? (byte)0 : (ordinal <= -0.5) ? (byte)255 : (byte)(ordinal * -1.0 / 0.5 * 255);
    return triplet;
}

The function takes an ordinal range from -1.0 to 1.0 per the JET color specification, though this function does no sanity checking if you're outside that range (I do that before my call here).

So make sure you do sanity/bounds checking prior to calling this function or simply add your own limiting to cap the value when you implement it yourself.

This implementation does not take luminosity into consideration so may not be considered a purist implementation but gets you in the ballpark fairly well and is much faster.