I've seen quite a bit of difference in how people are doing color balance of the Mastcam and MAHLI images, both here and in press releases.

I don't know how anyone is doing this specifically. Some results seem too gray or red/pink to me. Without something known to be neutral in the image it's hard to do white balance on any direct basis. Even in fairly dusty conditions, I'm skeptical that there's enough tinting by sky scattering to really affect the color of the scene much, especially near mid-day.

The top of the white pyro box where the Mastcam cal target is mounted seems pretty clean in the sol 3 images. To normalize that to neutral, I multiplied the red channel by 1.06 and the blue channel by 1.13 and the results seemed OK. This is a fairly subtle change, but it gets rid of the green cast in the raws; see example below (raw on the left, processed on the right.)

That definitely looks better. The black strut looks 'black' now as well.

Would be nice to have any paper/studie on this subject. I would believe that the light on mars is not a neutral white and with this in mind I wouldn't calibrate a white surface on mars to neutral white, at least not to get "true color" results.

Another thing I'm thinking about are exposure times - even at mid-day it would not be that bright on mars like on earth right? So the images are all to bright too (?).

original - white balanced - wb and darker

No, they're not. If anything, they are all too dark on computer screens. The eye is also amazingly adaptive and can compensate for vast brightness differences without you actually noticing them. I would guess that you wouldn't even notice that the Sun is around 50% dimmer there on average.

Why? Do images taken on Earth look blue because the sky is blue? AFAIK, it takes unusual conditions (dust storms or smoke) on Earth to change the color balance of daylight near mid-day. And of course sunlight isn't "white" anyway, so to zeroth order one balances an image so that something that looks white to the eye looks white when the eye looks at the image (more complicated than it sounds.)

This has been analyzed a lot on previous missions, inconclusively IMHO. MSL is the first time we've had broadband filters that approximate the human eye response, so there's some reason to believe that with minor adjustments we can get "natural color". I'm not certain how automatic white-balance algorithms work; see http://en.wikipedia.org/wiki/Color_balance for a start, but ironically, someone has used an MSL image on that page as an example of white balance, and I suspect that image was just tweaked in Photoshop.

No, of course not. But with a "wrong" white balance they do

I thought there is more dust in the martian atmosphere than under normal earth conditions? So you would say there are the same (or close) light conditions on mars like here on earth (white or grey would be more ore less neutral)?

Edit: I do not question your arguments - I just would use slight off-white instead of a neutral white. And for sure I dont want to split hairs here.

Short answer: I would guess so under average dust loading (that's what we assumed to compute typical exposure times, etc), but I am not exactly an expert. The idea of "what it would look like" is so slippery as to start to lose engineering meaning.

http://marswatch.astro.cornell.edu/Bell_etal_SkyColor_06.pdf is a nice try from MER but I'm not convinced it's conclusive or even what question it's trying to answer -- I think the sky color at the horizon.

Unfortunately this article is behind a paywall, but the abstract tells the essence of the tale:

http://adsabs.harvard.edu/abs/1999JGR...104.8795T

The ambience of the Martian sky is a factor in the color of the scenes shaded regions in particular. The effects of the ambient lighting on the changing apparent color of the 'Yogi' rock is mentioned in the article.

I wish there was within the field of view a stepped grayscale like the Vikings had. An abbreviated and/or reduced variation of the Macbeth color charts such as was used in the camera tests would be even better. That said, the color balance looks pretty good, the brightness levels seem justifiably 'brightened' somewhat to optimize the detail recorded.

The hypothetical question "How would our perceptions of color change on worlds with dimmer or stronger sunshine" has always interested me.
I've been lucky enough to witness two total solar eclipses in my adult life. Both of them observed under crystal clear skies and from an elevated location with a unobstructed view to the horizon. The first one was observed on feb 26 1979 from atop a small Mesa near wolf point montana with the sun about 25 degrees high. With this question in mind I made note of what the world looked like under Martian (50%), asteroid belt(10%), and Jovian illumination(4%). As ugordon states, I expected to not notice much until most of the sun was covered. But at 50% illumination the change in color and contrast was significant. It was as though I had a pair of weak sun glasses on. However when I asked my 3 companions( who had not been paying attention to this) if they noticed it I got a yes,no and a maybe.

I tend to be more on the side of expecting the sky to color the terrain, and wanting to see what the scene looks like with a context appropriate white balance (i.e., using the cal target). Under current dust loads, diffuse sky light exceeds direct sun light throughout the sol. That's never true under a blue sky on Earth--but rarely true for other sky colors and frequently true for gray skies. And, on Earth, the blue sky results in a yellowed Sun. On Mars, the dust slightly reddens (oranges?) the Sun as well as being colored by mineral absorption. That said, I doubt astronauts will come back talking about how much the sky changed their color perception--it's well within adaptability (but that's another debate).

Another small effect that can change the appearance of the same image as interpreted by different people is just that, while the broad filters sorta match the eye's response, the processing (e.g., companding to 8 bits vs. gamma=2.4 vs. perhaps expanding the bits without applying a gamma [?], no accounting for eye vs. monitor color) does not match the sRGB standard, and will not show up as the right color on typical monitors. These differences are small, I think--but can result in slight variations of color balance, even between white balanced images.

Really? I thought reddish dust would blue the sun. It certainly does at sunset. I think the one highly noticable difference in the quality of light on Mars would be the colour of the shadows. They'd be brownish not bluish because that's the colour of the scattered light there.

I agree 100% with Don, who has alway been for us, "Martians", the great "Master" of Mars colors since the old Viking times... (and also much respect to Deimos...)

At JPL we did accurate color processings using the 3 grayscale charts positioned on the VL1 deck with images taken during the Monitor & Long-Term missions. Here are the comparative results : we discovered that the color of the Martian sky changes a lot during the seasons. In fact NO one can tell which color exactly are the Martian sky and ground at a specific time. Sayings at JPL were that the Martian soil is of a "yellowish-brown" (refer to the JGR issues of 1977) and the sky is of a "moderate salmon pink"... globally but NOT on a specific day. Herebelow are accurate comparative processings showing the changes of colors of the Martian sky over the seasons...



Regarding the equalizing of raw images on grey values, please refer to posts #
http://www.unmannedspaceflight.com/index.p...st&p=191996
http://www.unmannedspaceflight.com/index.p...st&p=192007
Enjoy !

PS : Sol 1742 image was taken during a dust storm. See the inverted brightness due to higher concentration of dust in the plane of sight towards the horizon...

Interesting. As an ex-BBC camera operator, with a vested (professional) interest in correct white balance and lighting in general, my experience of the near total eclipse of August '99 in Scotland was different: what struck me at near totality was the sharpening of shadows. Not, particularly, an overall change in the amount of light. As (I think it was?) Heinlein once said, "a bit of sun is still a lot of light".

For Mars? A smaller sun will mean subtley sharper shadows - I doubt we would notice much in the way of a drop in insolation. And as to white balance, the eye copes with amazing variations all the time: we'd white-balance on any available clues. My gut reaction is Ronald's central frame in his post above is about right.

Andy

It's always fun to get accustomed to ski goggles or tinted sunglasses and then remove them. Pretty easy to become unaware of the weird lighting.

Mark, do you have a reference for this? I find it very surprising.

It's true that I've ignored the effects of the companding.