Hi all,
after lots of work (and frustration) with the two recently available hires surface maps of the Moon, I felt it may be useful to write up a résumé of my detailed comparisons of these two scientific imaging data sets.
The title of this threat anticipates my personal conclusions...
The maps to be compared are:
For specific comparisons, I use here just 16k versions. These exhibit already plenty of detail, while this image size can still be handled without big delays in GIMP, which has considerable advantages.
For Celestia(.Sci) applications, a crucial usability criterion is a good alignment of the surface map in question with the official
LRO-LOLA normalmap VT set (32k)http://www.celestiamotherlode.net/catal ... on_id=1549 Chang'e 2 map:
PROs ================
- The Sun is at a high angle in the Chinese imaging, so shadows are essentially missing. This makes the map perfectly suited for a dynamical shadow rendering via a normalmap!
- The vertical stripes throughout the dark central mare areas are considerably weaker than in case of the LRO-WAC map (see image below). De-striping (ISIS3) can be successful without too much deterioration of map quality.
Chang'e 2 map:
CONs ================
- The biggest drawback are the disturbing alignment problems with the
LRO-LOLA normal map! There are several issues that I have carefully analyzed:
The aspect ratio of the original Chinese map is NOT 2:1. After close examination I was able to locate the underlying reason. It demonstrates that the assembly quality of the Chang'e 2 images into a simple cylindrical map leaves much to be desired:
After scaling the original 218527 x 109166 (.jp2) map down to a standard 16k texture height of 8192 pixel with the original aspect ratio of 218527 / 109166, the corresponding width becomes 16399 pixel, i.e 15 pixel larger than the canonical 16384 pixel, corresponding to a proper width : height ratio of 2:1.
I then was able to spot the place where these extra 15 pixel went:
In this image I zoomed-in on part of the vertical line, along which the east and west ends of the simple cylindrical map are joined in 3d rendering. While the joining is much better hidden than in case of the LRO-WAC map, you see clearly a redoubling of features along a vertical line in the center of the original 16399 x 8192 map on the left. This is exemplified by the two orange arrows pointing to TWO tiny craters instead of only one.
Their distance is precisely 15 pixels!
AHA...Bingo.
The zoomed image on the right then shows how the image looks after cropping this excess strip of 15 pixels from the east end of the simple cylindrical map. The corresponding texture now has precisely 16384 x 8192 pixels. After adding some optimal global offset to readjust the global alignment optimally, there remained still various local areas with an intolerable misalignment w.r.to the LRO-LOLA normal map.
Sigh....
LRO-WAC map:
PROs =================
- The LRO-WAC surface map matches perfectly with the LRO-LOLA normalmap
(no surprise 
)
LRO-WAC map:
CONs =================
- The relatively low position of the sun in the LRO-WAC map leads to dark fixed shadows in many craters, and notably to unpleasant artefacts, when combined with dynamical shadow rendering through the LRO-LOLA normalmap!
Here is an explicit illustration:
First, with the normalmap deactivated, let us look at the (disturbing) line where the two ends of the simple cylindrical LRO-WAC surface map were joined in Celestia's 3D rendering:
You clearly note that the fixed illumination of the crater rims is in opposite directions left and right of that joining line! To the right of the line, the crater illumination is towards the left, while on the left of the line the crater illumination is towards the right. Without a normalmap this frequently leads to visual effects like "inverted craters" looking like mountains...
Now, with the LRO-LOLA normalmap activated, things get even worse, since we note a really disturbing artefact: A sudden drop in brightness of a whole surface strip to the right of the joining line! The normalmap now has placed all shadows in the same direction, which implies that it placed shadows over the previously illuminated areas on the right. Hence the dark overall impression.
- The vertical stripes throughout the dark mare areas are considerably more disturbing and more resistant to de-striping by a combination of lowpass and highpass filters (ISIS3), than those of the Chinese'e 2 map. The reason is that the stripes have irregular widths and sharp boundaries. They can certainly be eliminated but hardly without affecting the crisp oveall imaging quality of the original.
[click on image by all means!]
Personal Verdict:
=============
After much frustrating previous work with artefacts and misalignment of the early
Celementine surface maps, things have certainly improved with these two recent maps of our Moon. BUT, altogether, BOTH maps still leave a lot to be desired, at least as concerns their application in Celestia(.Sci).
No doubt, a number of the emphasized map deficiencies may be eliminated with an excessive amount of image manipulation work. John van Vliet's 64k VT maps represent such an example.
But unfortunately the crisp appearance of the original maps is lost by the required large number of correcting actions... In my view, Moon maps exceeding 16k - 32k are barely worthwhile at this time for rendering in Celestia(.Sci).
Fridger
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