There is no color balancing done in the camera at all. The CCD signal, with the IR cut and Bayer pattern filter throughput and the detector quantum efficiency, is directly converted, run through the square-root encoder, interpolated, compressed, and sent to the ground. No attempt was made to balance this, although because of the way it works out, it is pretty well-balanced for sunlight. For a terrestrial sunlit scene, the raw image is just a little bit greenish, which is more or less what my analysis above says.

As for brightness, perceived brightness is an even slipperier concept than perceived color. Most of these images are auto-exposed to mostly fill the 11-bit histogram prior to companding. What that means relative to how they "should look" is as much a matter of taste as anything else.

Absolutely right regarding percieved brightness and colors! Although I was more thinking from a photographers point of view.
Thanks for the hint with the auto-exposure, this explains why the surface ist darker when the bright rover top is in the view, giving a somewhat near white to the camera and doing a "good" auto-exposure then. When no bright parts are seen in the view the auto-exposure does a very well job for scientific purpose but maybe overall a bit too bright (for my gusto it is ).

Edit: Just to illustrate my thinking - compare these two images (same sol, same time): 1 2
And please don't get me wrong - I'm just curious and do enjoy all the fine images we got so far!

This is the crucial point. To get a proper sense of how accurate the mastcam colours are, I'd like to see some of those terrestrial sunlit scenes, treated in the same way as the Mars images. Then we could see for ourselves just how "greenish" the Mars images are. What's "a little bit greenish" to some might be substantially shifted to others.

The outdoor terrestrial views on this page are "colour balanced", so unfortunately we don't know what the originals were like. I don't know of any public, unbalanced, outdoor, terrestrial mastcam images.

True enough. But it is still an interesting question to ask "what would some Martian soil sprinkled on a sheet of white paper look like". My sense from MSL and years of MER images, where white rover parts are visible in the same frame as Martian ground, is that the answer is "brown" or "cinnamony brown", rather than orange or bright orange.

Of course, when we present an image showing only the Martian ground, displaying it as brown would mean we loose detail in the darker regions, so it makes sense to brighten it so it looks like some kind of orange. What we'd actually perceive standing on Mars looking at the ground is a whole other question, and would probably depend on what else is in the field of view, how long we've been there, etc...


Edit:
Another good comparison is this pair of images, taken from and looking at the same location, but at different times of day:
http://mars.jpl.nasa.gov/msl-raw-images/ms...2000E1_DXXX.jpg
http://mars.jpl.nasa.gov/msl-raw-images/ms...2000E1_DXXX.jpg
The degree of colour saturation seems to depend on the angle of sunlight.

Ronald's links reveal the shadows depict the sun is likely shining somewhat into the lens in image2.
Aethetically speaking, the autoexposure has seemed to darken it too much (my eyes get similarly brightness-averse when i look towards the sun), however the sky itself seems to actually be just about the same brightness and has been noticeably green-shifted as compared to image1 where the sun is at left.
perhaps the exposure algorithm did what was expected, might IR wavelengths have a more pronounced affect on the MSL exposure system given the sun angle?

When I said a little bit greenish, I meant that the Neutral 5 square on the Macbeth chart was this color (linearized, averaged, and rescaled to 8 bits):



#657761. Of course, the Macbeth neutral values are only neutral under CIE Illuminant C and these images were taken in bright sunlight (more like Illuminant B ) so there might be some small departure from neutral.

Martian sky color palette (10° above horizon). Enjoy

Found an interesting article on Slashdot on how Nasa/JPL is able to deliver all of the cool images and data we use here on UMSF.

NASA achieves data goals for Mars rover with open source software

Since it deals with Software and systems, I thought I would share it here.
Admins. if you think of a better place, please let me know.

Curious thing about this MAHLI image:

http://mars.jpl.nasa.gov/msl/multimedia/ra...0_DXXX&s=58

The time that image was taken is given as 2012-10-04 22:38:31 UTC (sol 58). According to my calculations, that was about 46 minutes after sunset on sol 58 (21:52). I cross-checked that with Eyes on the Solar System. Yet the scene seems fairly well lit with directional light--so is that the normal nature of the sky glow at dusk on Mars? Or maybe there's an error?

I would think this is the auto-exposure doing its job (or an exposure to get as much tonal information into the image). At the lower half you can see some grazing light from the low sun angle (?) so the time given seems to be right. If you tune down the brightness it looks like this:

Though it is interesting how the light situation is there on low sun angles.

Dust on the color calibration target on Sol 72...
http://mars.jpl.nasa.gov/msl-raw-images/ms...0000E1_DXXX.jpg
Enjoy (if I may say...)


(And also refer to post # http://www.unmannedspaceflight.com/index.p...st&p=191996 )

Maybe the LEDs were being used?

Looks like a bunch of full-res narrowband filtered image sequences were just released from sol 72. Now I understand more fully what mcaplinger said back in this post about greyscale full res images being filtered ones.

The thumbnails appear colored (i.e. not post-processed onboard to account for the filters, so for example the 525 nm thumbnails look green) but the full res versions appear greyscale (i.e. bayer-interpolated and compensated for the different pixel responses to the narrowband filter, though the JPG files are still color files). Pretty neat, and makes it easy to identify the sequences when sorting by time taken or filename.

The MAHLI image doesn't really look LED-illuminated. 46 minutes after sunset, the western sky is still relatively bright, while the eastern sky is much darker. I'd guess that is all that is going on (along with the much longer than daytime-normal exposure time).

I vote error. The more global (diffuse) illumination shouldn't cause such sharp distinction between light and dark.

A time error of around an hour would be a serious error.

Can someone identify the context of that image - it looks like a trench? If the trench is aligned roughly east-west, that might concentrate the bright, western sky into a smallish region as viewed from the trench floor, and so sharpen the shadows somewhat.

Otherwise, as Deimos says, the sky really will be quite a bit brighter in the west than in other directions 45 minutes after sunset, and maybe this lighting is what we expect. It would be good to see other images taken at similar local times - can anyone find some?