Processing an image of Mars in color must be done with care if one wants to correctly reproduce the real aspect and the planet. By taking a tour of the numerous Mars images produced by amateurs, it's obvious that the aspect of the planet varies greatly from an image to another (and much more than for Jupiter or Saturn). Such a wide variation proves that the planet is difficult to image. Mars's details are also quite desequilibrated as the surface details have an evidence and a presence that atmospheric details almost never have. As a result, during the last years we have witnessed a rush for getting the highest contrast and the highest - surface - detail possible that has degenerated into abusive processing innovation. At first rank is found the highly popular RRGB processing, which is certainly the worst idea that has emerged among the amateur community. I have been myself doing RRGB images during my first steps in CCD imaging, seduced as many by the technical quality it gave to the images. Experience and knowledge of what is seen on the planets soon lead me to give this up.

Various ways of processing a color images of Mars. RGB, RRGB, IRGB (with an near-IR image for luminance) and R(G)B with a synthetic green frame. No one of the special processings (RRGB, IRGB, R(G)B) is able to match with the RGB image (colors, contrasts, details). Where's the problem ? And is this really a problem ?

RRGB IMAGES : THE DEVIATION OF THE LRGB TECHNIC

The LRGB technic has been created years ago by the Japanese imager Kunihiko Okano . The method provoked a quick jump in images's quality and is still used today with much profit especially for Saturn. But then the desequilibrated martian details in favor of the red frame (most people simply don't see or aren't interesting in the details found in Green and Blue light) created the idea that this frame could be used as well as luminance. This had a strong visual advantage to seduce people : it produces highly contrasted and detailed "color" (see below) images of Mars. This is because the martian surface is best contrasted in red, and because the red image is usually better than the green and blue ones as the atmosphere is less perturbating in longer wavelenghts. But who had ever really thought about all the consequences on the images ?

Let's be clear with an important point. RRGB don't gives you a color image of Mars. Color is related to the human vision and is reproduced only by the RGB or LRGB methods (not to talk about the abandoned CMY method). RRGB produces a colorized image of the surface of Mars, and this is widely different !! Or, one could also argue that RRGB is already a false color image. That's again true.

Finally, RRGB has no objective justification. Its one and only purpose is to create images that most people will find better because it's really contrasted. There is no other reason. But is this enough to accept a given kind of processing ? No. If so, everyone is likely to use the most subjective processing freely. What about using the strongest unsharp mask or wavelet ? It will also produce an excellent contrast. But the image will still be wrong. Amateur's images should be astronomical images not artistics images, and this implies rules to be followed.

RRGB technic produces the following imaging errors :

1) The most important errors is that around 90 % of the atmospheric details disappear, except the brightest clouds (like the NPH). This is really harmful as atmospheric phenoma, although generally barely visible, is important, interesting, and add its own beauty to the image.

2) Colors are not the good ones. This is because the LRGB technic, to work well, must conserve an equilibrium between luminance and chrominance, so as every detail of the L image will find its color in the RGB frame. As RRGB uses only a part of the spectrum for the details, this equilibrium is broken, and the relative contrasts of both components don't correspond anymore.

3) There is also a loss of color tones. Martian colors are far more varied than it's generally believed. This range of color shading is visible in a good RGB image, but no more in RRGB, for the same reason than why it doesn't show the real colors of the planet.

4) Fourth, RRGB is likely in some occasions to provoke a loss of resolution. Few color webcams users know that the best resolved component of the webcam image is the green one, not the red one. This is because there are two green pixels for one red and one blue on the chip. Spliting a webcam image of Mars will show this easily, and the difference is very important. For CCD or B&W camera/webcam users, in good seeing a RRGB is also likely to reduce the resolution, simply because the resolution of a telescope is less important in longer wavelenghts. In that case the full-resolution green image, which also shows the surface details, will gives the superiority to RGB even in this terrain.

From all this we can conclude that the RRGB in reality is a processing that makes much details and information disappear from the image. This is really absurd : processing is supposed to reveal the information, not to cancel it ! From all this we can also conclude that there is no natural law that makes the RRGB more pleasing to look than RGB. Some people will find that true-color images are a beautiful sight to look at ; and that RRGB gives an awful, artifical view of the planet with unreal contrast and several losses of informations.

ARTIFICIAL GREEN IMAGES : A FALSE GOOD IDEA

Back in 1999, it was Antonio Cidadao who made a technical innovation by using only a red and a blue images of Jupiter to make a color one (see his page). Green was merely a sum of both R and B. The resulting R(G)B image (also called today RsGB) gave very satisfaying results for Jupiter. The idea was to reduce the time where the different components of the future color image were getting obtained, because Jupiter spins very fast. This was in a time where CCD cameras were much less efficient than today's tools. The idea has been also applied to Mars later but with another kind of argument (because the slower spinning of the red planet as well as its brightness really gave more room to obtain a true green frame) : the idea that because of the reddish color of the planet, and because of the apparent lack of green-based details, a green image wasn't really justified. Time was then saved to get more red and blue shots. This is a false idea (and for every planet). A simple comparison of RGB and RsGB images of Mars gives evidence that an artificial G doesn't give the same result than a true G. They're not equal.

Colors obtained by RsBG isn't the same than RGB. Especially, yellow isn't reproduced anymore and this is a strong error, as any dust-associated phenonema on Mars will appear yellow to an extent. This is because true color implies that every wavelenght reaches the CCD. With RsGB, more than 100 nm at the center of the visual spectrum aren't recorded...

GETTING RID OF THE BLUE IMAGE FOR COLOR : AGAIN A FALSE GOOD IDEA

The blue image is usually not as good as the two other R and G. This is because, as already pointed, the atmosphere isn't as steady and transparent in short wavelenghts ; but also because the B layer of color webcam images is often technically very bad, with much artefacts. Of course, it has a negative influence on the technical quality of the RGB composite... This is why some amateurs - again, I was one of them in my beginning monthes - simply get rid of it and buil a "color" image with the G layer replacing the B one. The new G image is obtained by doing a sum between R and G... !! Of course the result is even worth than any RRGB component. Colors are completely false, and all the details of the B image is lost.

CONCLUSION ?

Let then me give you the secret of the good CCD image ;-) : the quality of our planetary images isn't created by processing. It entirely belongs to the quality of the imaging phase : collimating the instrument, getting it cooled, imaging with adecuate settings. No, nothing has changed in this respect since the "dark ages" of classical photography. The easyness of numeric processing technics is really the dark side of today's astronomy, if not properly used. Don't be seduced by the dark side.

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