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The last image from DART only includes about the top ⅒ of the image, but that portion of the image is in full quality. That suggests that the image encoding/compression is mostly a raster encoding. Off hand this seems to me like a rather odd choice. It prioritizes information that isn't very useful over data that would be more useful.

Does anyone know (or have any informed speculation about) why the images were transmitted the way they were?

The first alternative that comes to mind would have been some sort of global/progressive encoding (e.g. something as simple "progressive JPEG") that first sends low quality image data for the full field of view followed by progressively more detail. The potential advantage of that would be that, regardless of where the transmission is cut off, you get some information from the whole field of view at the expense of not getting really high quality.

The second alternative that comes to mind is even simpler: just re-order the pixels to transmit the center of the image first. The point of impact is likely the portion of the last image that is most interesting and the encoding used has only about a 50:50 chance of showing that.

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    $\begingroup$ The real answer to this is probably something like “that’s what the guy who programmed it decided” $\endgroup$
    – Topcode
    Commented Oct 14, 2022 at 1:43
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    $\begingroup$ As a person who actually designs digital cameras for spacecraft -- basically, that's how the data comes off the imager. Clearly, for this mission, they would not have wanted any significant latency in buffering or processing the data onboard the spacecraft -- they pretty much dumped the pixels to the transmitter as soon as they had them. $\endgroup$
    – Dave Tweed
    Commented Oct 14, 2022 at 3:21
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    $\begingroup$ @uhoh: It's possible that there might have been some sort of line-by-line compression, but I agree with David Hammen -- probably not. The HDR images you get in space don't really benefit from it. $\endgroup$
    – Dave Tweed
    Commented Oct 14, 2022 at 12:09
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    $\begingroup$ @uhoh FWIW, I've implemented delta encoding (and RLE). Yes, it's fast, and can give substantial compression, but not with noisy data. The raw images on skyandtelescope.org/astronomy-news/… look rather noisy to me, so they wouldn't benefit much (if at all) from naive delta encoding. If you clean up that noise, then sure, delta encoding would be fine. But I'd rather have all the raw data. OTOH, I agree that the final frames of the impact would probably compress ok using delta. $\endgroup$
    – PM 2Ring
    Commented Oct 14, 2022 at 21:41
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    $\begingroup$ Regarding latency, I'd be surprised if the amount of time to compress an image (even using lossless compression) is anywhere near the time needed to transmit it. Latency (for any significant percentage of the image) would probably be reduced by compression. - Or is someone asserting that the bandwidth off the sensor is comparable to the bandwidth back to earth? That would be extremely surprising to me. $\endgroup$
    – BCS
    Commented Oct 15, 2022 at 4:11

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The first alternative that comes to mind would have been some sort of global/progressive encoding

Yes, and that's what you are looking at. The last frame is a sub-resolution image, and additionally contains only low frequency components.

It's just that even the progressive pass was cut off very early. That ~10% of a frame we see is less than 1% of the data from the CMOS sensor, which is leaps better than a simple scan line raster strategy would achieve.

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    $\begingroup$ Is that a fact or speculation? Please provide some sources. Do we have access to raw data? The published images seem to be heavily post-processed. $\endgroup$
    – asdfex
    Commented Oct 16, 2022 at 11:14
  • $\begingroup$ dart.jhuapl.edu/Gallery $\endgroup$
    – SE - stop firing the good guys
    Commented Oct 16, 2022 at 13:16
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    $\begingroup$ Where exactly can I find your information on that page? The caption of the image in question definitely doesn't help anything. $\endgroup$
    – asdfex
    Commented Oct 16, 2022 at 13:30
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    $\begingroup$ agreed with asdfex. I can believe that the final DRACO image doesn't have the detail of the others, but I wouldn't call it obvious. $\endgroup$
    – Erin Anne
    Commented Oct 16, 2022 at 16:06

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