HDR processing is renowned for producing a lot of noise due to the amplification of the local contrast. Therefore, noise reduction is a vital step in your post-processing work. There are different philosophies as to whether noise reduction should be applied at the very start or at the very end of the whole post-processing process. I prefer to apply it at the start. In this way, any noise artifacts are not amplified by any of the other processing steps and I have a clean image to start with. I usually apply Topaz Adjust or Topaz Detail as the very first thing and directly after that I apply noise reduction. Some folks recommend to reduce noise even before creating the HDR image – on the source images. I found that this increases the blur in the image after the tone mapping.
There are several tools out there that do a very good job at reducing noise. I will not discuss the differences. I use Topaz DeNoise. This is a simple, easy-to-use but yet extremely effective tool which preserves the edges in the image while removing the noise. One thing that most experts agree on however, is that the tool that Photoshop itself provides (at least until version CS4) do not deliver a good performance.
Selective Noise Reduction
I will not go into the details of how to use Topaz DeNoise. There are good tutorials for this. However, I would like to point out that noise reduction, just as many other techniques introduced in this cookbook, can and should be applied selectively. One example where I frequently use different noise reduction settings on different areas of the image is the sky and water surfaces. These areas are particularly susceptible to noise as they usually consist of large homogeneously toned areas where noise becomes particularly apparent.
There is not a lot of magic involved with selective noise reduction. You probably already have a group for the image region you want to apply the noise reduction on, if you followed my advice of structuring your projects. In our example, we will apply the noise reduction selectively on the water. The “water” group has a layer mask revealing the water and hiding anything else. In this case the work is trivial:
Make a copy of the image layer and move it to the “water” group.
- Apply the noise reduction to this copy. Ignore what happens to the rest of the image, only the water is important here. The rest is hidden by the layer mask. In this case, we want to apply a more aggressive noise reduction setting to make sure that the water looks clean. For the building for example, we want to apply a less aggressive setting to make sure that it is not blurred too much.
- Create a second copy of the original image layer and position it under the “water” group in the layer stack.
- Apply noise reduction with less aggressive settings to this layer to preserve the details in the building.