Noise Reduction Software - Part One

My friend, Russell Horn, in search of a good noise reduction (NR) software routine, asked my opinion recently regarding the best choices for astro image processing.

Obviously, there are many good noise reduction solutions for processing your astroimages. Because of this, it would be difficult to give ratings to various packages since the choices are largely made on the basis of the comfort level of the "processor," and the ability to put his experience into play.

So, what works well for me may not work so well for you.

But what I can do is tell you what I feel is important in such a solution. What features should a good noise reduction package offer? Are there different requirements for different camera types? How big of a learning curve can I expect? Will I be comfortable with the interface?

Today, I'll write about the features you will want as a part of any solution. In the next fews days I'll add parts that highlight specific noise reduction softwares that might be right for you and I'll cover some of the other features that these packages are marketing, particular as they concern specific camera types.

Feature Set

For the most part, any good solution will provide all or most of the following features...

16-bit Processing - You don't want to spend thousands of dollars on your 12-bit DSLR or 16-bit astro CCD only to be forced to truncate your data to 8-bits for the noise reduction steps. Not so much a problem today as it was a couple of years ago, but it's definitely a good reason to upgrade your older software, and it's certainly a good reason to opt for the "pro" version of many of the stand-alone NR packages.

Batch Processing - Do you ever need to apply the same noise reduction steps to many images at once? If so, then you'll appreciate the ability to load multiple images singularly into computer memory and automatically apply your routines to an entire stack. Truthfully, I fail to see the utility in this, since we are normally working with master channels of specifically luminance, red, green, and blue images, and thus would want separate applications of NR on my channels. However, I could see a need in some tasks to clean up individual sub-exposure frames prior to the master combine step. As a fan of early NR application in a processing workflow, this might be something worth exploring, particularly in low signal-to-noise ratio (S/N) data sets where either you lack enough subframes for outlier rejection combine methods (e.g., median, sigma) or those methods cannot guarantee sufficient clean-up of certain pixels.

Type of Package - Standalone "exe"? Integrated routine? Plug-in? Photoshop action? Often, the biggest decision when choosing a noise reduction solution comes in deciding the type of interface you need in your NR software. Adobe Photoshop is a big player in this decision, simply because it dominates the processing landscape, and a tidy add-on makes a lot of sense, whether it expands the core options in the form of a filter "plug-in" or it takes advantage of the "action" macro. Other software makers take a different approach and realize that there is a market for a solution that stands on its own and does not require prerequisite software. These often reasonably priced "executables" are powerful options because they afford the opportunity to investigate multiple, advanced, or "smart" algorithms and batch processing modes without the heavy encumbrance of bulky memory overheads of larger core processing kernels, such as with Photoshop. (How long does it take PS to load on your PC?). Finally, there are integrated solutions as standard fare with many of the processing software you already use (e.g. Photoshop and MaxIm DL). These options need to be investigated if only to discover if a more powerful, third-party solution might be required.

Manual Filter Controls and Multiple Undos - No matter the software, the ability to affect the amount of aggressiveness or to completely undo an effect is very important. This is the advantage of a program like Photoshop, as plug-in filters and actions can be controlled by the opacity settings and adjustment layers, or by retracing your steps in the workflow history. Or some people might like the ability of programs to offer a wide number of parameters to give weight to particular aspects of a filter, and then to preview the effect in a "preview" mode, after which dumping the result with the "undo" function should it not meet expectations. In other words, the option to control the parameters of a filter manually are important, and you will appreciate a program that will do exactly as you set it.

Intelligent or "Automated" Mode - The converse to controlling the NR process manually is to have a processing mode for total automation. The nice part about a lot of solutions is that, in many cases, the user can push a single button and remove noise as the program identifies it. Beginners are especially fond of filters that do all the work for them. But for some people, they are reluctant to give up that control, fearful that the aggressiveness cannot be tamed, or that it might not apply NR to the right areas. But for the most part, especially when coupled with a good preview mode or undo button, automation can be a nice feature to have. Also, such an algorithm is commonly called "noise profiling," so be on the look-out for this when examining the specifications for various tools.

Grayscale vs. RGB Modes - I'm not as big on this aspect as others might be since I believe NR should be done early in a workflow and/or primarily in grayscale channels only. However, a one-stop chrominance NR tool is certainly helpful at times. Global color noise algorithms, if intelligently identified and removed, are nice tools for the arsenal, especially if they do not detract from the fidelity of the luminance data. However, because RGB images are 3 times the file size of grayscale images, noise reduction for chrominance noise will require a little more computer memory and processing power.

Multiple NR Algorithms - The problem with old versions of Photoshop, among others things, was that you were limited on the types of filters for noise reduction. "Despeckle" and "Gaussian Blur" are the standard old routines, which can be effective if used on small selections in the image; however, they convolute pixels by averaging groups together. The result is normally a blurring of image details in the areas you do not want. They lack sophistication and global control. Newer versions of Photoshop actually have more intelligent filters geared specifically for "noise removal." These do a much better job by differentiating noise within the lower S/N areas, allowing you to set the parameters for reduction and thereby applying its function more selectively. Many other software NR solutions will bundle lots of nice algorithms for a wide variety of uses. As such, there can be more versatility and power to the user. However, more functions can mean a steeper learning curve.

Extra Algorithm Bonuses - Aside from giving noise removal tools, many packages, especially Photoshop Action sets, can give a wide-range of additional tools as well. Noel Carboni's Astronomy Tools Actions for Photoshop represent a tremendous value once you account for all the other slick and powerful actions. So, such solutions, while giving excellent NR algorithms, will also beef up the rest of your processing toolset as well!

Astrophotography Specific - Many packages, particularly those in the stand-alone category, may not be designed specifically for astroimaging. The reason is obvious: noise infests terrestrial images too and that market is much larger! Keep in mind that some NR algorithms may have adverse effects based on the fact that it might have been designed with terrestrial images in mind (and this makes manual programming of the filters even more important). Similarly, these packages may not have as many usable tools as one that has "astronomy" as its focus. For example, Photoshop actions designed for astronomy could add routines for handling star clean-up, removal of column defects, and vignette removal as well. Lastly, be careful of file format options with your non-astronomy specific solutions. Most all solutions will offer output to 16-bit TIFF formats, which is my preference in Photoshop anyway. But for those who like to work their images in FITs, you might be limited to certain NR packages, else you will be forced to convert to another bitmap format such as TIFF.

User-Friendliness and Learning Curve - Often going hand-in-hand, these two aspects are probably the most important choice. Inherently, Photoshop actions will get lower marks in this area simply because many people do not understand them. Similarly, the simple task of loading a Photoshop plug-in may prevent a person from ever using it. Photoshop is synonymous with "learning curve," and this fact alone might have people seeking easily loadable "executable" programs with their own interfaces. For example, I have many friends that own the affordable and powerful Astronomy Tools Actions mentioned above, but seldom use them either because they find applying actions to be confusing or they have a limited knowledge of Photoshop adjustment layers.

Part One of this article has dealt with aspects of good noise reduction software. In the next Part, I will focus on some specific packages.

More later...