Recipe 8.10. Creating Noise

Problem

You want to create a random noise pattern in a bitmap.

Solution

Use the noise( ) method of the BitmapData class.

Discussion

A noise filter or pattern is simply sets each pixel in an affected area to a random value. Although you could loop through each pixel in a bitmap and use setPixel( ) to randomly change its color, there is an easier and more powerful way of doing the same thing in one line: the noise( ) method.

By itself, noise( ) creates a random, speckled image, like what you see when a television is tuned to a channel with no signal. However, by combining noise with other filters, such as the blur filter, you can get some very useful effects.

The noise( ) method is called directly on an instance of BitmapData, and its parameters are straightforward:

bitmap.noise(seed, low, high, channel, grayscale);

Use the seed parameter to determine the random pattern; it can be any integer. If you call the method twice with the same seed, you get the same noise pattern. To ensure you get a different pattern each time you call it, pass in a random number like so:

Math.random(  ) * 100000

The low and high parameters determine the minimum and maximum values for each pixel. They can range from 0 to 255. Setting them higher gives you a brighter noise pattern; setting them lower makes darker noise.

The channel parameter specifies to which color channel of the bitmap the noise will be applied. You can specify 1, 2, 4, or 8 for red, green, blue, and alpha, respectively; or use the static properties of the BitmapDataChannel class: RED, GREEN, BLUE, and ALPHA, to guard against typos.

Finally, grayscale is a Boolean value. true applies the random value to the three color channels equally, resulting in a colorless noise.

Here are some examples. First, create a bitmap, add the noise, and then display it:

bitmap = new BitmapData(stage.stageWidth, stage.stageHeight,                      false, 0xff000000); bitmap.noise(1000, 0, 255, BitmapDataChannel.RED, false); var image:Bitmap = new Bitmap(_bitmap); addChild(image);

This creates a random pattern in the red channel. Upping the low value gives you a lighter noise pattern:

bitmap.noise(1000, 200, 255, BitmapDataChannel.RED, false);

You can easily convert that to a grayscale image by setting the last parameter to true:

bitmap.noise(1000, 200, 255, BitmapDataChannel.RED, true);

If you have set grayscale to true, it doesn't matter which channel you create the noise on. You can also combine various channels to create multicolored noise, using the | operator:

bitmap.noise(1000, 0, 255, BitmapDataChannel.RED |                         BitmapDataChannel.GREEN |                         BitmapDataChannel.BLUE,                         false);

Finally, as mentioned earlier, perhaps one of the best uses of noise is when you combine it with other filters or effects. The following code creates some noise in a bitmap, and then applies a horizontal blur filter, producing a passable brushed metal effect:

bitmap = new BitmapData(stage.stageWidth, stage.stageHeight,                      false, 0xff000000); bitmap.noise(1000, 128, 255, BitmapDataChannel.RED, true); bitmap.applyFilter(bitmap,                 bitmap.rect,                 new Point(  ),                 new BlurFilter(30, 1, 3)); var image:Bitmap = new Bitmap(bitmap); addChild(image);

This is just one example of combining noise with other effects. With some experimenting and practice, you can come up with many more.

Problem

Solution

Discussion

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