This is fascinating stuff.
Regarding Io, as discussed earlier in the thread, creating synthetic B from G and V results in more accurate color than using V. The main reason is that V images are far more contrasty than the B images. To determine how to do this I used Voyager B images as a guide (this is described on my webpage here
). I'm convinced the resulting color is more accurate than what you get using RGV. However, there are some caveats:
* I haven't done a careful comparison of the Voyager and Galileo filters (this needs to be done for G and V).
* I haven't checked the 'real' color of these filters, for example whether the G filter is actually closer to yellow as seems to be the case with Cassini's G filter (see ugordan's messages above).
* I wasn't using calibrated Galileo images but simply 'tweaked' the brightness of the raw PDS images using publicly released color images as a guide. This isn't as bad as it sounds because calibrating Galileo images doesn't result in big changes, unlike for example Cassini (many of the Cassini images need to be scaled nonlinearly from 8 to 12 bits and dust rings and other blemishes are more obvious in the Cassini images). Now my software can calibrate Galileo images (it couldn't when I originally did this) so the accuarcy can be improved slightly.
I haven't tried sRGB yet but doubt it would be effective when you're using IR7560 instead of R. The problem is that the reddish areas brighten significantly with increasing wavelength. This can't be seen in G images but is obvious in R images. This is also the reason the Voyagers didn't 'see' this, the O images are more similar to G than R images in this respect (in other words, it's impossible to use O and G to create realistic, synthetic R). This also means that you cannot create true color images by using IR7560 instead of R. Synthetic R is needed. Using a linear combination of IR7560 and G yielded bad results in the examples I tried. In particular I wanted a 'true color' version of the well known C21 global mosaic. However, only IR7560 is available. So using G2 images (where both R and IR7560 are available) as a guide I came up with the following nonlinear formula:
R=(IR + abs(d)^s * sign(d) * 255 x w) * m
IR=The IR7560 intensity (0-255)
(255 because the intensity range is 0-255).
What's remarkable is that this resulted in an almost perfect R image. However, the values of the coefficients probably need to be modified for use with images of the Jupiter-facing hemisphere (the values here apply to the anti-Jupiter hemisphere).
A comparison using the C21 mosaic:Click to view attachment
The version at right is more realistic in that the the red areas appear more subdued. It can be improved further using synthetic blue and by adjusting the color balance a bit.
QUOTE (elakdawalla @ Mar 30 2007, 10:38 PM)
Cool. I just played with this and before your histogram fudging, the image does look quite a bit more like Bjorn's versions [of Io with synthetic B - BJ].
I haven't tried this (sRGB colorspace processing) yet (obviously I need to try it ASAP) so it would be interesting to see this image.
QUOTE (4th rock from the sun @ Apr 1 2007, 01:50 AM)
For now, I'll just leave you with a Voyager OGB image of the Great Red Spot processed with colorspace transformation (Ok, this is going very very off topic but at least is Jupiter related!). Click to view attachment
This looks great, the color is probably more realistic than in most of the Voyager color images I have seen of Jupiter. It would be interesting to see a global Voyager 1 or 2 image of Jupiter processed this way.