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414 CHAPTER 10 ADVANCED COMPOSITING Adjust the Layer Mask The gradient in your layer mask was great for a first step, but now it s time to eliminate parts of the image that don t match well. You don t want to replace the gradient, just add to it. In this case, the first priority is getting rid of those mesa tops that don t line up. To do that, a bit of black added with the Paintbrush tool in the upper-left of the second image s layer mask will make the ghostly floating mesa disappear. (Remember, black in the layer mask makes that part of the image invisible.) Watch the double images gradually go away as you scribble it s fun. If you need to get rid of a whole corner of an image, you can add another gradient to the existing gradient. Use the Mode menu in the Gradient tool: any of the darkening modes, such as Multiply, Subtract, or Darken Only, will do (they ll give slightly different results at the edges where the two gradients meet, but you probably won t see the difference). Tip Remember that you can use Show Layer Mask from the Layers dialog (or Alt-click or Alt-Shift-click on the layer preview). When you re finished adjusting the mask, click on the layer preview in the Layers dialog so the mask won t remain selected. That way, you won t be surprised if you try to do anything with the layer later. Figure 10-39 shows the mesa-top area cleaned up (I cheated a little and made a new butte). Notice that the sky now looks patchy. Don t worry, you can clean up the sky later. Figure 10-39. The area around the mesa top is cleaned up. The sky still needs work.
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CHAPTER 10 ADVANCED COMPOSITING 413 Figure 10-38. Draw a gradient on the layer mask so that the left edge of the second image fades into the first. Fine-Tune the Position of the Second Image Now that the second image has transparent areas, you can position it more accurately. Switch to the Move tool, but there s a trick: if you simply click and drag with the Move tool, you ll move the layer mask rather than the layer itself, since the mask is still selected in the Layers dialog (indicated by the white boundary around it in Figure 10-38). Click on the layer preview in the Layers dialog to select the layer and not just its mask. Zoom way in so that you can see details, and scroll so you can see the region of the image where the two layers overlap. Then use the Move tool to move the second image around until it matches the first image as well as possible. (Don t forget about the arrow keys.) It ll usually be obvious when the images mesh. When you have two overlapping images (one partially transparent) that don t quite match, the image will look a bit blurry, as though you had a few too many beers before beginning the stitching process. When you find the right position, suddenly the image will seem to snap to focus, and all the blurriness and double vision will go away. The catch? It won t happen everywhere in the image. Most of the time, the two images won t match up perfectly. If you line them up in the center, the top and bottom won t match. You re seeing lens warp. If you overlapped the images a lot (50% or more) or zoomed in when you shot the images, you ll see less lens warp, but there will always be some. Usually, you re best off matching two layers at the vertical midpoint, and not worrying about the top and bottom (you can fix that in the next step). But if your panorama has a pronounced horizontal line somewhere say, the horizon in an ocean shot you may want to use that as the location to match.
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412 CHAPTER 10 ADVANCED COMPOSITING Load the First Two Images Open the first image as a new layer and move it (using the Move tool) all the way to the left side of the image (assuming your panorama is left to right). The panorama image will look something like Figure 10-37. Figure 10-37. The first image is loaded. Next, load the second image as a new layer, just as you did the first, and use the Move tool to move the new layer to where it approximately overlaps the first one. Don t worry about getting it perfect . . . yet. Make a Gradient Layer Mask How do you get rid of the sharp edge between the first picture and the second? By using a layer mask. In the Layers dialog, right-click on the second layer and choose Add Layer Mask…. Initialize it to White (full opacity), the default. Now use the Gradient tool to draw a gradient on the layer mask. It should be white everywhere to the right of the overlap, shading to black at the left edge of the second image. If you have the normal black foreground and white background, and the Gradient tool is in normal mode (not Reverse), this means starting your drag from the left edge of the second image, and then dragging right for some distance, not all the way to the point where the first image ends. You ll end up with a result like Figure 10-38. Tip It helps to use the Control key while dragging your gradient, to help get a gradient that s exactly horizontal.
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CHAPTER 10 ADVANCED COMPOSITING 411 a panorama is more difficult than it sounds, and you can save yourself a lot of work by shooting them level to begin with. It s less important that you keep the horizon at the same height for each photo. You can have more sky in one shot, more ground in another, and the panorama will still look fine. The downside: to get a rectangular picture you may have to trim more from the top and bottom than is ideal. Match Exposure Levels If you can shoot all the images at the same exposure setting, that may save a little work later. Otherwise, the individual photos may vary in color or brightness. Notice the different colors of the sky and the ground between the left and middle pictures of Figure 10-36. Some cameras offer a panorama setting which does this automatically, or manual exposure controls. If yours doesn t, you may be able to simulate it by metering on a point somewhere around the middle of your panorama: hold the shutter button halfway down, rotate the camera (still holding the button halfway), and press the button the rest of the way to take the picture. Repeat as necessary. This isn t crucial. You can adjust brightness in the GIMP, of course. And a panorama whose components have slightly different brightness levels may still look okay. Try it both ways and see whether it s worthwhile for you. Once you ve shot all your images and uploaded them to GIMP, it s time to start stitching. Decide on a Resolution Although it s normally best to work at the full resolution of your camera and scale the final image down after you re finished, that s not necessarily true for a panorama. Panoramic images combine a lot of photos into one humongous GIMP image. Using three-megapixel or larger images for a big panorama (say, five or more images) can take up so much memory that it slows the machine to a crawl. You might use full resolution if you need an image that large (for instance, if you plan on printing to a banner three-feet long). But otherwise, if you notice slowness, consider scaling down a bit. Once you know the resolution of the individual images, how many of them there are, and the overlap you used, you can calculate how big an image you ll need to hold the panorama. Calculate Your Expected New Image Size The next step in building a panorama is to make a new image of the right size. What is the right size? That s something you ll have to figure out. The easy way out is to simply take the width of your images and multiply by the total number you ll stitch together. There s not much penalty for starting with a new image that s too big you ll just crop it in the end anyway. (GIMP may complain if you try to create a new image larger than the parameters set in your preferences. The default, which you probably didn t change, is 64MB. Should you need to change this setting, you ll find it in Preferences under Environment.) If you re feeling lucky, subtract whatever you think you can get away with, say 20 to 25%. Don t stress over it. If it s too big, crop it later. If it turns out you need more space, a simple Image . Canvas Size will fix it in a jiffy. Once you ve decided on a size, create the new image. I ll refer to this image as the panorama image.
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410 CHAPTER 10 ADVANCED COMPOSITING Figure 10-36. An old-style panorama, before GIMP Combining several images into a panorama is called stitching. The process is straightforward, but it includes several steps, and requires some patience to get a result that looks truly seamless. Let s take it one step at a time. Shooting the Images Perhaps you already have a collection of images you d like to stitch together. If not, here are some handy tips for taking a panoramic series. Some people find this stage the most intimidating part of the process. It s really not a problem if you follow a few guidelines. Overlap Photos for Panoramas Overlap quite a bit. At first, you might be tempted to put the left edge of image 2 right where the right edge of image 1 was. There are two problems with that. First, camera lenses (especially wide-angle lenses) distort the image (a phenomenon known as lens warp). The distortion is especially pronounced at the edges. If you use a lot of overlap, you ll be using mostly the centers of each frame where there isn t so much distortion. Second, a lot of overlap means that if one image turns out to be bad, you can discard it and still make a usable panorama from the remaining images. Overlap at least a third of each image (as in Figure 10-36), but more is better, especially if you re using a digital camera: bits are cheap. Keep the Horizon Level Keep the camera level: don t tilt it between shots. If you have a tripod, or a flat surface like a picnic table, that makes it easy. If you have to hold the camera by hand (and let s face it, most of the time when you see a spectacular vista worthy of a panoramic shot, you probably won t have your tripod with you), try standing in a stable position and twisting your body as you take each shot. Pay attention to the horizon in the camera s viewfinder: try to keep it parallel to the top and bottom of the frame. If you accidentally tilt the camera, you can use GIMP s Rotate tool to make it level after the fact. However, rotating images so that they all match well enough to make
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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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408 CHAPTER 10 ADVANCED COMPOSITING Figure 10-34. Saturn s moon Titan, from NASA s Cassini spacecraft Figure 10-35. Stacking several layers in Multiply mode boosts contrast, so you can see Titan s cloud details.
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CHAPTER 10 ADVANCED COMPOSITING 407 Figure 10-33. A 24-layer stack of the same fence is much less noisy. You can even see some stars! Increasing Contrast by Multiplicative Stacking Those great shots that you see from NASA spacecraft aren t just snapshots downloaded straight from the camera. They take a lot of processing before NASA puts them up on public websites. (If you watch NASA TV, sometimes you can even see GIMP on screens at Mission Control! Of course, NASA uses a wide assortment of tools, not just GIMP.) Figure 10-34 shows an original photo of Saturn s largest moon Titan, taken from the Cassini spacecraft. (You can find unprocessed images by looking for raw images on the Cassini web- site.) You can see that there s a little detail there, but it s hard to see it well. The contrast is very low. But Cassini actually shot four of these Titan shots, and you can download all four and stack them. Register the frames, and then set each layer s mode to Multiply (Figure 10-35).
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406 CHAPTER 10 ADVANCED COMPOSITING Note Even the best tripod will not give you perfectly registered photos when you take pictures of the night sky. Why? The earth is turning, which means, from the camera s point of view, that the sky is moving about a degree every four minutes. Once the images are approximately aligned, select the Move tool in the Toolbox and set the mode of the upper layer to Difference. This will show every place the two images differ. Two perfectly registered, identical layers will look totally black when the top layer is in Difference mode. Otherwise, move the upper layer around (remember the arrow keys for moving by one pixel at a time) to make the combined image as black as possible. Now it s time to make a stack, using one of the methods below. After you ve stacked one layer, make the next layer visible, register it, and stack it. Repeat until you re out of layers. Increasing Light by Additive Stacking There are times when a flash just isn t appropriate. Did you ever go out at night with your little digital camera and wish you had a fancy SLR that could take time exposures? With stacking, you can even if your camera can t. Just take a lot of short exposures, and then stack them in Addition mode. For instance, I shot some photos of a fence near a local reservoir. The camera I was using wouldn t do an exposure longer than four seconds. At that setting, the original images just look black. It s hard to tell that there s anything in the shot at all. If I take a single image and use Curves or Levels on it, I can see that I do indeed have a shot of the fence. But I have to enhance the image so much that it becomes extremely noisy and grainy. I can do a little better by making 10 or 12 copies of the image and compositing them all in Addition mode. But it s still very noisy. But what if I take 12 different images of the fence, and then stack those? Stacking different images allows you to increase the light level a lot with hardly any increase in the noise. You can even duplicate some layers: Figure 10-33 shows a stack of 24 layers total, two copies each of 12 different layers, all in Addition mode. This technique reduces noise so well that you can boost the brightness even more using Levels or Curves without hurting the image much. So grab a tripod (even one of the little plastic ones with legs a few inches long is fine if your camera is small enough), wander off to a dark place, and try taking some shots!
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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.
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