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Flex Thermal Colormap

January 23rd, 2009 Leave a comment Go to comments

As a user of Matlab, I have never had to generate a “heat” colormap, since I always use the built in one. It just so happens that among other ongoing projects I have not listed on this website, I will attempt to write some computational examples in Flex/Actionscript 3.0. In order to visualize the data, of course one needs to have some type of plotting features.

I believe I figured out how the “heat” colormap that is generally used for thermal visualization (I’m a Mechanical engineering student) works. It’s actually just HSV. Now depending on how you define HSV, the conversion seems to require people to do strange things (aka the implementation on Wikipedia, that uses HSV and HSL or whatever interchangeably).

However, using GIMP, I found out that the heat colormap is easy to generate from HSV. If you load up the color wheel in any graphical editor that supports choosing colors in HSV, you will notice that it’s generally a wheel, or that the values go from 0 to 360 (obviously a circle). Now the heat map color range we want is usually from [0:240]. We also want to have the higher values map to Red (Hue=0) and the colder or lower values to Blue (Hue=240). The mapping on the color wheel makes it fairly clear what the other algorithms are trying to do.

The mapping algorithm from HSV to RGB is as follows (assuming a S and V=1.0 – a.k.a. heat map colors).

From H=0-59 R=255 G=0-255 B=0

From H=60-119  R=255-0 G=255 B=0

From H=120-179 R=0 G=255 B=0-255

From H=180-239 R=0 G=255-0 B=255

From H=240-299 R=0-255 G=0 B=255

From H=300-359 R=255 G=0 B=255-0

As you can see, it’s a fairly simple linear mapping if you go by HUE. The permutations go according to ROYGBIV along the circle. S, V are selected to be 1.0 simply because that’s what the thermal color map is.

Anyways, the following is just a bit of flex code that displays the value of a 2D function on a domain of [0,1]x[0,1]. It’s my first application with flex and so I’m a little unfamiliar with the data types. So if anyone needs an easy to understand heat/thermal colormap, here’s a “crippled” version of HSV to RGB specifically made for thermal maps.

< ?xml version="1.0"?>
<mx :Application xmlns:mx="http://www.adobe.com/2006/mxml" 
  backgroundColor="#123456"
  creationComplete="init();" width="600" height="600" verticalAlign="middle" backgroundGradientAlphas="[1.0, 1.0]" backgroundGradientColors="[#FFFFFF, #AAAAAA]">
 
  </mx><mx :Script>
  < ![CDATA[
 
 
	var plot:Shape = new Shape();
	var domain_width:Number = 1.0;
	var domain_height:Number = 1.0;
	var domain_xelem:uint = 100;
	var domain_yelem:uint = 100;
	var color_value_max:Number = 1.0;
	var color_value_min:Number = -1.0;
 
    public function init():void 
    {
 
 
    }
 
    public function heatmap(value:Number):Array
    {  	
 
 
    	var RGB:Array;
 
    	// Remap hot color to red and cold color to blue
    	// 240 is the angle range [0:240] for red to blue in HSV
    	var h:uint=(1-(value-color_value_min)/(color_value_max-color_value_min))*240;
 
    	var g:Number=(h%60)/60;
    	var inc:Number=255;
    	var up:uint=int(g*inc);
    	var down:uint=255-up;
 
    	switch(int(h/60))
    	{
    		case 0: return new Array(255,up,0);
    		case 1: return new Array(down,255,0);
    		case 2: return new Array(0,255,up);
    		case 3: return new Array(0,down,255);
    		case 4: return new Array(up,0,255);
    		case 5: return new Array(255,0,down);
    	}
 
		return new Array(255,255,255);
    }
 
    public function RGB(RGBArray:Array):uint
    {
    	return RGBArray[0]*65536+RGBArray[1]*256+RGBArray[2];
    }
 
    public function showMap():void
    {
    			// Define height parameters
		var dx:Number=domain_width/domain_xelem;
		var dy:Number=domain_height/domain_yelem;
 
		// Define color gradient
		var RED:Number = 0xFF0000;
		var BLUE:Number = 0x0000FF;
 
		var scrn_width:Number = myCanvas.width;
		var scrn_height:Number = myCanvas.height;
 
		for(var i:uint=0;i<domain_xelem;i++)
		{
			for (var j:uint=0;j<domain_yelem;j++)
			{
				var x:Number = i*dx+dx/2;
 
				var y:Number = j*dy+dy/2;
				plot.graphics.beginFill(RGB(heatmap(Math.cos(2*Math.PI*x)*Math.cos(2*Math.PI*y))));
				plot.graphics.drawRect(((x-dx/2)*scrn_width),((y-dy/2)*scrn_height),(dx*scrn_width),(dy*scrn_height));
				plot.graphics.endFill();
			}
		}
 
      myCanvas.rawChildren.addChild(plot);
    }
 
  ]]>
  </mx>		
  <mx :Canvas width="400" height="400" id="myCanvas" backgroundColor="#903636">
  </mx>
  <mx :Button label="Generate Color map" id="btnColorMap" enabled="true" click="showMap()"/>
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