MSL Images & Cameras, technical discussions of images, image processing and cameras |
MSL Images & Cameras, technical discussions of images, image processing and cameras |
Aug 16 2012, 11:05 PM
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Senior Member Group: Members Posts: 2228 Joined: 1-December 04 From: Marble Falls, Texas, USA Member No.: 116 |
I'm still trying to figure out a number of things about the new images we are trying to work with. Assuming others are likewise trying to learn, I thought I would open this thread to create a place for such discussions.
I'd like to start out with a comment about raw image contrast. There have been several postings in the main threads about whether or not the MSL raw images have been stretched like those from the MER missions. I am certainly no expert on this, but it looks to me as if the MSL images have not been stretched at all. I haven't tried to analyze all of the image types, but the hazcams and navcams have pixel brightness histograms that are very different from their MER counterparts. This attached image compares MER and MSL navcams along with their luminosity histograms. The MSL images clearly are not using the entire, available range of brightness values, whereas the MER raws do. For this reason, the MSL raw images can usually be nicely enhanced by simply stretching the distribution of brightness across the full 256 value range. -------------------- ...Tom
I'm not a Space Fan, I'm a Space Exploration Enthusiast. |
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Aug 28 2012, 10:08 AM
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Senior Member Group: Members Posts: 1465 Joined: 9-February 04 From: Columbus OH USA Member No.: 13 |
I wonder what the effective bits per pixel of the MASTCAM raw images is. The cameras use the KODAK KAI-2020CM sensor. Is it sampled with a 12-bit ADC like MARDI and NAVCAM? One reference for NAVCAM gives an SNR of >200 for certain conditions. So that would mean effectively 7-8 bits per pixel for that camera anyway?
It was interesting that yesterday Mike Malin referred to stacking images. -------------------- |
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Aug 28 2012, 01:48 PM
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#3
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Senior Member Group: Members Posts: 2542 Joined: 13-September 05 Member No.: 497 |
I wonder what the effective bits per pixel of the MASTCAM raw images is. The MAHLI paper I've referred to several times before is an excellent source of this kind of information. From section 7.5.1: "Acquired as 12-bit images, MAHLI data are converted onboard the instrument, without loss of information, to 8-bit images using a square-root companding look-up table." -------------------- Disclaimer: This post is based on public information only. Any opinions are my own.
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Aug 28 2012, 07:47 PM
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Senior Member Group: Members Posts: 1465 Joined: 9-February 04 From: Columbus OH USA Member No.: 13 |
The MAHLI paper I've referred to several times before is an excellent source of this kind of information. http://rd.springer.com/article/10.1007/s11...4/fulltext.html From another paper.... QUOTE MAHLI shares common electronics, detector, and software designs with the MSL Mars Descent Imager (MARDI) and the two Mast Cameras (Mastcam). Ahh, good to know. Wonder what the (optimum) SNR is. -------------------- |
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Aug 28 2012, 08:25 PM
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#5
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Senior Member Group: Members Posts: 2542 Joined: 13-September 05 Member No.: 497 |
Wonder what the (optimum) SNR is. I often tell people who are fond of such numeric metrics as SNR, MTF, ENOB, etc, that no matter how optimal those numbers make your camera sound, anybody can tell a good image from a bad image. I think the MMM images hold up pretty well. That said, you could work out the best possible SNR we could get from the Truesense datasheet. You can't ever do better than sqrt(fullwell) for a single measurement due to shot noise and these sensors have a fullwell of 20K-40K electrons so 140:1 to 200:1 is as good as it could be. -------------------- Disclaimer: This post is based on public information only. Any opinions are my own.
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Aug 29 2012, 12:32 AM
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Senior Member Group: Members Posts: 1465 Joined: 9-February 04 From: Columbus OH USA Member No.: 13 |
these sensors have a fullwell of 20K-40K electrons so 140:1 to 200:1 is as good as it could be. 200:1 is about 46 dB SNR which according to the method on ctecphotonics gives effective number of bits (ENOB) of (46-2)/6 = 7.3 bits. That could be improved if desired by stacking? The images are very impressive to see--just wondering what the limits are when the images are considered as abstract data, perhaps to be analyzed by machines teasing out the last bit of information. -------------------- |
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Aug 29 2012, 06:44 PM
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#7
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Member Group: Members Posts: 408 Joined: 3-August 05 Member No.: 453 |
That explains it all Joe - thanks! This made me wonder if the public "raw" images we see are uncompanded (and rescaled into 8 bits) again or not?
As an experiment, I used some full size ML and MR color images of the sundial, which has gray rings of 20%, 40% and 60% reflectivity, and measured the corresponding grayscale pixel values using "sample merged" data point of appropriate radii in the Gimp. Making various assumptions about JPEG accuracy, lighting and dust etc. (i.e. ignoring them!), the resulting data shows that the JPEGs we see appear to be quite linear in response to the different grayscale rings. This suggest that the images we see has been decompanded - unless either my measurements are flawed (for instance, the extrapolation of the trend does not go through the origin), or the CCD is not really linear in response, and the MSL side companding has now made it appear linear? Comments welcome, as if I could stop them anyway :-) Airbag |
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