Web hosting reseller - CHAPTER 10 ADVANCED COMPOSITING 409 Increasing Resolution
CHAPTER 10 ADVANCED COMPOSITING 409 Increasing Resolution by Averaging When the Titan images were stacked, they didn t get sharper, merely more contrasty. But stacking, carefully done, can also increase sharpness. Technically, this is known as increasing resolution, the size of the smallest detail that can be distinguished in the photograph. In recent years, video astrophotography has become very popular. Astrovideographers take cheap webcams or security cameras, modify them to mount on a telescope, and then take videos of bright objects such as the moon, Jupiter, or Mars. Each individual image is awful, and it doesn t make a very interesting movie. But when the video is split into its component frames several hundred separate images they can be stacked to make a very sharp photograph. Using these techniques, amateurs are now getting images as sharp as the professional observatory photos from a few decades ago. First, the bad frames are removed. Turbulence in the atmosphere (even on a clear night) means that some images in these movies will be too blurry to be worth keeping. Then, the remaining frames are stacked using an averaging technique. Each layer is in normal mode, and the stacking is done with GIMP s Opacity slider. The bottom layer, of course, is at 100% opacity. Select the layer above it, and change the opacity to 50%. Now you re seeing an image that comes half from the first layer and half from the second: they re averaged. The resulting image won t be any brighter or darker than the components, but it will share pixels from each one. Now make the third layer visible and select it. Set its opacity to 33%. That means that it ll be contributing only a third to the final image; the combination of the first two images will contribute two-thirds. So you re still seeing an average of the three. Turn on the fourth layer, and set its opacity to 25%. And so on through all the layers. The rule you re following is that layer N gets opacity 1/N: 1/1, 1/2, 1/3, 1/4, and so on. Of course, you wouldn t want to do this by hand if you had 200 layers. Astrovideographers usually use dedicated programs that handle both stacking and registration. Within GIMP, for a large number of layers, you d be better off writing a script (you ll learn how to write simple scripts in Chapter 11). Stitching Panoramas Ever find a beautiful sweeping view and feel frustrated that your camera s lens isn t wide enough to fit it all in? Even if you have an SLR with replaceable lenses, sometimes you ll want to shoot vistas that are just too wide for the camera to capture in one shot. Back in the film days (remember film?), I sometimes used to shoot several overlapping pictures, and then take the prints and tape them together and pin them up outside my office. They might have ended up looking something like Figure 10-36. That might have been good enough a decade or so ago, but you can do a lot better than that now!
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