Juno Perijove 42, May 23, 2022 Options

[Brian Swift]

PJ42_01. From upper left: Europa, Io, Jupiter. (Orientation, Io North Up)

[Bjorn Jonsson]

Very interesting discussion. As suggested in earlier posts here, I think it would be a very interesting and good idea to take at least one test image with lossless compression over the highly photogenic area near latitude ~40 deg north. This would be an image obtained near perijove - probably not the highest resolution image but close to it. The exact location should be selected to maximize the probability of imaging a photogenic, high contrast area with various cloud features visible. If I have understood everything correctly this would have to be done soon since perijove will be occurring over Jupiter's nightside only a few orbits from now.

In contrast, I doubt there is strong need for more images near perijove because even though everything is moving pretty fast there has always been nice overlap between adjacent images (the emission angle is not optimal for some areas though but there's always overlap).

I took a quick look at compression artifacts. This is a heavily processed example comparing PJ42_34 (lossy compression) and PJ42_35 (lossless) by rendering them using identical viewing geometry. This is comparable to what Brian did earlier.



Image 34 lacks many of the details visible in image 35 and has obvious artifacts that are very probably due to the compression - some very uniform areas and far less 'dithering-like' appearance. There are some differences because the emission angles in the source data differ for obvious reasons but I don't think this is the reason for any the artifacts. It is also obvious from the PJ42_34 image (and also obvious in Brian's image) that the 'color discontinuity' that was discussed in the PJ6 thread has now appeared again because the images show an area fairly close to the terminator.

Here is a different example of compression artifacts. This is a contrast stretch of a small part of the 34 and 35 raw images without any other processing (i.e. this is not decompanded):



Here image 34 does not show the same area as image 35 but the compression-related differences are nevertheless obvious. There are no completely uniform areas in image 35 whereas image 34 has some completely uniform areas.

Images 34 and 35 show a relatively low contrast area near the equator. I also took a look at image 28 which shows a high contrast area and was obtained near perijove (the resolution is ~2.5 km/pixel at the nadir). This image was obtained with lossy compression and unlike the 34/35 comparison, no image with lossless compression shows this area. Therefore a comparison to lossless data is not possible. Here is a contrast stretched crop from image 28:



Some 'contouring' is visible at various locations, e.g. lower right and upper left and more subtle in the dark, reddish/brownish spot at left. What's happening is more obvious when the color channels are examined separately:



The compression artifacts (I'm assuming the uniform areas are compression artifacts - this seems very likely but it is not completely certain) are relatively subtle in the red channel but very pronounced in the green and blue channel. I think it would be very interesting to know what a photogenic, high contrast area like the one in image PJ42_28 looks like at very high resolution without compression artifacts.

Interestingly, I have found that it *might* be possible to make these artifacts slightly less obvious by adding a small amount of noise to the raw image before decompanding. These experiments of mine are at a very early stage though so I don't know exactly how well this would work. A different amount of noise is probably needed for the different color channels. This plus the images above also implies that it might make sense to use different compression parameters for the different color channels (higher compression for red than for green/blue) but I would be *very* surprised if this was possible.

[mcaplinger]

... it would be a very interesting and good idea to take at least one test image with lossless compression over the highly photogenic area near latitude ~40 deg north.

FWIW, I raised the possibility of doing this in today's PJ43 planning telecon, but we can't do it for PJ43 because of data volume constraints from Io imaging. We will certainly consider it for PJ44.