MEX VMC - Back on, and online! Options

[Juramike]

For those of you who've been doing work on the VMC images, have you developed any specific workflow for boosting the Mars-like color in these images? Is it just as simple as increasing the saturation or do you do any channel-specific processing?

(on vacation in Savannah, but secretly using the lobby computer...)

IIRC, for most of the images I played with, I did a Gaussian blur to help eliminate the "spottiness" of the color blobs. I didn't do any funky channel mixing but I did adjust the saturation and hue, then adding Ted Stryk's half-phase Mars image as the reference and adjusting the individual color channel curves to try to make it match Ted's image. If there was a cloud I tried to balance it out as a whiter shade.

[ugordan]

I wonder if by virtue of imaging far closer to mars, on more featureless areas, possibly during dust storms - we might be able to generate a flatfield of some sort.

Also, such an observation would probably best be carried out as a rapid-fire, single exposure setting sequence letting Mars slide underneath and capturing as many frames as possible. A dust storm or even high cloud cover would be perfect.

If we can then constrain observations to that one exposure setting - then we can have just one dark field image, one flat field image, and be much better equipped to process the images.

I was thinking two best exposed images and leaving the one or two underexposed ones out. What I imagine the team has problems with is using generic exposure settings for all observations as Mars' surface is backscattering and its brightness varies with phase angle. The several exposure settings are probably just safeguards against severe over/under-exposure

[djellison]

Also, such an observation would probably best be carried out as a rapid-fire, single exposure setting sequence letting Mars slide underneath and capturing as many frames as possible.

Bingo - we then sum the frames ( slightly under-exposed ones ) any features cancel themselves out- and we have a flatfield.

[ormstont]

All really nice work! Where to start?!

Colour enhancement
Really cool enhancements of the colour of the images, we've been thinking for a while of boosting the colour to get more like the Mars people are used to seeing. If you get something you're happy with showing off it'd be great to post it to the blog and hopefully introduce it into our workflow so all images would be enhanced before upload.

Lens Artefacts
I'd love not to have the artefacts on the image and in fact I don't think we had them at launch (check out the pre-separation Beagle images). My theory is that debris from the ejection of Beagle (VMC sits right underneath the Beagle separation mechanism) might have hit the lens but I'm not sure it would produce the artefacts seen - anyone that can "reverse engineer" what sort of thing would cause them might make for an interesting blog article.

Flat-fielding
First off, I think the initial flat-field test by ugordan already seems to be a huge improvement on what we've got at the moment! I'm not sure if there are any images in the library that would at least get us closer to the data we need, perhaps some of the early exposure tests and crescent images or previous low altitude observations. Generally though we could consider doing a flat-field observation, although the chances to do low-altitude images are very rare so would be good to use them for proper imaging. What sort of image would we be looking for? Good exposure, under/over-exposure? Planet filling the image, only part of the image? etc. etc.

Exposure Settings
The exposure settings we use at the moment (the loop of 4) is as you guessed meant to be a good approximation to catch any lighting conditions regardless of phase angle. The middle two exposures are usually the best but I put the darker one in for extremely bright conditions and the brighter one for dim features like faint clouds. There have actually been two loops of 4 used: 0.4, 6.8, 14, 22.8 and the tightened range (currently in use): 2.8, 6, 10, 14 (all times in milliseconds). We actually have available a range from 0.4 to 191.6 milliseconds in steps of 0.8 milliseconds. If anyone can demonstrate from past images that tighter settings or even a loop of 2 or 3 exposure settings would still give good results then please let us know and we can change it! We have the ability to set the exposure individually for each image.

Image frequency
We've actually slightly improved the image frequency recently and are now really at the maximum frequency that VMC can support. This means an image every 44 seconds which is a hard constraint caused by the amount of time it takes the rather basic VMC to write the captured image to the on-board storage of the spacecraft.

Hopefully this answers some of the questions cropping up, of course please do send us by e-mail any results, processing or analysis done on the image for us to post on the blog. djellison sent us those wonderful movies he created and hopefully they'll be going up soon! Thanks again for your interest in VMC!

[elakdawalla]

I think others here would probably agree that generating a useful flat field would be so valuable that it would be worth the cost of sacrificing "proper imaging" on a rare low-altitude imaging opportunity. The flat field would improve the appearance of every image in the catalog.

[ugordan]

Here's a preliminary version of the VMC2RGB with flatfield support:
 vmc2rgb_v03b.zip ( 40.59K ) Number of downloads: 805

You'll need to place this file into the same directory as VMC2RGB.EXE. It's only a preliminary flatfield, has some artifacts and is cleaner in the lower center half of the image due to the pointing used in the images I used. I'm not really sure it does much other than reduce an artifact here or there and reduce color noise somewhat so your mileage may vary. I've seen older images that have one or two pixel offsets (probably readout errors or something) that completely negate the flatfield so be warned.

[elakdawalla]

Wow, Gordan. Although I'm sure the flatfield can and will be improved, the difference that this preliminary version makes in the processed images is huge. Here's my best version before flatfielding (the first one) compared to a new version from after flatfielding. Artifacts remain, but they no longer distract from the actual albedo features visible in the image. Well done!

[djellison]

Holy **** - Awesome work. I'm going to have to do all my movies again when I get back from a few days out the office

[ugordan]

I just noticed a few images here and there still have a 2-pixel shift to the left compared to the majority and thus the flatfield increases the noise. Take a look at images #10 and #11 from Nov 20 for example. I don't know if I'll be able to automatically deduce this from the image - seems like checking the rightmost pixel column might work as it appears zeroed out in that case.

In the meantime, you have been warned...

[mcaplinger]

You might want to do a little filtering now. This is a GIMP unsharp mask, radius 40, amount 0.7.

Note that this is the only working wide-field camera at Mars until MRO gets fixed.

Attached thumbnail(s)

 

[nprev]

Fascinating to watch you guys working out the processing of these images.

There seems to be some subtle pale banding centered on the pole, more noticeable on the left half of the image. Am I correct to assume that these are real (high-latitude clouds), and not artifacts?

[elakdawalla]

You might want to do a little filtering now. This is a GIMP unsharp mask, radius 40, amount 0.7.

Note that this is the only working wide-field camera at Mars until MRO gets fixed.

Cool, I'll try that out next, thanks for the suggestion.

Given its status as the only wide-field camera at Mars, is there something Mars Express could be doing with this camera that would actually have some value for weather monitoring, despite its limitations?

--Emily

[mcaplinger]

Given its status as the only wide-field camera at Mars, is there something Mars Express could be doing with this camera that would actually have some value for weather monitoring, despite its limitations?

I don't know enough about the particulars of when VMC can be operated relative to the eccentric orbit of MEx to know how close to global coverage it could get. It'd be interesting to see these images map-projected. I expect dust storms could be tracked, at least large ones.

[ormstont]

Yet more great work, UMSF never fails to impress me! I've taken a look at possible opportunities and we may have some low altitude VMC opportunities but these are still not at our pericentre. At these altitudes we'd nearly fill the frame with the planet but there'd still be dark areas around the edges. Would this be sufficient to generate the improved flat-field? Also what are we looking to do? Blur the planet but keep the artefacts visible? I agree completely though that it's worth doing a flat field which will improve all future VMC observations, I was just playing devil's advocate Once again on that front, do we think a real flat-field observation would bring significant improvements on the already stunning improvements just shown by ugordan?

For the 2 pixel shift in some images I'll look into what might be causing that, could be a bug in the processing software I've written. Certainly in older observations there was an anomaly within the camera that caused sometimes severe shifts in the image pixels. This should have been solved for some time now though.

Finally, I hadn't thought about it but I guess we are the only wide-field camera currently in operation at Mars, although I read on the MARCI website that they'll start imaging again in December. While it'd be great to do more systematic weather monitoring we are quite constrained by the Mars Express orbit and primary science activities. As such that's why we slot into the apocentre observation slots where the spacecraft has just performed orbit maintenance. This gives us a gradually shifting phase angle on the planet (it's a crescent as you know at the moment) as Mars moves round the Sun. The orbit is roughly polar, highly elliptical and with a precessing argument of periapsis (for those that know orbital elements), which gives us the varying central latitude of the VMC images.

For the flat-field script update by ugordan, can we start using this routinely to process VMC images? Would be quite straightforward to slot into the automated workflow after image downlink. Also would be great if we could post a small article about how flat-fielding works and how you derived it for VMC (also an image of the flat field to see how messy the camera really is!). Thanks in advance!

Hope this helps with the info, let me know if you have more questions!

[elakdawalla]

Since the flat field may improve with time, my inclination is that it should not be applied to the raw images posted on the website, though perhaps it wouldn't hurt to apply it to the PNGified versions? What do others think?