Making Selections For Composite Images

[Kevin Childress]

This post is to follow up with a recent Critique Forum discussion regarding composite images, or more specifically, about making foreground selections when layering in a new sky for a composite image. I've had a bit of success in creating such composites and wanted to share a couple techniques that work best for me in hopes the information will be useful to others. As with many topics in post processing, there are many different ways to achieve similar results. This article is written with using Photoshop.  This post is not about whether or not I chose the correct sky for my composite - the primary intent here is to demonstrate that if you're willing to invest the time, you can make incredibly tight selections, which is quite necessary for creating good composites. 

The first image below is a before-and-after screenshot of one of my favorite composites which was created a few years ago using Photoshop CS5. As a matter of fact, this image was my first really successful composite. I think the key to making really good landscape/sky composite images is being able to make incredibly tight selections, especially if your composite sky has new cloud formations. Those tight selections are quite necessary for layering in your sky behind trees, leaves, grass, etc. The grass in the reflection in the water below was the toughest part of creating this composite. 

The next image is a 1:1 screenshot of one area of the grass and lighthouse. I show this image for two reasons: 1) To show a 1:1 view of how the clouds are indeed behind the grass. 2) To show a huge mistake that I made in creating this composite. In the 'before' image, look down the right edge of the lighthouse and you'll see a lot of nice ripples in the water right along the edge. I was so focused on making selections of the grass that I initially ignored the reflection of the lighthouse and completely missed including those ripples in my first composite. The point being: when you're making these tight selections, particularly where reflections are concerned, you need to be really observant of nice detail like those ripples in order to create a more believable composite. I've never been questioned on the missing ripples so I don't know if anyone has ever noticed it or not. Whatever the case, the ripples should be in the composite.

In my experience there are two requirements for making these tight selections. 1) A good understanding of how the selection tools work in Photoshop (or any image editing program for that matter). In Photoshop we have the Quick Selection Tool, the Magic Wand Tool, the Lasso Tool, the Polygonal Lasso Tool, and several others - all of which can be manipulated to make these tight selections. Within the Magic Wand Tool we see options for sample size, tolerance, anti-alias, and contiguous. Here is a good starting point for understanding the function of these features. One of the more powerful selection tools that Photoshop gives us is the 'Refine Edge' feature which is used to improve the edge of whatever selection you've made. Here is a definition of each of the options within the Refine Edge dialogue. 2) But the most important requirement is patience, patience, and more patience. For me, post processing is just part of the fun in creating images that meet my vision. I enjoy taking the time to learn and practice image editing so I don't necessarily mind putting a lot of time into my projects.

When selecting fine objects like blades of grass or clusters of leaves, it is rare that you'll get everything you want selected in a single swipe of the selection tool. For this reason I choose to make my selections in small areas of the image. As I get my selections refined for a small area I will promote that selection to a new layer so that I don't lose that selection. I'll give an example of that process using the Magic Wand tool. Notice that in the screenshot below I am zoomed into a pixel-level view of the image. In this case I was viewing the image at >500%. Zooming into the image at very high levels is very useful for making your selections. At that level I can clearly see the blue sky between the blades of the grass. Always pay attention to the sample size and tolerance settings when using the Magic Wand tool. If your sample size is set to "Point Sample", you're only going to select the color of a single pixel when you click into the image, and that is typically not what you want. Notice in the selection below I have the sample size set to a 3-by-3 average. Using the 3-by-3 average will now select colors using the average of a 9-block sample when you click into the image. I find the 3-by-3 sample size to work quite well for fine objects when there are clearly defined colors in the foreground layer (like the clearly defined blue sky and green grass shown here). But also pay attention to the tolerance of the sample size. I think Photoshop defaults to a tolerance of 32. And while 32 sounds like a relatively low number you might be surprised at the number of pixels that get selected at that tolerance. The higher the tolerance setting, the more pixels Photoshop will include in your selection that are similar to your sample size. A lower tolerance setting will restrict the the selection to colors that are closer to the average of your sample size. In the screenshot below I only wanted to select the greens and yellows of the grass and ignore the blue sky so I have the tolerance set to a very low number. With these options I was able to get a fairly tight selection of the grass, which of course included all of the grass in the surrounding areas of where I clicked into the image. It only took a couple of minutes to this point.

Now you should improve this selection using the Refine Edge feature. One of the greatest tools within the Refine Edge dialogue is the View feature, which allows you to view your selection against different overlays. I preferred the black and white overlay here which helped me to view a high contrast sample of my selection. Be careful with the Smart Radius feature when selecting grass and leaves. The Smart Radius feature works well with many subjects but I find it somewhat counterproductive for refining the edge around grass and leaves. Its best to simply experiment with a very small radius to determine for yourself if the Smart Radius feature benefits each selection you make. To improve this selection my objective was to further restrict the selection to the grass and remove as much of the sky as possible. I primarily used the Shift Edge feature to do this. Moving the Shift Edge slider to a negative value will shift the selection closer to your object while a positive value will move the edge farther away from your object. You'll see here that I shifted the edge 27% closer to the blades of grass. I also used the Contrast slider here to some effect to improve the selection. Naturally the Contrast slider works to greater effect when there is greater contrast within the selection. Looking at the image above you'll see there isn't great contrast between the tones of the blues and greens, but there is just enough contrast there to use the tool. My suggestion is that you make a test selection and move the Contrast slider to great extremes to see what effect it has and determine for yourself how it benefits whatever project you're working on.

Once I have my selection refined I will typically promote that selection to a new layer. The new selection layer only helps me preserve my selection to that point so that I don't lose it as I continue working in different regions of the image. Below is a screenshot of one such selection layer. One of the benefits I have found in using this technique is being able to identify gaps in my selections as I work. You can clearly see gaps in the blades of grass below. While the selection tools in Photoshop are very good, they're not absolutely perfect. And when viewing the background layer it isn't always possible to see every pixel you have selected, but creating the new selection layer helps with that. Now that I can see the gaps I can go back to the background layer and select the missing bits and merge the additional selection(s) into this new layer. And when the time comes I simply merge all my selections together to completely isolate the foreground from whatever composite I am creating. I probably have ~15 minutes into the project now which included whatever few experiments I needed for the Refine Edge tools. HELPFUL TIP: Learn as many keyboard shortcuts as you can for Photoshop. For example, "Ctrl + Z" (CMD + Z on Mac) is a keyboard shortcut to quickly undo the last action you made. When making really tight selections you will swipe the image many times with the selection tool. In doing so you will inevitably select many pixels by mistake. I probably used the Ctrl + Z shortcut here as many times as I actually made selections!

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Using Levels adjustment layers: Many times we are creating landscape/sky composites because our original photos include dull, featureless skies. And those types of photos are definitely the easiest to work with for making selections of trees (etc) in the foreground layer. Below is a before-and-after screenshot taken from a recent project where I created my selections using Levels adjustment layers. Again you can see the tree and leaves are clearly in front of the new clouds and the trees and leaves have nice, clearly defined edges. Note the high contrast between the tree/leaves and the sky in the 'before' image. The high contrast makes the initial selection process much, much easier but it also helps to incorporate additional tools to restrict your selection to incredibly close pixels. 

The image below shows the initial selection I made using the Polygonal Lasso tool. As before, I use this technique on small regions of the photo so that I can control the process in small junks rather than trying to consume the entire image all at once. And as before I will promote my selection to a new layer when using the Levels adjustment layers to improve my selection. Notice the selection includes all of the sky that is amongst the tree limbs and that I didn't try to use the Magic Wand tool (yet) to make a selection of only the tree/leaves.

Once I promote the Polygonal selection to a new layer I will apply a Levels adjustment layer as a clipping mask to the new layer. Setting the Levels adjustment layer as a clipping mask ensures the adjustment applies only to the new layer and will not take into account all layers in the stack. The idea with using the Levels adjustment layer is to create extreme contrast between the blank sky and the tree and leaves. The Levels dialogue shown in the image below demonstrates how the contrast is achieved by moving the black and white Levels sliders towards the middle of the histogram shown in the dialogue box. Ideally you will achieve maximum contrast by moving the two sliders to the apex of the histogram but doing so will also ignore tones that fall in the gray regions. Each image/selection is different so you'll need to position the black and white sliders accordingly to include as much of the tree/leaves as possible depending on each particular selection. With the Levels adjustment made, now use the Magic Wand selection tool directly on the Levels adjustment layer as detailed previously. Since you're now dealing with (almost) pure black and white, you can typically increase the sample size and tolerance to make the selection process faster.

And as before, use the Refine Edge features to improve your selection. The image below shows an incredibly tight selection by using the contrast slider alone to refine the edge. Once I have the edge refined I will select the background layer (which contains the original pixels I want to use) and promote that selection to a new layer. You now have a nicely selected tree with nice, hard edges to put in front of your new sky.

Happy selecting!  :)

[kenfowkes]

Thanks Kevin! Wonderful tutorial.

[hvbemmel]

have put a sticky on it :-)

[Steve Ross]

Excellent tutorial Kev, this has really improved the finish on my post processing projects.

Regards

[hemantwps]

Thanks for sharing the great information.Very nice -mod edit hvb- link removed

[markolavrovcina]

I'm also very thankful on this tutorial! :)

On Wednesday, July 22, 2015 at 1:44:08 AM UTC+2, kenfowkes wrote:

Thanks Kevin! Wonderful tutorial.

[Joseph Ray]

Thanks so much for this thoroughly informative tutorial. I will definitely start using these tips with my panoramic images.