I hope this hasn't previously been explained, because I haven't found the answer. This is something I have wondered about for a long time, and I am sure someone here can explain it. Most of the JPGs we all download from JPL are grayscale images, yet they are three times the size they need to be, because they are in RGB format. The additional channels do not provide any more information. It's simply annoying to people like myself who are forced to use dialup, and possibly more expensive for everyone than it needs to be.

The only explanations I have been able to come up with are that, it is just an oversight of the webmasters, NASA/JPL gets its bandwidth for free and doesn't care about the rest of us, or... (someone fill in the blank).

What's the deal, here?

I dont know why they do it - Cassini images are Greyscale..

BUT

You're not having to download 3x as much data. If it were BMP's or PNG's then perhaps you might - but JPG's are a little cleverer than that - instead of 200-300kb, they'd be about 150-200kb instead, not <100k. Perhaps 1/3rd less ( I tried a few in photoshop )

Doug

djellison is correct. Color JPEGs typically work in YCbCr color space, and typically use a sampling factor of 2 or 4 in the Cb and Cr channels (JPEGs can use other color spaces and sampling factors, but YCbCr 122 is the most common format for color images, followed by YCbCr 144).

That means even an untransformed color JPEG only take up 50% more space than a grayscale JPEG of the same width and height (1x1[Y]+0.5x0.5[Cb]+0.5x0.5[Cr]), assuming a CbCr sampling factor of 2.

Then you go on and transform the image into frequency space, and you usually find that even after the previous downsampling, the Cb and Cr channels contain less noise than the Y channel, making it possible to compress them harder, so that the actual size increase when going from grayscale (Y only) to color (Y+Cb+Cr) can be even less than 50%.

This, of course, is the very idea of the JPEG image format, and the reason why it's so popular.

Thanks for the replies, folks. I'm glad to learn that the excess baggage isn't as big a deal as I initially thought. That being the case, I suppose it is possible that it's merely convenient for the folks at JPL to generate RGB JPEGs of grayscale images, and they are not worried that the files are 50% larger than they need to be. But it also seems that it would not be difficult to write an automated routine that optimizes the file size to reduce their bandwidth costs. Maybe the total cost for the MER images is not that large now that the initial public excitement over the rover missions has died down.

I wonder though, they surely are not downlinking the original images from Mars with excess bits, as I have got to imagine that is bandwidth they are very frugal with. The conversion to JPEG must take place shortly before the images are posted to the web sites.