Wednesday, 2 January 2013

Tutorial: removing halo artifacts in a HDR photo

This is quite a simple shot but I like how this one tree stands out from the crowd.

Halo artifacts. One of the nightmares in HDR photography. They appear between regions of different luminosity and virtually ruin any photo. You might have great composition, light and colours but if you have halos in your photo - it won't be considered good - rather poorly executed.

There are many ways of dealing with them and here are just a few:

In Photomatix Pro

Make sure to:
  • Have Strength around 70 or less,
  • For Lightning Adjustments use Natural+ or Natural,
  • If you still get halos make sure to increase Smooth Highlights.
This should keep your halos at a low level. Unfortunately sometimes it's still not enough. Also several Photoshop filters (like Topaz Adjust) might amplify halos so there are a few more things to do.

But for that you will need to go into Photoshop.
In Photoshop

There are a few ways of dealing with halos in Photoshop. First open your image with halo artifacts:


At full size it doesn't look very bad but hit CTRL + MINUS keys several times to see nasty halo artifacts. Ugh...

As you see highlights in the sky are a way too strong and also shadows are a way too deep. So the easiest thing to do would be to adjust both by using Curves adjustments layer.

First make sure to select sky only (eg. with Wand selection tool) as the halos are present only in it. Then add Curves adjustment layer.

For this image I used following settings:

Note I darkened highlights and brightened shadows a little bit. After that adjustment the image looks like this:


It's slightly better but still far from good.

So what we will do is to manually Dodge & Burn the sky to get rid of too dark regions and too bright ones too. Fear not - this tool is very easy to do (and powerful too!) but I admit one has to get used to using it.

You can use Dodge and Burn tools from the Photoshop toolbox for this but they have one serious disadvantage - they both are destructive tools. I prefer editing my images in a non-destructive way so I do the following:
  1. Press CTRL + SHIFT + N to create a new layer. In a dialog box that appears, change Mode to Soft Light and also check the box at the bottom to fill the new layer with neutral grey colour.
  2. Press Ok, to create the layer.
  3. With your new layer selected select Brush tool and change its Flow to around 2 - 3%. Also make sure your brush is soft.
  4. Paint over dark regions with White colour and over bright ones with Black. Using White colour has effect of brightening the image, while using Black - of darkening it. So when you use White on Shadows you brighten them up. Similarly when you use Black on Highlights you darken them.
  5. While painting make sure to zoom out from time to time because it might be easier to see halos at smaller magnification.
Here is the final result:

Although still not perfect, it now does look a lot better :)

For your reference, here is also the Dodge & Burn layer I used. Note that the brightest parts of the image were painted with black and darkest with white colour.

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