HDR Cookbook – The Three Rules of Noise Reduction
One of the main problems of any HDR workflow that involves tone-mapping is noise. The most widely used tone-mapping operators are based on local contrast enhancement. Unfortunately, they also “enhance” the noise in your images. Thus, noise reduction (NR) is imperative. But what is the best way of doing this, and at which point of your workflow should you apply NR? In this recipe, I will show you the best way of reducing noise. I will present three simple rules that help you optimize your own workflow, and I will show you the results of a series of experiments in order to prove my point.
The Three Rules of Noise Reduction
Probably everybody who creates HDR images will agree that you need to apply proper NR techniques to your HDR images. But what is the best point in your workflow to do so? Which software should you use? And how should you apply this software? There are different answers to each of these questions, and it is not obvious which one is correct.
Let us start with the When. In the following, I will briefly list the steps in a fairly standard HDR workflow, and I will indicate the possible points at which you may apply noise reduction.
- Take your source exposures in RAW format
- Develop 16-bit TIFF images from your RAW files using a RAW converter like Adobe Camera Raw (ACR)
- NR opportunity no. 1: ACR (and other RAW converters) offer NR capabilities. You could reduce noise at this early stage.
- NR opportunity no. 2: You could also apply a more sophisticated noise reduction software one step after that – on the TIFF files ACR generates
- Load your TIFF files into Photomatix
- NR opportunity no. 3: Photomatix (and other HDR software) offers noise reduction capabilities. In Photomatix’ Processing Options dialog that opens immediately before you can create the 32-bit HDR, there is a Reduce noise option.
- Create the 32-bit HDR image
- Tone-map the image back to a 16-bit image
- Load your tone-mapped image into Photoshop for post-processing
- NR opportunity no. 4: You could apply an arbitrary noise reduction plugin right after you loaded the tone-mapped image and before you do any other post-processing work to get a clean start. Alternatively, this can be done using a stand-alone NR software before you load the tone-mapped image into Photoshop.
- Apply image enhancement software like Topaz Adjust
- NR opportunity no. 5: You could apply noise reduction after applying Topaz Adjust to get rid of any kind of noise that is produced by this kind of software.
- Apply arbitrary enhancement (contrast, saturation etc.)
- Apply sharpening
- It is common sense that sharpening should be the last thing you do. This is definitely too late for noise reduction.
That leaves us with 5 possible points in your workflow. You could come up with other opportunities, but let us stick with those 5 for the purpose of this discussion. Wherever you apply NR, it is always a trade-off between two things: Reducing the noise and retaining the details to get a clear and sharp final image. Therefore, you should not apply NR too heavily. However, you should also not apply it too often to the same image. For sure you will agree with that statement, but there is a good chance that you (yes, I am talking about you) are applying NR three times (or more) in your workflow: When you use your RAW converter, when you merge the HDR, and finally in Photoshop. This mix of at least three different NR technologies inevitably leads to images that have fewer details and more blur than necessary. This leads us to our
NR rule no. 1: Apply NR only once if at all possible!
When you look at the different technologies that come into play in our little workflow above, you will notice that they are very different in nature. ACR has a very simple NR interface and not a very convincing quality. Photomatix gives you an even more simplistic interface with merely a single Strength slider and without any visual feedback. Dedicated NR technologies, on the other hand, have become very sophisticated and powerful. They specialize on differentiating between the noise and the details in the image, removing the former and retaining the latter. There are a number of such products on the market and opinions on which one is the best vary. But one thing is common sense: All of these products are highly superior to the NR capabilities offered by ACR, Photomatix or anything built into Photoshop (at least up to CS4) or other image editing tools. There is a special technique called image stacking that is supported in Photoshop since version CS3. It allows you to stack equal exposures of the same scene on top of each other such that the random noise averages out. This is a very effective way of reducing noise that is used e.g. in astronomy photography. However, you would need multiple independent exposures for each exposure value to apply this technique, which is usually not the case in a standard HDR workflow. Thus, our next NR rule is
NR rule no. 2: Use dedicated NR technology!
Under these two rules, the possible points of applying NR in our workflow are reduced to 3 (opportunities 2, 4, and 5). But there is still the question as to where the dedicated NR software should be used. There are two philosophies with respect to this, and both have their followers:
- Apply NR as early as possible to avoid that subsequent post-processing steps amplify the noise making it much harder to remove it without losing details.
- Apply NR at the end in order to really kill all the noise in the image and avoid reintroducing noise through products like Topaz Adjust.
One can debate this question to great lengths, and people are doing this in diverse discussion groups all over the Internet. Personally, I tended more towards second philosophy. But I have learned that the opposite is true. Let me jump ahead here and present my NR rule no. 3:
NR rule no. 3: Apply NR as early as possible!
Why is that? Well, current NR technology is very good at recognizing the noise patterns produced by today’s cameras. If you process an image before applying NR, you change those patterns into something that the NR software cannot recognize that easily. Hence, It will have a tougher job of removing it, and in the attempt it will remove more of the details as well.
The consequence of NR rule no. 3 is that only NR opportunity 2 stays in the race. In the following, I will prove my points by presenting a series of experiments and showing you the results in a visual comparison.
The Experiments
I am approaching this comparison as scientifically as possible. In the following, I describe 6 experiments I conducted, with the NR step in different positions of the workflow. In order not to overwhelmed you with the details, I have omitted them here. You will find all the details in the appendix at the very end of this recipe.
In summary, I have conducted the following tests:
- Test #1: NR is applied only once using dedicated software as early as possible.
- Test #2: NR is applied only once after the tone-mapping and before any other processing steps.
- Test #3: NR is applied only once right before the sharpening at the end of the workflow.
- Test #4: NR is only applied once, in Photomatix.
- Test #5: NR is applied twice – in Photomatix and early in the post-processing workflow
- Test #6: NR is applied twice – in Photomatix and late in the post-processing workflow
Results of the Experiments
In the figure below, you see 100% crops from different regions of the test image for each of the tests (one test per column). Please click on the image with your middle mouse button (mouse wheel). This will open it in full resolution in a new tab. If necessary, zoom in to 100% in your browser window to compare the results.
Experiment results: Compare the different NR methods (columns) for each selected region of the image (rows) (click with middle mouse button to open large version in new tab)
Conclusions from the Experiments
Upon careful inspection of the results, you will notice that the early NR with a dedicated product (Test# 1) yields the best results. The crops in the left-most column are still detailed but yet most of the noise has been removed.
Applying NR early in the post-processing (Test #2) leaves homogeneous and visible noise (resembling film grain) across the entire image. This grain was added by Topaz Denoise (very mildly) to achieve a more natural look in the denoised image. Topaz Adjust has amplified it to a level where it becomes visible. Note also that Test #2 is much less sharp than Test #1. Applying NR late in the post-processing work (Test #3) produces a cleaner and also somewhat sharper result. The conclusion here is that if you apply NR only in the post-processing, do it after applying any image enhancement tools (such as Topaz Adjust).
In Tests #4, #5 and #6, Photomatix’ NR technology was applied. Using this technology exclusively (Test #4) yields a lot of noise and produces the worst result in the test. This is interesting as our winner (Test #1) applies the NR basically at the same point in the entire workflow – before the tone-mapping. This shows that Photomatix has some homework to do in terms of NR. In Tests #5 and #6, Topaz Denoise was used in addition to Photomatix’ NR tool (deliberately breaking our first rule of noise reduction). The results are comparable to Test #2 but notably more blurry than Test #1.
In summary, the experiments show that using a dedicated NR tool before merging and tone-mapping the HDR is the best solution. It reduced the noise and retains details in a way such that the resulting image does not need any further NR step in the post-processing work. The details are preserved best. If you thought that Photomatix would take care of your noise problems, this test may be an eye-opener for you. My advice is: always turn off NR in Photomatix!
How to Reduce Noise the Right Way
Now you know when to apply NR, but what about the How? Of course, this is highly depending on the product you are using. Different vendors use vastly different technologies and strategies. Some offer camera profiles and enable you to train the software to deal with the noise characteristics of a specific camera model. Since I cannot cover all the products here, I will give you a few basic tips:
- Don’t underestimate the effect of NR. You may think that NR is something for pixel peepers. But keep in mind that according to our findings above, you will apply it very early in your workflow, leaving a lot of room for other tools and post-processing steps to amplify any noise that was left. So you should invest some time to get the NR step right!
- Learn to master your NR product. Yes, that may seem obvious, but do you really understand all the sliders of your tool? Read the manual carefully and experiment with different images until you know what you are doing.
- Use presets and automatic settings with care. I know that this is the most convenient way – you just have to press one button and the software does the rest. But in most cases, you will end up with suboptimal results because no software can really analyze the visual appearance of your image correctly. You may start by applying a preset, but you should always fine-tune the parameters to the specific properties of each image. This is where tip #2 comes in again!
- Reduce the noise in each of your exposures separately. The noise will be different in each of your exposures. Therefore, you should not simply apply the same setting to each one of them. Optimize the settings for the specific exposure and keep in mind that different regions of each exposure will end up in the final image. Thus, there is no need to work extensively on the darkest regions of the darkest exposure. These will not make it to the final image.
- Pay attention to the shadows. Noise is always most intense in the shadow regions since the signal-to-noise ratio of your image is very low there. That is, the actual pixels and the noise pixels both are dark. Brightening those areas boosts both and makes the noise really come out. A good NR product should let you adjust shadow and highlight areas separately. Work on the shadow areas in the brightest exposure (see tip #4).
- Do not overdo it. Any NR software has a main strength slider, and the noise will disappear completely if you only push this slider high enough. But that will also remove the fine details that make your image appear sharp. You should work on those details to retain them. That may mean that some of the noise will remain in the image. You can fix this later.
- Apply NR selectively. If you fail to hit the right compromise between reducing noise and retaining details, go for the details. This will leave noise that is most obvious in homogeneously colored regions (e.g. the sky). Photoshop’s layer masking allows you to blend different versions of your image in post-processing. So you can apply NR again in post-processing to optimize those noisy regions and then blend them with the rest of the image. This breaks rule no 1, but only a little. 😉
Concrete Final Recommendation
I have not tried all the NR products out there. There are at least three major competitors: Noiseware, Noise Ninja and Topaz Denoise. Personally, I use Topaz Denoise, and I am pleased with the results. It does a very good job of reducing noise and retaining the details if used right. It offers separate controls for shadows and highlights, a way of recovering details, and a debanding technology that avoids color banding effects.
Denoise offers several standard presets that are a good way to start. Additionally, it has a noise estimation technology that adjusts every preset to the specific image. This is very useful. Usually, I go through the RAW presets starting at the lightest to see which one works best. At some point I see all the noise disappear in the preview. This is too much, and I go back to the previous preset. Based on this preset, I optimize the settings.
APPENDIX: Details of the Experiments
The setup was the following:
- The Source Images:
- Shot in RAW format with a Nikon D7000
- Settings: 1/25s, 1/100s, 1/400s, f/4,5, ISO800
- Development
- The source images were developed in Adobe Camera Raw (ACR) using neutral settings. That is, no noise reduction or sharpening was applied at this stage.
- Chromatic Aberration was reduced in ACR.
- In each test, the results were saved as 16-bit TIFF images and fed into Photomatix.
- Tone-mapping
- All images were tone-mapped with the Detail Enhancer option using identical settings.
- The result of the tone-mapping was saved as 16-bit TIFF
- Processing in Photoshop
- The tone-mapped image was directly loaded into Photoshop without any further intermediate steps.
- In each test, a Levels layer was applied to correct the contrast and a Saturation layer was applied to correct the colors.
- No other processing was applied except for the Topaz plugins as described in the test descriptions below.
- All Topaz plugins were applied with identical settings in all experiments.
Test #1
Dedicated pre-tone-mapping NR
- Preparation
- The TIFF images produced by ACR were loaded into Photoshop and the Topaz Denoise plugin was applied to each of the three exposures separately with different settings for each on of them.
- The results were saved back to 16-bit TIFF images and fed into Photomatix.
- Photomatix
- NR in Photomatix was turned off
- Photoshop
- Topaz Adjust was applied
- Topaz InFocus was applied (for intelligent sharpening)
Test #2
NR only in post-processing (early)
- Preparation
- No noise reduction was applied to the images produced by ACR. So the three exposures were not subject to any noise reduction prior to being fed into Photomatix
- Photomatix
- NR in Photomatix was turned off
- Photoshop
- Topaz Denoise was applied for early NR
- Topaz Adjust was applied
- Topaz InFocus was applied (for intelligent sharpening)
Test #3
NR only in post-processing (late)
- Preparation
- Same as in Test #2
- Photomatix
- Same as in Test #2
- Photoshop
- Topaz Adjust was applied
- Topaz Denoise was applied for late NR
- Topaz InFocus was applied (for intelligent sharpening)
Test #4
NR in Photomatix only
- Preparation
- Same as in Test #2
- Photomatix
- The three exposures were merged into an HDR using a NR strength of 150% (max. possible value)
- Photoshop
- Topaz Adjust was applied
- Topaz InFocus was applied (for intelligent sharpening)
Test #5
NR in Photomatix and early Post-processing NR
- Preparation
- Same as in Test #2
- Photomatix
- Same as in Test#4
- Photoshop
- Topaz Denoise was applied for early NR
- Topaz Adjust was applied
- Topaz InFocus was applied (for intelligent sharpening)
Test #6
NR in Photomatix and late post-processing NR
- Preparation
- Same as in Test #2
- Photomatix
- Same as in Test#4
- Photoshop
- Topaz Adjust was applied
- Topaz Denoise was applied for late NR
- Topaz InFocus was applied
Very useful article. Thank you again. BTW, NR algo in the newest ACR is much more powerful than in its predecessors.
Hi Lucas,
thanks for the info about ACR. Still, I think that the dedicated products on the market should be superior. I would recommend getting one of those.
Cheers
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Had to try this out straight away, seems to work really well. AAg to see the difference in amount of NR neded dependent on exposure level. Thank you very much for sharing this.
I would really like to know how you make your wonderful watermarks. Maybe your next tutorial:) ?
Hi,
your’re welcome. Good to hear that it works outside my small world. 🙂
The watermarking is actually a very personal touch, and it can be quite tricky. However, if you watch the making-of videos carefully, you should be able to understand most of the technique.
Cheers
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Very complete article, thanks again fr sharing notu jst your art, but your knowledge
Hi Miguel,
it’s my pleasure. I love sharing things and helping others. If it helps you in getting better, that’s all the reward I need.
Keep stopping by for more!
Cheers
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Thanks for this article. It helps to improve my workflow for HDR very much. This is the second time I will change my workflow due to an article on your blog.
The experiments are great and make me understand why I’m doing things in my HDR workflow.
Hi Jan,
great to hear that. I hope I did not mess up your workflow too much through those changes. 😉
Cheers
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Thanks for the tutorial Klaus, I have read many websites with tutorials/articles and yours give the most in-depth workflow and details I have ever seen. Thank you for your passion for the art and your willingness to share with all of us,You are one of a kind!
Cheers
Helgard
You are welcome, Helgard!
Thanks for the nice feedback. I appreciate this a lot! I hope you enjoy your stay here and take some new ideas with you.
Cheers
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Great post, which noise reduction products would you recommend?
Hi “noise reduction” (is that your real name?) 😉
I have not tested all of the NR products out there and I have not attempted to compüare them either. I am working with Topaz DeNoise, and I am really pleased with it. So, that would be my recommendation.
Cheers
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and Google+ gplus.to/farbspiel
Many thanks again Klaus very informative and educating.
Fraser
Very clearly explained and thank you very much. I purchased Topaz
Denoise with trepidation, but now feel a lot more confident.
Hi Paul!
don’t worry! You bought a great piece of software. Just make sure you understand how to use it properly.
Cheers
I enjoy and respect your site immensely. I’m a user of DeNoise and Photomatix Pro. Since reading this article I’ve gone back to some of my older HDR panos and manipulated them differently using the RAW editor to set white balance and noting the value. I then set that value as a custom value in the preliminary stages of Photomatix. That does produce good results in my eyes, however I cannot determine how I can employ DeNoise on a RAW image. Perhaps someone can advise me.
I cannot engage DeNoise while in the RAW editor and I cannot save out a RAW image when passing my white balanced images, I can only save TIF of JPG or and few others out of the RAW editor. What am I missing here. Perhaps you are suggesting I open the RAW image out of Bridge into CS5.
Hi Dick! Thanks a lot!
As for the “DeNoise on RAWs” issue: You cannot and need not employ in directly to the RAWs. The trick is to develop the RAWs into TIFFs in ACR (or a similar RAW converter) and, afterwards, load each of the TIFFs into Photoshop to apply DeNoise. Then, save the denoised images back to the TIFF files and use those as input the Photomatix. Yes, that sounds like a lot of work, and it is. Sorry! But if you use Photoshop, you can create an action to speed up this process.
I hope this resolves you problem. If not, feel free to ask.
Cheers and have fun!
I was of that understanding after reading your article but the thing that threw me a curve was after adjusting the RAW bracketed images to my white balance liking in ACR and saving those changes in TIFF format, also in ACR, I opened them in CS5 and engaged plugin DeNoise. The presets shown in DeNoise are JPG and RAW. I was expecting to find a selection of TIFF presets as well. I read through the user manual for DeNoise where there appeared to be no mention of TIFF presets so I must assume I must use either of the two mentioned above and will get satisfactory results. I’ve written to DeNoise tech support for an explanation and don’t expect you to labor over this, but I would image there may be others of your readership that after reading these comments may find some resolution. I anticipate you will say that the RAW presets are the way to go.
Hi Dick,
yes, that is a bit confusing. The solution is: Use the RAW presets whener you work on a TIFF developed from a RAW file. Also note that You should turn off any noise reduction and sharpening in ACR. As far as I remember, that is somewhere in the manual, or maybe I picked that up in one of the video tutorial on the Topaz website. I don’t remember.
I hope this resolves the problem. Have fun, mate!
Cheers
Klaus
Great article Klaus, and thanks for sharing all the knowledge that you have!
Great information Klaus. This is somethig I definately need to look into. I am using Noiseware but I am finding that this cannot be applied to TIFF’s using the Noiseware Photoshop Plug-in. I may need to consider investing in Denoise. My photos will definately benefit from this as I am losing too much detail in them currrently. Keep up the great work. It is really helping us newbies to HDR.
This is really helpful and knowledgeable to me since I am new to HDR photography. Will keep reading your other articles and may have some questions soon.
Thanks,
Sarith
Hi Klaus.
There are conflicting opinions about whether High ISO NR should be turned off in-camera or not.
In my case the Camera is an entry level Canon 450D. The argument goes that NR in current post-processing software is more advanced than that in the camera’s firmware.
Also with the camera’s NR turned on, there is a huge lag between shots while the camera is processing the image to reduce noise.
Do you have an opinion on that?
Thanks, Wally
Hi Walter,
well, any image processing done in the camera only takes affect on the JPEGs, not on the RAW files. I would really advise you to shoot in RAW mode (maybe in JPEG + RAW). This way, you can turn on the image processing functions of your camera (including noise reduction) to get good JPEGs straight out of the camera. Any post-processing should be based on the RAW files as they give you full control over all the aspects (including noise reduction).
If you only shoot JPEGs, the answer is not so simple: If you want acceptable results straight out of the camera, turn on NR. If you want to do some post-processing, do the NR in a dedicated software where you can fine-tune the strength and apply different NR settings to different sections of your images. But if you really do this kind of stuff, why not shoot RAW in the first place?
I hope this is a satisfactory answer to you. 🙂
Cheers
Klaus
Hi! Thanks for the awesome tutorials! I have been reading all your tutorials and they are amazing!
I an wondering have you ever gotten a chance to play around with Gimp? If yes, what aspects do you like in that and what you would not recommend?
Hi Pannag,
I have been using GIMP some years back. At that time, it was nowhere near to Photoshop in terms of flexibility and usability. What they are still lacking is adjustment layers that let you edit an image non-destructively. That is a show-stopper for me. That’s all I can really say.
I hope this helps you.
Thanks for your tips and the steps. This is very clear.
I’m using Noise Ninja ©PictureCode. Did you use that? I’m curious whether this software is great by your opinion.
I was using this software at the end of my process.. but I definitely have to use your approach with making NR at the very beginning. Thanks for directing me 🙂
I will put a link to your site in my Friends and Partners section on my own site. I believe my visitors have to learn some good tricks from your site.
Hi Roman,
I have used Noise Ninja, and it is a great tool. It is certainly one of the best and most used. So, you took the right decision.
Thanks for the link!
Regards,
Klaus
BTW, I have used your method of NR in my new vertorama but using Noise Ninja instead of the software proposed by you.
I have set sharpening and NR by ACR to 0 and then applied NR to the files, then merged them to HDR images and applied NR again and then merged files to vertorama. After making all postprocessing things I’ve done sharpening using High Pass (as you described in your another article).
I’m very satisfied by the result. Would you give me your opinion on it? I’m very curious whether you think it is blurred or maybe something else..
http://roman-shymko.com/digest/church-of-the-icon-of-the-mother-of-god-recovery-of-fallen/
Even though somewhat dated now, this is an excellent compilation of test results. Given the advancements in noise reduction software since 2011, I’d be interested in an update of this article. In particular I’d like to see how DxO Optics Pro 9 stacks up against some of the more traditional plug-in approaches.
Paul
Thank you! A very complete discussion with lots of useful detail. Can’t wait to practice with it.