BitmapData Average Colours 25 comments

If you want a very simple way of extracting a colour palette from an image, one technique would be to average the colour values within specific areas. Averaging colour values is almost identical to averaging numbers, except with the added initial step of finding the red, green and blue components of the colour. To do this we can use bitwise operators, in this case bitwise shift, to perform fast operations on each bit inside the unsigned integer returned by getPixel or getPixel32. If you want to know more about bitwise operators, Moock has written a detailed and, as ever, very clear article on where, when and why to use bitewise operations. You can read it here.

So once you’re familiar with how to shift the bits of an integer, you can easily get the RGB values from a 24 bit hexadecimal by moving the bits to the right by a certain amount using the bitwise right shift operator (>>).

For example, getting the red, green and blue values from a 24 bit integer would look like:

var colour:uint = 0x33CC99;

var R:Number = colour >> 16 & 0xFF;
var G:Number = colour >> 8  & 0xFF;
var B:Number = colour & 0xFF;

And finding the alpha, red, green and blue values of a 32 bit integer:

var colour:uint = 0xFF33CC99;

var A:Number = colour >> 24 & 0xFF;
var R:Number = colour >> 16 & 0xFF;
var G:Number = colour >> 8 & 0xFF;
var B:Number = colour & 0xFF;

So we know that by shifting the bits to the right, we can effectively break an unsigned integer representing a colour into its specific components, but what about changing these components back into an RGB or ARGB value? Well, it’s simply a matter of shifting the bits in the other direction, using the bitwise left shift operator, which looks like this << (two less than operators next to each other).

So using this bitwise shift left operator, you can take your RGB or ARGB values and cram the little buggers back into a usable format, with a little help from bitwise OR:

// RGB
var colourRBG:uint = (R << 16 | G << 8 | B);

// ARGB
var colourARBG:uint = (A << 24 | R << 16 | G << 8 | B);

Right, so we know how to take an unsigned integer, for example one that’s been returned from getPixel or getPixel32 and break it down into its RGB or ARGB components – and then for the sake of universal harmony, put it back together again and release it back into its 24 or 32bit world with all its binary friends.

So now, finding the average colour within a set of colours is simply a question of averaging the red, green and blue values of all the colours, and then turning these averages into a new colour.

The method bellow does just this, by looping through the pixels in a BitmapData object, adding up all of the red, green and blue values, dividing them by the total number of pixels and then creating a new colour from the results.

public static function averageColour( source:BitmapData ):uint
{
	var red:Number = 0;
	var green:Number = 0;
	var blue:Number = 0;

	var count:Number = 0;
	var pixel:Number;

	for (var x:int = 0; x < source.width; x++)
	{
		for (var y:int = 0; y < source.height; y++)
		{
			pixel = source.getPixel(x, y);

			red += pixel >> 16 & 0xFF;
			green += pixel >> 8 & 0xFF;
			blue += pixel & 0xFF;

			count++
		}
	}

	red /= count;
	green /= count;
	blue /= count;

	return red << 16 | green << 8 | blue;
}

So taking this a step further; say we want an array of 64 colours from an image, we can break the input image down into 64 chunks and find the averages of each. The easiest way of doing this is to build a grid, copy the pixels in each cell and use the averageColour function to return the average colour value of that cell.

public static function averageColours( source:BitmapData, colours:int ):Array
{
	var averages:Array = new Array();
	var columns:int = Math.round( Math.sqrt( colours ) );

	var row:int = 0;
	var col:int = 0;

	var x:int = 0;
	var y:int = 0;

	var w:int = Math.round( source.width / columns );
	var h:int = Math.round( source.height / columns );

	for (var i:int = 0; i < colours; i++)
	{
		var rect:Rectangle = new Rectangle( x, y, w, h );

		var box:BitmapData = new BitmapData( w, h, false );
		box.copyPixels( source, rect, new Point() );

		averages.push( averageColour( box ) );
		box.dispose();

		col = i % columns;

		x = w * col;
		y = h * row;

		if ( col == columns - 1 ) row++;
	}

	return averages;
}

In terms of building an accurate colour palette from an image, this isn’t a great method, mainly because averaging the colours doesn’t give you a fair representation of the range of colours in the image. The palette returned can often be somewhat muddy; however it is still a technique that I’ve found a use for on a surprisingly regular basis and so thought it worth sharing. Becoming familiar with bitwise operators is also a useful exercise and so, if they aren’t already part of your life, I suggest you begin to woo them with your nerdy charm.