Clouds? |
Clouds? |
Jul 8 2008, 01:38 AM
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#1
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Senior Member Group: Members Posts: 1465 Joined: 9-February 04 From: Columbus OH USA Member No.: 13 |
Was looking at some MGS imagery:
Kind of a low-contrast scene with some darker splotches here and there. Running that through an equalizing filter gives this image: Some surprising detail brought out there--it looks to me like there are small clouds casting shadows on the surface. Can that be right? They could be little hills I suppose. -------------------- |
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Jul 8 2008, 01:49 AM
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#2
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Member Group: Members Posts: 128 Joined: 10-December 06 From: Atlanta Member No.: 1472 |
QUOTE Some surprising detail brought out there--it looks to me like there are small clouds casting shadows on the surface. Can that be right? They could be little hills I suppose. Likely JPEG lossy-compression artifacts. |
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Jul 8 2008, 01:55 AM
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#3
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Senior Member Group: Members Posts: 1465 Joined: 9-February 04 From: Columbus OH USA Member No.: 13 |
Likely JPEG lossy-compression artifacts. The original images are from IMG files (256 pixels per degree MGS images http://www.msss.com/mgcwg/mgm/), only converted to JPG in the final step. I believe IMG files are lossless. -------------------- |
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Jul 8 2008, 11:42 AM
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#4
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Member Group: Members Posts: 128 Joined: 10-December 06 From: Atlanta Member No.: 1472 |
The original images are from IMG files (256 pixels per degree MGS images http://www.msss.com/mgcwg/mgm/), only converted to JPG in the final step. I believe IMG files are lossless. I'm not sure. I agree, IMG per se is lossless, but MOC had the ability to compress images on board using Walsh-Hadamard or Discrete Cosine techniques, which are lossy. I wonder whether you can see the same low-contrast, semi-regular patterning in other MOC wide-angle images. |
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Jul 8 2008, 02:02 PM
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#5
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Senior Member Group: Members Posts: 1465 Joined: 9-February 04 From: Columbus OH USA Member No.: 13 |
I wonder whether you can see the same low-contrast, semi-regular patterning in other MOC wide-angle images. Here's one of interest, from a simulated view from MRO as it was approaching the Phoenix site the day after the landing: So there's a bit of the "cloud" effect on the left but not so much elsewhere. Could that be an area where ground ice is subliming and condensing again at height? Unfortunately there's a bit of a dataset problem around the Phoenix site, approximately at the center of this MGS mosaic: Looks like some inconsistent lighting on the stitched-together sections. Also, maybe clouds were there for one pass and absent the next? Here is what MRO was looking at just a minute or so later: Some pretty large clouds? Anyway, there are plenty of different kinds of artifacts (stitching, posterization, dropouts, etc.) but not compression per se, IMO. Note that the HIRISE field of view is much less than this image (like 1/50th the height). -------------------- |
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