Reducing Digital Camera Noise and Moiré
Digital cameras are based on a variety of technologies, including scanning, peizo, three-shot, and the one-shot cameras. Every digital camera technology has its pros and cons, which have been addressed in Real World Digital Photography, which I coauthored with Deke McClelland.
The one-shot cameras work just like your film camera, except that a CCD (Charged Coupled Device) with a color mosaic filter replaces the film. The CCD structure, in combination with the filter, can cause color artifacts that look like little twinkles of colored lights or rainbow-like moiré patterns might be visible in areas of high-frequency, fine-detail information, such as eyelashes, flyaway hair strands, specular highlights, small branches of trees, or woven fabrics, as seen in Figure 5.34.
Even if you don't use a digital camera, you might pick up moiré patterns when scanning images with fine fabrics, and you can use the following techniques to take care of those problems.
Blurring and Sharpening in Lab Color Mode
Moiré and color artifacts like those in Figure 5.35 show up as problems in the color channels. By separating the color information from the black-and-white image information, you can fix what's brokenthe color artifactswithout affecting what's notthe tonal information, as seen in Figure 5.36.
When using any of these techniques to remove moiré or color artifacts, make sure you view your image at 100% monitor view to accurately see what is occurring.
Figure 5.34 Extreme rainbow moiré problems might be impossible to eradicate completely.
Figure 5.35 Before
Figure 5.36 After
Choose Image > Mode > Lab Color to convert the RGB file to Lab.
Make the "a" channel active and then press ~ to see the full-color image. Run the Gaussian Blur filter with a high-enough radius to soften the artifacts. Don't try to eradicate all artifacts at once because you will be repeating the Gaussian Blur filter on the "b" channel in the following step (see Figure 5.37).
Select the "b" channel and run the Gaussian Blur filter on the "b" channel. Use a higher radius than you used on the "a" channel as the "b" channel usually has more color artifact problems.
Select the Lightness channel and choose Filter > Sharpen > Unsharp Mask to sharpen up the black-and-white information, as seen in Figure 5.38.
Make the composite channel active and inspect the image for any stray color artifacts. Use the Sponge tool set to Desaturate to brush away any artifacts that the global approach might have missed.
Using this Lab blur and sharpen technique can sometimes desaturate the entire image, making it look flat and faded. To offset this undesired side effect, use a Hue/Saturation Adjustment Layer to boost the saturation, as seen in Figure 5.39.
Figure 5.37 Blurring the a channel to lessen the color artifacts.
Figure 5.38 Sharpening the Lightness channel to sharpen the black-and-white image information.
It would be unwise to use my values as the right values for all image problems. The Gaussian Blur and Unsharp Mask settings shown here offer good starting points. You might need to increase or reduce values to fine-tune the artifact removal in your images.
Figure 5.39 Boosting overall saturation.
This technique might sound like a lot of steps but, thankfully, the entire procedure and settings are actionable. By making a Photoshop Action, you can batch process an entire folder of digital camera images in a fraction of the time it would take to do this step-by-step. Go to http://www.digitalretouch.org/ch5 and download the Action moiré_removal.atn. Another practice file, ch5_costume.jpg, is also on the site.
Using Quantum Mechanics Software
If you work with a lot of digital camera files, making a modest investment in Quantum Mechanic Lite or Pro (http://www.camerabits.com) to do all the previously described work for you and do it better makes a lot of sense. Both Quantum Mechanic Lite and Quantum Mechanic Pro are designed to remove color noise and artifacts. They work very well with Kodak's cameras and high-end cameras that don't do any similar kind of filtering in their host software.
Quantum Mechanic Lite (see Figure 5.40) uses a quick filtering mode with simple controls for busy workflows, such as newspapers. QM Pro (see Figure 5.41) has complete control over all the parameters of color filtering, and includes a more advanced filtering mode to retain color detail. Best of all, both flavors of the plug-in work in 48-bit mode and the sharpening is much more effective in 48-bit mode, producing less artifacting with smoother results and better shadow noise reduction. After selecting specific areas, use the Quantum Mechanic Moiré Eraser to take care of any stray color artifacts and color moiré, as seen in Figure 5.42. The Moiré Eraser is a heavy-duty filter and must be used with care.
Both Quantum Mechanic Lite and Quantum Mechanic Pro will remove some color moiré, but only if the moiré pattern waves are close together. As the waves get farther apart, the radii values need to be increased, and you can run the risk of desaturating other color detail in the image. Figure 5.43 shows an extremely bad example of wide-band moiré. As Figure 5.44 reveals, the moiré problem has contaminated all three color channels, making this a tough, tough job to fix. After separating the red sweater onto its own layer, I was able to minimize the color artifacting of the moiré with repeated use of Quantum Mechanic Pro and selective use of the Quantum Mechanic Moiré Eraser. The results in Figure 5.45 are certainly better.
Figure 5.40 Quantum Mechanic Lite simplifies the moiré reduction process.
Figure 5.41 Quantum Mechanic Pro offers additional controls to reduce moiré.
Figure 5.42 Quantum Mechanic Moiré Eraser is used to clean up stray color moiré problems.
Figure 5.43 An extreme case of color moiré with very wide bands.
Figure 5.44 By inspecting the three color channels you can see that the moiré problem is throughout the entire file. This will make it especially tough to retouch.
Figure 5.45 After using Quantum Mechanic Pro and Quantum Mechanic Moiré Eraser, the color moiré has been reduced.
Avoiding Moiré Patterns When Shooting Digital Pictures
If you notice moiré problems while you're taking pictures with digital cameras, you can lessen them with the following tips:
Move the camera closer or farther away from the subject by just a few inches to change the relationship between the grid of the CCD color filter and the high-frequency information that is causing the problem.
Open the aperture to use a larger ƒ-stop. This decreases the depth of field, which means that less of the subject matter is in focus. The out-of-focus parts have lower spatial frequency content and hence fewer aliasing artifacts. The in-focus areas might still reveal some artifacting.
Some people recommend defocusing the camera a smidgen and then over-sharpening the file in Photoshop. I'm not a huge fan of that approach, but offer it as a technique you can experiment with.