Web site design - CHAPTER 10 ADVANCED COMPOSITING 405 Although stacking
CHAPTER 10 ADVANCED COMPOSITING 405 Although stacking techniques were developed first for astronomy, you can stack earthbound images as well. You can boost the light level of a dim image, improve contrast, or even increase resolution. It can also be fun to stack several different images into an average. But stacking will always add another benefit: reduced noise. Reducing Noise Every photograph includes some unwanted interference, from thermal activity in the camera s CCD chip or various other sources. (Such as cosmic rays. Really!) Collectively, these spurious signals are called noise. Noise is random: it shows up in unpredictable places. If you take two images of the same scene, the scene may not change, but the noise will be different. Suppose you take an image that has a light speck due to noise. If you composite that image with itself, or use a tool such as Levels or Curves to boost the contrast, you will increase the speck of noise as much as the object you re trying to capture. If you ve tried boosting contrast on a very dark photograph, you ve probably seen that already. But if you composite two different images of the same object, chances are that the second image s noise will be in different places. The noise in the first image will be partially cancelled out by the second image, and vice versa. The result is a less noisy image. So how do you go about making a stack? There are three steps: 1. Open the images as layers. 2. Register the layers. 3. Combine the layers. Loading All the Images as Layers Load each image in the stack as a separate layer. Open the first image normally, and then use File . Open as Layer… for the rest. If you have more than two images, use Shift-click or Control- click in the file selection dialog to select all your images at once, and they ll all open as separate layers. Once all the layers are loaded, make all but the first two layers invisible. Next, before you stack the two layers, you have to register them. Registering the Images Using Difference Mode Stacking won t work well unless the images fit over each other exactly. If the subject of the photograph isn t in the same place, or isn t the same size, then it will look blurry when the two images are combined. You can make this step much easier if you use a tripod when shooting images you plan to stack. If you try to shoot hand-held, you will almost certainly move or rotate the camera. That can be fixed, but it s tedious: use GIMP s Measure tool to measure sizes and angles in the two images, and then Scale and Rotate by the ratio of the two measurements. Eek! It s much better to use a tripod and get them right in the first place.
We recommend you use shared web hosting services, because many users agree that it is cheap, reliable and customer-satisfying webhost.