Demosaicing

Most digital cameras today use a color filter array over their sensors. To display such raw files their data needs to be demosaiced. Cameras with a Foveon X3 sensor (Sigma) do not have color filter arrays and so do not need demosaicing. Demosaicing is why opening a raw file always takes a bit longer than opening a JPEG or TIFF file, where the data is already 'display-ready'. RawTherapee offers several demosaicing algorithms, each with its own characteristics. The differences between them are often very subtle - one might need to zoom in to 200-400% to discern them - but since the program works on a pixel-by-pixel basis and demosaicing is the basis upon which all other tools work, the choice of demosaicing algorithm can have a visually significant effect when combined with other tools, such as the sharpening ones. The choice of a certain algorithm influences, among other things, the quality of very fine details in the image, whether false maze patterns will appear, and decides how well colored edges are rendered.

RawTherapee supports demosaicing images from sensors with Bayer filters and Fujifilm X-Trans filters. If you take a look inside the "Raw" tab, you will notice there are two tools: "Sensor with Bayer Matrix" and "Sensor with X-Trans Matrix". The settings in one of these two tools have no influence over the settings in the other - if you open a raw image from a Bayer-type sensor, only the settings from the "Sensor with Bayer Matrix" tool will be used, the settings from the "Sensor with X-Trans Matrix" tool will be ignored, and vice versa if you open a raw image from an X-Trans type sensor. For simplicity, we will describe both here.

Method
The following demosaicing algorithms are available for raw files from Bayer sensors:
 * AMaZE
 * IGV
 * LMMSE
 * EAHD
 * HPHD
 * VNG4
 * DCB
 * AHD
 * Fast
 * Mono
 * None

The following demosaicing algorithms are available for raw files from X-Trans sensors:
 * 3-Pass
 * 1-Pass
 * Fast
 * Mono
 * None

Which demosaicing method should you use? This page aims to tell you as much about the various algorithms as is relevant to a photographer, but there is no much to say as the explanations would quickly become technical and of a programmatical and mathematical nature. After reading though this article you should know that LMMSE and IGV are to be used on high ISO photos and that for the majority of other cases you should stick with the default AMaZE method, but of course you are free to explore for yourself and to test each method out on your own raw files. It is of no use reading an article somewhere on the internet which claims that some specific method is best because in their test that method was the sharpest, as the performance of each method depends specifically on the sensor in your camera and even on ISO, so keeping our suggestions in mind, run your own test and make up your own mind!

RawTherapee versions 4.2.91 and newer always use the demosaicing method you choose regardless of zoom level. RawTherapee versions older than 4.2.91 use the Fast algorithm to initially open the image for editing. After this, the selected demosaicing method is applied when the image is zoomed to 100% magnification or when the detail window is opened. The selected method is also used for batch processing. It is not recommended to select the Fast method for the final conversion, as it is a low-quality algorithm for display purposes.

AMaZE (Aliasing Minimization and Zipper Elimination) is the default demosaicing method, as it yields the best results in most cases. In RawTherapee versions 2.4 and older VNG4 used to be the preferred algorithm for Olympus cameras, as AMaZE didn't exist yet and VNG4 eliminated certain maze pattern artifacts that might have been created by the other methods, but with the introduction of the AMaZE method in RawTherapee version 3.0 Olympus users might prefer that instead.

DCB produces similar results to AMaZE. AMaZE can often be slightly superior in recovering details, while DCB can be better at avoiding false colors especially in images from cameras without anti-aliasing filters.

When working with very noisy, high ISO images in conjunction with the Noise Reduction tool, it is recommended to use the LMMSE or IGV demosaicing methods. They will prevent false maze patterns from appearing, and prevent the image from looking washed-out due to heavy noise reduction.

If you use a medium format technical camera with near-symmetrical wide angle lenses such as the Schneider Digitar 28mm or 35mm it's likely that your file will contain some crosstalk, especially if the lens is shifted (due to the low angle of incoming light from these lenses some light leaks over to the next pixel on the sensor), and in this case you can get mazing artifacts with AMaZE and DCB because of green channel separation caused by the crosstalk. If you, via adapters, combine a mirrorless camera with a wide angle lens designed for film, you may also get crosstalk. It can then be better to use the more robust VNG4 algorithm (Variable Number of Gradients) which handles this situation well, at the cost of some fine detail. An alternative is to enable green equilibration to even-out the green channel differences.

AHD (Adaptive Homogeneity-Directed), EAHD (Horvath's AHD) and HPHD (Heterogeneity-Projection Hard-Decision) are old methods which are generally slow and inferior to the other methods.

None means no demosaicing is performed. This can be useful for diagnostics, but you would not use it for photography.

Mono is only useful for users of either monochrome cameras, or cameras with the color filter array removed.

Fast is a very fast but simple and low quality demosaicing method, not recommended.

3-Pass is a demosaicing method for cameras with X-Trans sensors (Fuji). It runs three passes over the image which leads to sharper results though you can only see this on low ISO photos. It is slower than 1-Pass.

1-Pass is a demosaicing method for cameras with X-Trans sensors (Fuji). It is faster than the 3-Pass method though a bit inferior in quality, though this difference is only visible in low ISO shots, so if speed is an issue you can use this method on high ISO shots with no visual difference in quality.

False Color Suppression Steps
Sets the number of median filter passes applied to suppress demosaicing artifacts when applying the demosaicing algorithm. False colors (speckles) could be introduced during the demosaicing phase where very fine detail is resolved. False color suppression is similar to color smoothing. The luminance channel is not affected by this suppression.

False colors are generally more apparent in images from cameras without anti-aliasing filters. Note that it is foremost the chosen demosaicing algorithm which is the deciding factor in how prominent will be the false color problem with which you will have to deal. In some situations it may be better to change the demosaicing algorithm than to enable false color suppression, as the latter reduces color resolution.

How to Find the Best Demosaicing Method
Zoom in to at least 100% (1:1) and try all the demosaicing algorithms, see which works best for you. Try them on sharp photos with fine detail and tiny patterns, such as the wavy and repetitive fabric of a sweater (watch for maze pattern artifacts), a distant brick wall, a distant round road sign (watch for aliasing along the round edges), and test with both low and high ISO shots. Use photos from your own camera; what's best for Nikon raw images may not be what's best for Olympus ones.

Monochrome Cameras
A monochrome camera has the same light filter in front of all pixels, that is you get a black-and-white image and no demosaicing is required. Some of these cameras have no infrared filter and are thus sensitive to infrared light, which can be used for creative black and white photography.

RawTherapee supports monochrome cameras, but the user interface is not adapted for it so when you load a monochrome file all color tools will still be enabled (they won't do anything meaningful of course). You will have to live with that, monochrome cameras are rare so we won't put any major effort into making the user interface morph into reduced monochrome-only version.

There are a few additional factors to consider when working with monochrome files: some monochrome cameras report that they have only a single monochrome channel and a neutral color matrix, like Leica M Monochrom, while others report RGB channels in a bayer configuration (like Phase One IQ260 Achromatic, or IR-modified cameras). If the camera reports only one channel, RawTherapee recognizes this and won't perform any demosaicing (the demosaicer selection is still enabled but does not do anything), and everything works normally. However, if the camera reports as an RGB bayer camera, demosaicing will be performed and a color matrix will be applied. To disable this, you should select the "Mono" demosaicing option, and select "No profile" as input profile in the Color Management panel.