News and Views from Dave Wilson

HDR Tip #7 – Magic Blue Sky Halo Removal

by on Mar.06, 2011, under Photography

Here’s the last of my HDR tips for this week. I’ve been saving this one for a while but, if you shoot in climates where you can rely upon clear blue skies, it’s a killer tip and could save you hours (even if I do say so myself).

Do you often find yourself with a mess like the following after tone-mapping? Frankly, this is an image I really don’t like – the tone mapping is wildy unsubtle but it serves as a great example for this tip.
Fort Bend County Courthouse (before)

The foreground looks the way it was intended to look but the sky is a horrible collection of haloes and weird areas of different shades of blue. Wouldn’t it be great if I could just drop in a new sky without having to spend a week masking around all those blasted leaves? Well here’s a tip that will fix your sky in about 2 minutes flat as long as it’s predominantly blue. This will work if there are some clouds but it definitely works best when the sky is clear.

Take a look at the original brackets you used and decide which one has the best sky tone. I typically like the -2EV image since I like nice, dark, saturated skies but it’s entirely up to you. Open the tone mapped image and the original image with the good sky as layers in Photoshop with the tone-mapped image below the original bracket.

Now we’re going to use a bit of magic that very few people seem to know about. Make sure your magnification is set so that you can see the whole sky area of the image then right click on the top layer (with the original image) and select “Blending options…” (in CS5 at any rate. It used to be called “Advanced Blending…” or something similar in some previous versions as far as I can remember and you accessed it from the layer effects list I think). The dialog that you will see allows you to control how this layer blends with the one below it and one of the great features it offers is the ability to blend based on the colours in either this layer or the one below it.

As you will notice, the bits of the tone-mapped image we want to replace correspond with the blue sky in the original image so let’s tell Photoshop to show the original only where it is predominantly blue. Do this by setting the “Blend If” option to “Blue” then drag the slider under “This Layer” to the right. As you do this, you will see that the underlying layer starts to bleed through and become visible. Move the slider to the right until you start to see chunks of the tone-mapped sky appearing then stop. At this point, you will notice something along the lines of the following:
Advanced Blending

The original sky is definitely visible but you will also see a lot of really horrid fringes around all the tone-mapped leaves. There’s another secret to get rid of these. Hold the “Alt” key and click on the slider you just dragged. You will notice that it splits in two so now grab the left hand half-slider and pull it back towards the left. As you do this, you should see the fringes get smaller and, eventually disappear leaving you with something like this:
Advanced Blending (after splitting the slider)

You will likely have to juggle the left and right half slider positions to get the best overall effect. It’s a good idea to zoom to 100% to check out the fringes since it’s tricky to see how the individual leaves (or other edges) look when you are zoomed out.

Once you have marveled at how the sky looks, you may be disappointed to notice that this blending has had a muting effect on other sections of the image too. This can, however, be corrected very easily by adding a layer mask to the top (original image) layer and, using a soft-edged brush, painting in black over the areas that you don’t want to be affected. Doing this, you are effectively allowing the underlying tone-mapped layer to shine through completely unaffected. If you accidentally paint any of the sky, just switch to white and paint the mask with that to erase the effect. Typically, the effect on non-sky sections of the image is reasonably subtle so you don’t have to be enormously careful when you are doing this final masking step. Painting this mask is, of course, enormously easier than trying to figure out how to mask around all the leaves!

Here’s the final version of the image I started with after about 2 minutes of slider juggling and some easy mask painting.
Fort Bend County Courthouse (after sky replacement)

Hopefully you’ll find this tip useful. It’s saved me hours here in Texas where blue skies are the norm. If you live up north or across the pond, you may find this less helpful but the basic approach can still be used for many other cases where you want to blend based on the colour of particular regions of an image.

I hope you’ve found this week of HDR tips helpful. Thanks for reading!

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