Mar 27, 2007

Image Stacks in Photoshop CS3 Extended


Of all the new features in Photoshop CS3, those that stand out for me most are the ones that have been built around the new Align Content feature, engineered by Jeff Chien.

For example, you can use Align Content in Perspective mode to align group portraits and it is now possible to create really accurate Photomerge composites automatically.

Now that Adobe has announced Photoshop CS3 Extended alongside the regular version of Photoshop CS3, we can let you know about what some of the extended features will allow you to do. Of these, the Image Stacks rendering is, in my view at least, one of the most interesting new features in Photoshop CS3, better than Live Filters.

The new Stacks feature was engineered by Chris Cox and was originally designed as a tool for analytical work, where you could place a series of images together in alignment and apply a Stacks rendering to the layers and use this to process them in such a way that Photoshop will highlight the differences found between the layers, or as is shown below, blend the layers according to where there is a high frequency of recurring pixel values to display only the most commonly occuring pixel values. But I have also managed to discover several key creative uses for Image Stacks.

For this first demo, I had a series of six pictures where there was always at least one person walking through a scene. Using image stacks, I was able to automatically remove them from the shot.


Figure 1- Here is a sequence of photographs that were shot hand held over a time period of a minute or so. There were a lot of people walking in front of the fountain and I just made sure that I captured enough shots so that each portion of the picture had two or more frames where someone wasn’t in front of the camera.


Figure 2- I opened all of these photographs in Photoshop and went to the File ➯ Scripts menu to choose: Load Files into Stack. This opened the dialog shown here, where I chose Use: Open Files and checked the Attempt to Automaticallly Align Source Images and Create Smart Object after Loading Layers options.


Figure 3- Depending on how many pictures you have and how large they are, it may take a few minutes to process all the photographs. What you will end up with will be a new document with a Smart Object layer that contains all the previously open image documents as layers grouped within the smart object. If you double-click on the Smart Object layer you will see the full expanded list of layers contained in this Smart Object.



Figure 4- And now for the clever part. If you have the Smart Object open, make sure you close it again. You will want to start with the Smart Object selected (see the Layers palette top left). Go to the Layer menu and choose Smart Objects ➯ Image Stack Mode ➯ Median. Again, the stacks rendering may take a little while to complete. In the result shown here, the Median rendering managed to blend the layers such that nearly all of the people in the merged picture disapppeared completely.


Figure 5- The image stack median rendering did a pretty good job of removing the people, but there were still a few ghost outlines left. Obviously some people were having too good a time in the sun to want to move around much. Plus there were a few bits of rubbish and artifacts around the edges of the picture where the frames had overlapped. I tidied up the final picture by adding a little bit of spotting on a new layer and added some masked curves adjustment layers to provide some dodging and burning to produce the final version shown here.

Tips for getting the best results
When you see this technique demonstrated it does at first look quite magical, but there is a logical explanation for how the process works. The technique relies mainly on the use of the auto-align command to align a chosen set of sample images together and place them within a grouped smart object. After doing a little bit of experimentation I have found that if you record at least five or six (or more) exposures, this should provide enough separate images for Photoshop to process in order to work out which pixels appear most frequently at any particular spot in the picture and use the most commonly occurring pixels only to produce the finished blend shown here.

Removing noise using multiple exposures
Here is a technique that makes use of the Stacks feature in Photoshop CS3 to merge a set of identical exposures and obtain a smoother-looking image.


Figure 6

Figure 7
Figure 8- I started by going to the File menu and choosing Scripts ➯ Load Files into Stack and selected a set of five images to open that had all been shot at identical exposures of a subject with the camera fixed to the tripod. These were photographs that had been shot at a high ISO setting using a long exposure in low light conditions. I checked the Align Source images and Create Smart Object options and clicked OK.


Figure 9- The selected images opened as a single image document grouped together as a single smart object. If I were to double-click on the smart object icon, this would open the smart object in a separate document window and allow me access to all the individual layers, which wasn’t necessary in this case, but would be if you wanted to edit any of the individual layers.


Figure 10

Figure 11- Back in the original Smart Object document, I went to the Layer menu and chose Image Stacks ➯ Image Stack Rendering ➯ Median. The processing may take a little while, depending on the size and number of layers, plus bit depth. Once completed, you will notice how the Smart Object layer has a ‘stacks’ icon indicating the smart object has been rendered using the stacks feature.

Figure 12


Figure 13- Here is a comparison showing a close-up view of a single exposure (Figure 12) and a rendered version (Figure 13) where five separate exposures were merged to produce a smoother, noise-free image. The Median rendering was used here because it analyzes the image content on all the layers and averages out the pixel values to use the most commonly occurring pixel values only, thereby elminating nearly all of the noisy pixels that occur on each of the layers.

Median versus Mean
If you are processing a series of still life captures, then a Mean stacks rendering can remove more noise than Median. For example, if you were processing astronomy pictures, you would want to use a Mean rendering.

The techniques shown here are fairly easy to accomplish. The Align Content feature is so good at recognizing areas of similarity and aligning images together as layers, that you can quite easily get away without having to use a tripod to shoot the pictures that you want to combine together. So anytime you are in a situation where you think it might be useful to remove people from a shot or you want to improve upon the image quailty capture potential of a lowlight scene, just shoot a quick sequence of shots with the camera hand held, keeping it as still as possible.

About Martin Evening

evening.jpgMartin, if you don’t know, is a London based advertising photographer and noted expert in both photography and digital imaging. As a successful photographer, Martin is well known in London for his fashion and beauty work. Check out Martin’s web site.

Martin also works with the Adobe Photoshop engineering team consulting on new feature development and alpha and beta testing. He worked alpha & beta for Photoshop CS3 and Adobe Photoshop Lightroom and was influential with the new Adobe Bridge 2.0 and Camera Raw 4.0.

In addition, Martin is also a principal of PixelGenius where he designed and was product manger for the recently released PhotoKit Color 2. PhotoKit Color 2 applies precise color corrections, automatic color balancing and creative coloring effects. PhotoKit Color offers a comprehensive set of coloring tools for Photoshop 7.0, CS, CS2 (and soon CS3) for both Macintosh and Windows.

9 Responses to “Image Stacks in Photoshop CS3 Extended”

  1. Andrew Rodney Says:

    Wonderful tutorial as usual Martin.

    It would be useful if readers could download low rez copies of one or both projects to play with these new features on their end.

  2. Martin Evening Says:

    But of course, you would only be able to test this out if you had a copy of the extended version of Photoshop CS3 (not the current public beta). As usual I will be supplying certain images on the DVD that comes with my Photoshop CS3 book and making them freely available from the website. And that will include the pictures shown here. But I probably won’t get around to updating the sample image pages on the website till just before the book goes on sale or just after CS3 extended goes on sale.

    If anyone is intending to upgrade to the extended version of Photoshop I would suggest that they take the opportunity to shoot extra frames when photographing subjects like the ones I have shown here.Whenever I am out with my camera at dusk photographing landscapes, I know it is a good idea to fire off a quick succession of pictures knowing that I can blend these together in Photoshop C3 extended to get a smoother looking capture. Or whenerver I want to remove those tourists out the shot!

  3. curious Says:

    Thats great !

    So, if you can use this technique to filter something out of a photo and leave the background, could it work the other way around – remove the background and leave the subject ?

  4. SeanMcFoto Says:

    Fantastic Martin.
    A short time with extra images for much better control and quality.

  5. paulwasserman Says:

    This is a great technique, but I wonder if I need to get CS3 Extended to make use of it. I understand from the Adobe web site that stacks are also available in Photoshop CS3 standard, with lesser functionality than extended. Does any of what you’ve described here require the Extended version?

  6. Martin Evening Says:

    The Adobe site displays a semi-available icon, but I have never been able to see any explanation as to what that refers to. Image stacks are in essence Smart Objects that you can assemble with the auto align option switched on. The key feature presented here is the ‘Stacks rendering’, and in all the correspondence I have seen, this ships with extended only. All I have to check this against are my Extended version beta copy and standard version public beta copy. I don’t believe there has been any shift in plan here.

  7. solly Says:

    I can see a lot of potential for my still photography especially with the noise reduction in difficult light. Thanks for the explanation. The likelihood of forking out for extended after upgrading and getting into Lightroom in hopes that it becomes what we dream of is not too high (as in zilch). This should have been in CS3 (regular). It looks like a bad marketing decision re still photographers and one that makes me very grumpy. On the other hand, I can’t wait for your new book if extended isn’t requisite for making it useful. Thanks again.

  8. Martin Evening Says:

    The two image stacks techniques described here are indeed useful, especially for noise reduction. I agree, it is a shame that the median and mean rendering were not available in the standard version. When it came to writing an update for the book I did want to avoid devoting space to features that were only in ‘Extended’. I did keep to that plan mostly, but this particular feature was just too interesting to leave out of the book, especially since there will be professional users who are going to be getting the premium upgrade.

  9. kseidman Says:

    I am wondering if this image stacking technique would be useful and practical in my situation.

    I shoot medium format Velvia or 100 TMAX and then scan the 6×7 transparencies/negs with a film scanner (Minolta MultiPro, 3200 ppi). Resultant files are 350 MB each. Images are landscapes, always shot on a tripod.

    CONCERN/QUESTION: Can CS3 Extended successfully align and stack such large files? I am not sure just two images would reduce noise much (1/sqrt(2)) but perhaps stacking four (noise reduced by factor of 2) would be useful? Note, at present I have a PC with 2GB of RAM. I would upgrade to 4 GB (and use the 3GB boot.ini switch) to give more RAM to CS3 Extended if I thought this technique could work.
    Any thoughts or advice greatly appreciated.

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