I've been looking at mostly Navcam images so far, so I guess I can't and shouldn't try to say anything about Mastcam. But for MER Navcam images, the JPGs on the web usually are about 200K or more in size; MSL Navcam JPGs on the web seem to be usually around 100K or less. And there just seem to be more and stronger JPG artifacts, at least in images I've looked at of the deck. That could be factors other than compression for the web; for all I know they're being compressed more heavily on the rover in some cases. All I really know is, trying to take a MER Navcam image and make it about the same file size as the MSL Navcam images we're seeing on the web, I have to use a JPEG 'quality' setting of about 40%, which is high compression.

Here's what I typically see (random section of the image that happens to be in front of me):

Yes, an example - good idea.

Again, this is Navcam; I admit the Mastcam images are probably completely different. And I don't know that this is because of heavy JPEG compression for the web. That's just my guess.

Damn.... that looks bad.

Well, that speaks for itself. Assuming full 1024x1024 frames, 200K would be about 5:1 and 100K would be about 10:1. I did a quick spot check of some recent Navcams and they were more like 120K, but close enough.

I don't know how they chose the JPEG quality for MER and I don't know how they chose it for MSL, but I would think that 10:1 would be about quality 50 and 5:1 would be about quality 75. My own personal opinion is that 75 would be a more appropriate choice, but nobody's asking me.

As for your example, at that scale they both look pretty crummy (the MSL one has more JPEG artifacts, clearly), but I don't think zooming the image is really a fair test. That said, I wouldn't pick a fight with anyone who says the web release images are compressed too much, but I'm not sure I would use the phrase "compressing the living daylights out of" -- YMMV.

For the navcams, here's my guess. We've discussed before (image thread?) that the MSL public navcams appear to be stretched/lut'ed/delut'ed differently from the MER navcams. In practice it looks like the histograms are typically concentrated into the lower half (0-128 or so) of the full 8 bit range. So there's less information to begin with than in MER navacams. But then the details on MSL navcams tend to be dark with low contrast, so maybe jpeg doesn't capture the details as well. Then you need to stretch them to show the detail, and you end up enhancing the jpeg artifacts. You could test this by jpegging an original image, and then halving the intensity of the original, jpegging, and then doubling the intensity, and comparing the two.

Or they may be compressed more heavily on MSL. MER uses various compression levels depending on the need.

Nah, navcams use wavelet compression, but these are blocky artifacts suggesting JPEG compression on the ground is destroying data.

Fair enough. Here's a more typically-sized comparison, as good as I can make it at the moment. The MSL version still seems lossy to me. But if it is compression that's being done on the rover to get the images back faster, I'm perfectly happy with that - I'm just so happy to be seeing images at all. I only bring it up because I care.

Normally I would JPEG-compress these images, but well, this is a special case for obvious reasons.

I don't about MER, but as far as I can tell there is no stretching being done on any of the MSL images. They are typically autoexposed, which by its nature might be a little on the dark side but shouldn't really need much processing.

At this point I'm tempted to give into my bias and say that all Navcam images look bad to me. (Sorry, Justin. Not all, but some.)

I've always been a bit surprised that the MER Navcams were so grainy-looking. This might be dark current from the relatively long readout time. They're a bit blurry but what can you do with only four elements and fixed focus?

As for wavelet compression, having ICER artifacts interact with JPEG artifacts isn't going to improve the images.

But to recap -- are the public release images on MSL being compressed on the ground more than for MER? Could well be.

When comparing the left and right Mastcams of the sun I see a couple of things that I don't understand. The first is the large artifact at the bottom of the Mastcam 100 images. Is it a lens flare? The second is the color. The Mastcam34 thumbs are grey but the Mastcam100 thumbnails show as blue. I figured that this might have to do with the neutral density (sun) filters being different wavelengths, but looking at http://www.msss.com/msl/mastcam/MastCam_description.html the Mastcam34 sun filter is 440 nm which should be blue/violet and the Mastcam100 is 880 nm which should be near infrared. Sorry if this is obvious to others, but it seemed strange to me.

Blue filters on a bayer filter can often be nIR transparent.

The web page is in error. http://www.lpi.usra.edu/meetings/lpsc2012/pdf/2541.pdf is correct.

Thanks. That paper cites Malin, M.C. et al. (2010) LPS XLII, Abstract #1533. as having more detail in regard to the near-IR characteristics of the Bayer pattern, but I can't find that abstract and the diagram in http://www.lpi.usra.edu/meetings/lpsc2010/pdf/1123.pdf is hard to read. Still I learned a bit about how narrowband imaging will work (or rather is already working) with the MastCam. Thanks for directing me in the right direction.

The diagram is easier to read at http://msl-scicorner.jpl.nasa.gov/Instruments/Mastcam/

Oh nooo, what have they done ?

Auto-white balance of the navcam images



I liked the previous version a lot much because it was like working with true raw image, only degraded by the jpeg compression. But now… I hope this is just some kind of test, or something, because to be honnest, I don't like it.