[X] My Assistant

[CosmicRocker]

I'm still trying to figure out a number of things about the new images we are trying to work with. Assuming others are likewise trying to learn, I thought I would open this thread to create a place for such discussions.

I'd like to start out with a comment about raw image contrast. There have been several postings in the main threads about whether or not the MSL raw images have been stretched like those from the MER missions. I am certainly no expert on this, but it looks to me as if the MSL images have not been stretched at all. I haven't tried to analyze all of the image types, but the hazcams and navcams have pixel brightness histograms that are very different from their MER counterparts.

This attached image compares MER and MSL navcams along with their luminosity histograms.


The MSL images clearly are not using the entire, available range of brightness values, whereas the MER raws do. For this reason, the MSL raw images can usually be nicely enhanced by simply stretching the distribution of brightness across the full 256 value range.

[Holger Isenberg]

Hi forum community,

I've heard on the last MSL press conference, in about 1 to 2 weeks, MSSS will publish some details about the color calibration. Until then, we have the following information on the topic so far:

Mastcam Multispectral Imaging on the MSL Rover:
Wavelength Coverage and Imaging Strategies at the Gale Crater Field Site.
43rd Lunar and Planetary Science Conference (2012)
http://www.lpi.usra.edu/meetings/lpsc2012/pdf/2541.pdf

Mast Camera (Mastcam) (no date)
http://msl-scicorner.jpl.nasa.gov/Instruments/Mastcam/

CCD sensor used on all MSL cameras:
http://www.kodak.com/global/plugins/acroba...020LongSpec.pdf

Test images taken on Earth daylight with flight Mastcam before delivery to JPL:
http://www.msss.com/science/msl-mastcam-pr...unch-images.php

The two documents about the Mastcam show very different IR cutoff filters, especially concerning the cutoff off the blue wavelengths. Normally you would expect an IR cutoff filter only removing IR and not blue and UV. Removing UV is of course also useful if you have an sensor responding to those short wavelength. The sensor used for MSL however, is not responsive in UV as the bayer pattern already blocks everying below 400nm.

Now the question is: Which is the actual transmissin curve of the IR cutoff filter? The one (black dotted line) in the 2541.pdf or the one (thick dark green line) on the JPL-Mastcam web page?

Then there is the MAHLI. This camera normally used for near distant macro imaging on the "hand" of the robotic arm, it is also capable of focusing to infinity and has a nice wide angle lens. The test image taken on Earth shown in this report [1] makes hopes for a really nice overview color image from Mars. MAHLI has no additional filter except for the bayer pattern and an IR cutoff coating on the saphire camera windows creating a bandpass with 395–670 nm transmission. So this is a true color camera with excact the same characteristics as we know from consumer digitcal cameras on Earth. This is the best camera on MSL for a color picture to evaluate the color temperature of the Martian daylight.

[1]
(Curiosity’s Mars Hand Lens Imager (MAHLI) Investigation, 21 February 2012)
http://www-robotics.jpl.nasa.gov/publicati...12published.pdf

[mcaplinger]

Now the question is: Which is the actual transmissin curve of the IR cutoff filter? The one (black dotted line) in the 2541.pdf or the one (thick dark green line) on the JPL-Mastcam web page?

The plot in 2541.pdf is not a transmission curve, it's a response curve and thus modulated by the detector QE. The JPL web page looks pretty close.

[Holger Isenberg]

The plot in 2541.pdf is not a transmission curve, it's a response curve and thus modulated by the detector QE. The JPL web page looks pretty close.

Thanks for pointing that out! So the overall effect is like adding a strong UV filter to a normal digital camera, like a LP 420, which is actually helping to reduce visual haze. However, what explains then the decrease of the blue response function maxima at 470nm to almost 30% with the green response being the 100% reference on the black dotted line in 2541.pdf?

[mcaplinger]

However, what explains then the decrease of the blue response function maxima at 470nm to almost 30% with the green response being the 100% reference on the black dotted line in 2541.pdf?

I'm not sure what the dashed line is really supposed to mean. The IR-cutoff response is shown in blue, green, and red, but all the filters have been normalized to their peaks, so you can't, for example, compare the R2/L2 narrowband response to the IR-cutoff blue response, or the IR-cutoff blue to the red. This plot was just intended to give an outline of where the bandpasses are.

[ugordan]

This is just for fun, I tried to implement an adaptive correction for the JPEGged (ugh!), raw Bayered images to get rid of artifacts in image areas that are smooth in appearance. The artifacts come from the JPEG algorithm trashing the Bayer pattern, introducing this kind of pattern:



After correcting for that, the smooth areas became more smooth as illustrated by this comparison, although obviously this approach can't ever come close to an image already returned from the spacecraft in de-Bayered, color form:



I had to make the algorithm adaptive in picking which DCT blocks it will apply this to, because if I apply that correction invariably across the image, some uniform-color areas which originally already looked good had these artifacts introduced afterwards...

[Airbag]

Here is a poor man's way of de-Bayering Mastcam images using the Gimp (or similar tools) for those wanting to experiment a little - obviously not intended for the experts here! I realize it is not as sophisticated as proper implementations, but in the absence of a Gimp plugin, this way has the advantage of simplicity at the expense of ending up with a half-resolution image. It does not for instance use just the green pixels for luminosity, and it performs the chroma filtering by the simple expedient of scaling the image down by a factor of 2 (thus merging each set of red, blue and two green pixels together).

1. Open the Mastcam Bayer pattern (1536x1152) image in the Gimp
2. Change the mode to "RGB"
3. Drag the attached Bayer pattern color map onto it (forming a new layer)
4. Change the mode of that new layer to "multiply"
5. Flatten image
6. Scale image to 50% of original size
7. Colors->Auto->Normalize (or use Levels or Curves tool) to brighten the image.

Airbag

Bayer pattern color map image:


Sample result: