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Hi, gang! This is Deke McClelland. Welcome to Deke's Techniques. This week we are going to talk about a feature that most folks don't even know exists inside Photoshop Extended, by the way--CS3, CS4, or CS5--and it's called image stacks. Now, ultimately image stacks are a collection of statistical analysis tools, as sexy as that sounds. However, one of the many things you can do with it is take a bunch of images that you have shot of the single scene and wipe out all of the moving stuff, which is to say all the people. So for example, I shot these images at this place Teatro Olimpico in Vicenza Italy, and I just could not shake that woman in the foreground; she kept remaining inside the frame.
Well, I figured that would be pretty easy for Photoshop to get rid of, so I shoot up the ante. And this is the part where you might think I kind of lost my mind, but I decided I should add a big floating orangutan head, and it could grow increasingly larger like so, or it could become very small indeed. And despite all these things appearing inside the scene, I was able to merge them together using image stacks and completely eliminate everything automatically and end up with this pristine scene here.
Let me show you exactly how it works. All right, we are starting things off inside the Adobe Bridge which ships with every single version of Photoshop out there, and I'm looking at these photographs that I shot in Vicenza Italy in this place called Teatro Olimpico, which is this amazing work of art, the last work of Palladio, also the oldest enclosed theater in the world. And if I press the spacebar in order to preview these images, what you're seeing in the background is this forced-perspective backdrop that declines away from the viewer in five different directions.
It's absolutely astonishing, I have to say. It's made of wood and stucco in the 16th century and it's in mint condition today. And yet as I try to shoot this image, I've got this woman wandering around in the foreground, checking me out. I don't know what's going on here, but she can't be there. I need to go ahead and eliminate her from the scene. Well, fortunately because I have these four shots of the scene with her wandering from left to right here. I can go ahead and eliminate her automatically using this feature called Image Stacks which is exclusive to Photoshop Extended.
It is not included with the standard version of the software. Anyway, let me show you how it works. I am going to go ahead and select the first image, Shift+Click on the fourth image in order to select it as well, go up to the Tools menu--again, I remind you I'm inside the Bridge. I'll go up to the Tools menu, choose Photoshop, and choose Load Files into Photoshop Layers. That's step number one, and what that does is it assembles all those selected images into a layered composition. Now, check this out. I want you to see that these layers are not actually aligned with each other. I will go ahead and zoom in a click here, Alt+Click or Option+Click on the eyeball associated with the rear layer, make sure the rear layer is selected, and then if I press Alt+Right Bracket or Option+Right Bracket on the Mac, I'm basically jumping from one layer to the next. And you can see that the layer itself is jumping around as well.
So the image is not, strictly speaking, aligned. That's because I was not using a tripod; however, I was not moving either. So you don't want to be moving forward or backward or back and forth. That will ruin this effect. All right, step number two is to go ahead and turn everybody on, so make sure all the layers are selected, click on the bottom layer, Shift+Click on the top layer in order to select all of them, and then go up to the Edit menu and choose Auto-Align Layers. You will get this big dialog box; just make sure Auto is selected. That's all you need to do and then click on OK.
And now what Photoshop is doing is it's searching for similar pixels and aligning them together, and now if you click on the bottom layer, Alt+Click or Option+Click on its eyeball to view that layer independently, and now press Alt+Right Bracket or Option+Right Bracket on the Mac, you can see that nothing is moving in the scene except for this woman going back and forth. That's it. All right, step number three is to go ahead and turn everybody back on again, click on the bottom layer, Shift+Click on the top layer to make sure they're all selected, go to the Layers panel flyout menu and choose Convert to Smart Object.
This is a Smart-Object-dependent effect. And now you have got everybody assembled together. You can rename that layer if you want to. I am just going to call it Teatro Olimpico. And then step number four, exclusive to Photoshop Extended, you go up to the Layer menu, you choose Smart Objects, you choose Stack Mode, and then you are going to see this list of statistical analysis tools. Now, most of them are designed for scientists. They are not designed for regular old image editors. So I doubt you'll ever have any need for entropy or kurtosis or many of these guys.
However, there are two of these commands that are of great use, even though they are still useful for statistical analysis as well, they are of great use to general photographers. One of them is Mean. Mean is going to find the average color of the pixels that are stacked on top of each other. So in our case, it's going to go ahead and create these ghost versions of this woman walking around in the foreground. That's not really useful for this exercise; however, imagine this situation: imagine you have a bunch of night shots and you've got a high ISO, so you have got some very noisy photographs, or relatively noisy anyway.
And you shoot multiple images of that same scene, hopefully using a tripod in that case, then you merge them together using that Mean stack mode, and you're going to eliminate a lot of that noise because you're averaging away the noise inside the scene. All right, now I also want you to see, over here in the Layers panel, if you go ahead and click on this down-pointing arrowhead, you will see that we have a stack mode and it's Mean. You can only have one stack mode per Smart Object, although you can also keep on Smart Filters, layer effects, and so on, but just one stack mode. That means if you want to switch to a different stack mode, as we do, then you go up to Layer menu, you choose Smart Objects and you choose Stack Mode, and then you choose the command you want to use.
Now, in our case, it's going to be Median. Now, for now, I will just show you that it absolutely works. It wipes out all the different stuff in that scene. In this next example that I am about to show you now, I will explain what's going on with that tool and when it doesn't always work and what to do about it. All right, so I am going to go back to the Bridge by clicking on the Bridge icon up here in the application bar, and them I am going to switch down to four other variations of this image. Now, my reasoning for creating these goofy variations was that this woman walking across the scene, it's not really that difficult to solve for; I could have just stacked a couple of images on top of each other, layer masked one image over the other, and so forth.
However, what if you've got bigger problems? Like what if have a bus driving through your scene? You can still eliminate it using the Median mode. So what I decided to do was infect these images with this detached monkey head right there. So it's appearing in different positions, sometimes very large as you see it here, sometimes relatively small in this orchestra pit or whatever. But we still have to contend with this bizarre monkey head coming into the scene. So what we do? Well, same thing. I will go ahead and escape out and Shift+Click on the first image so all four of them are selected, go to Tools, step number one of course, choose Photoshop, and then choose Load Files into Photoshop Layers.
And that'll go ahead and assemble a composition that contains all four of those images. Then you already know that they're not aligned with each other, so click on one, Shift+Click on the other in order to select all those layers, go up to the Edit menu, choose Auto Align Layers and go ahead and click OK, because Auto is the default setting. Wait for Photoshop to do its thing, to evaluate the images an align them with each other. Even though there's is this ginormous monkey head in the scene, Photoshop has still managed to do the right thing. So if I click on his bottom layer, Alt+Click or Option+Click on the eyeball to see it by itself and then press Alt+Right Bracket or Option+Right Bracket on the Mac to cycle from one layer to the next, you can see that the scene is utterly aligned, totally awesome.
All right, go ahead and turn all those layers back on, click on one, Shift+Click on the other to select them all, go the Layers panel flyout menu, choose Convert to Smart Object. That's step number three. And then finally, step number four is to go to the Layer menu and choose Smart Objects and choose Stack Mode, and I want you to see a couple of the other ones here. We've got a few others that are useful to general photographers; for example, Minimum is going to go ahead and find the darkest pixel throughout all of the images, and so in that case we are keeping all versions of this woman here in foreground as well as in some of these dark details from the monkey heads.
If you want the opposite, if you want the lightest details from the various layers, then you go to Layer menu, you go to Smart Objects, you choose Stack Mode, and you choose Maximum instead, so the maximum luminance level, which is going to be your brightest luminance level. In this case it keeps mostly the monkey head details. We have a few little details from the woman, some highlights in her face and her hands, but that's about it. And another one that's kind of interesting in my opinion, if you go up to Layer > Smart Object > Stack Mode and you choose Range, you are going to subtract the minimum information from the maximum information, and what you are going to find is all the stuff that changed throughout the scene.
So anything that's the same is going to stay black, and anything that's different is going to be some other color. So it's almost like a Difference blend mode that's run across all of the images it wants. Now, what we really want of course is to go up to Layer menu, choose Smart Objects, then choose Stack Mode and then finally choose Median, and I want you to check this out. It's fairly amazing. Given these big huge monkey heads that have somehow populated the scene, once we choose Median, they go away. All vestiges of monkey heads, and the woman of course, are gone.
That is to say almost all. Check this out. If you zoom in, you do have to check out your scene. Take a careful look at it. Notice there is this kind of bizarre ear form right there, and then it goes around like so. Darn it, that's a monkey head. So what you need to do, if you find something like that that sticks around, it's probably not going to be a monkey head. It's probably going to be some other detail, but like some part of a buzz, for example. Go ahead and double-click on the thumbnail for your Smart Object in order to open that Smart Object, so that you can investigate the contents of the scene. Now, what's going on with Median? Now, this is kind of hard understand.
It easy to understand Mean, right, the one that is going to get of your noise. It's just finding the average color of all pixels. That's it. Median is more of a popularity contest, so it's trying to find that one pixel color that has as many other pixels that are identical to it as are either lighter or darker. And so what that means is as long as any given pixel only varies once across your images, then it's going to get canceled out by the good pixel. However, if you've got a person that appears in front of another person at some point in time, or the monkey head appears in front of itself at some point in time, then that's going to ruin things. And in our case, if I go head and turn off this top layer, notice where the monkey head is here, and I'll turn off the top layer, and we've got the ear.
Well, it's at about the same location as the chin of the top-layer monkey, and that's the problem. The big monkey over here on the third layer, he's fine because he is away from everybody, and then the monkey in the orchestra pit, he's fine, too, because he is away from everybody. But these two top guys kind of overlap each other, so what I am going to do is just eliminate the second layer. So I will just turn it off and leave the other three on. And then I'll go up here-- I don't have to throw it away or anything-- go ahead and close this image window, then click on the Yes button here on the PC, or the Save button on the Mac, and notice our problem totally goes away because now the popularity contest is working in our favor.
And that, folks, is how you eliminate people and other moving objects from your multi-shot scenes here inside Photoshop Extended.
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