29-30 August 2007 Icy Satellites (rev 49), Last stop on the road to Iapetus |
29-30 August 2007 Icy Satellites (rev 49), Last stop on the road to Iapetus |
Aug 9 2007, 10:40 PM
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SewingMachine Group: Members Posts: 316 Joined: 27-September 05 From: Seattle Member No.: 510 |
CICLOPS' Rev 49 Looking Ahead page is up. Highlights include a fourth monthy Voyager-class encounter with Tethys with 500m resolution over Odysseus (finally!)
Detailed mosaics of Rhea's prominent ray crater and points west are on tap for Old Scabby's second closeup. This should be a really cool periapsis passage to tide us over until the 10th of September. -------------------- ...if you don't like my melody, i'll sing it in a major key, i'll sing it very happily. heavens! everybody's all aboard? let's take it back to that minor chord...
Exploitcorporations on Flickr (in progress) : https://www.flickr.com/photos/135024395@N07/ |
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Sep 1 2007, 02:00 AM
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Administrator Group: Admin Posts: 5172 Joined: 4-August 05 From: Pasadena, CA, USA, Earth Member No.: 454 |
Jason, I've seen this explained before but I keep forgetting the explanation. Can you explain the origin of the every-other-line truncation that appears in Cassini images that have lots of detail?
I love the sharp peaky shadows cast by the peak ring of Odysseus. --Emily -------------------- My website - My Patreon - @elakdawalla on Twitter - Please support unmannedspaceflight.com by donating here.
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Sep 1 2007, 08:49 AM
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Senior Member Group: Members Posts: 3648 Joined: 1-October 05 From: Croatia Member No.: 523 |
Jason, I've seen this explained before but I keep forgetting the explanation. Can you explain the origin of the every-other-line truncation that appears in Cassini images that have lots of detail? The truncation comes from the "LOSSLESS" compression scheme (obviously a bit of a misnomer since it can and does lose spatial information). The algorithm operates on line pairs and guarantees a compression ratio of at least 2. It's a variant of a Huffman encoder and if it figures out that the two lines it's currently encoding will turn out to be compressed more poorly than 2.0 ratio, it just stops and truncates the rest of the second line. In an absolute worst case scenario the second line could be completely truncated if the first line turned out to expand rather than shrink, which could happen since the algorithm uses fixed statistical encoding tables, they are not optimized for each frame. I believe Voyagers also used something similar. Not sure if Galileo originally had it as well. It's a rather dumb and unflexible encoding scheme, but is computationally inexpensive so it's often used. The fixed 2.0 limit is probably due to ease of data policing on the onboard encoder - with LOSSLESS encoding you can guarantee that the frame will be at most half the original size so you can allocate space for each frame accordingly and not worry about losing the whole lower part of a frame in case your encoded frame turned out bigger than you predicted. Here's another one for the photoalbum: -------------------- |
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