[X] My Assistant

[Nirgal]

Hi all,

motivated by the impressive raw imagery discovered by n1ckdrake and others in the PDS archives I'd like to do more work with the CTX
images ... however, when I'm download und unpack the images from the PDS archives (via img2png) and looking at the histogram then it seems that the greylevels of each image only occupy a rather limited range of about 60 DN , so could it be that the dynamic range of
the CTX camera is in fact only 6 bit, or is part of the original range lost in the course of the processing for the IMG files ? This seems strange considering the very high spatial resolution of the CTX.

Especially when looking at some of the
CTX JPG images published by MSS themselves
which all look like a considerably higher dynamic range than the streched IMG raw counterparts from PDS

Unfortunately I couldn't find much specification about the CTX image processing chain ob the web (www.msss.com)
So it is my hope that some of the experts in this forum experienced with PDS raw data could shed some light on this.

Thanks for any answers in advance.
Bernhard

[cartrite]

ISIS3 is available to process these images. You only need a few steps. mroctx2isis, spiceinit, ctxcal, ctxevenodd. The first step, mroctx2isis converts the 8 bit to 12 bit data. I did many images and did a couple mosaics. I have one scaled down mosaic at my ISIS3 CTX web page along with some full resolution ( 6 meters per pixel ) 2kx2k tiles.
I have used a pds plugin for the gimp and opened some of the raw img ctx files and it is like night and day. You really need to process them with ISIS3 or an equivalent software package. I would post a couple full resolution tiles here but I'm not sure how to go about it on this forum.
cartrite

[Nirgal]

Thanks cartrite, for the link to the ISIS software ... now this alone would be another good reason to switch to LINUX

Am I understanding you correctly that you too are starting from the standard .IMG files from PDS ?

Then I still don't understand how you arrive at the higher dynamic range of the final images because in all the IMG files I have
examined closer the histogram of all samples consists of considerably less than 256 distinct integer values (often around 60 or so , thus suggesting some kind of 6 bit bottleneck?).
So with 60 distinct integer values to start from, no matter which kind of reverse lookup function is applied, the output will still
have only 60 DN ...
A possible explanation would be the use of some kind of neighboring pixel avaraging scheme involved somwhere in the
processing pipeline which would increase the number of greylevels but also introduce some blurring ...

Nirgal

BTW.: many of those CTX images are really breathtaking ...
the perfect complement to the magnificent HiRISE detail views

[mcaplinger]

Then I still don't understand how you arrive at the higher dynamic range of the final images...
A possible explanation would be the use of some kind of neighboring pixel avaraging scheme involved somwhere in the
processing pipeline which would increase the number of greylevels but also introduce some blurring ...

CTX has a 12-bit ADC whose range is tuned to the fullwell of the sensor. Mars is low-contrast so the full 12-bit dynamic range is usually not occupied. I just looked at a contrasty image of the south polar region with frost and dark dunes, and it has 8-bit DNs from near 0 to 200, but that's fairly unusual.

One usually does the pixel response and flat fielding in 12-bit space, which results in some filling between the discrete steps of the 12-bit values. This is not averaging, though, but per-pixel scaling, and should not produce blurring.