Chris wrote:I still maintain that cube maps could be useful in Celestia . . .
Using cyber_space_doc's most recent code above, I did some further investigations of hires quality of cubemaps.
First I used a hires 8k x 4k lossless TIFF image of the BlueMarble Earth
http://earthobservatory.nasa.gov/Newsro ... _2002.html
and converted it into PPM.
Next I wrote a small script to convert earth8k.png into the 6 required 2k x 2k cubemap patches with correct orientations as required by the code. The 6 cubemap reprojections were performed with mmps(project) by Matthew Arcus
as usual. The format conversions and rotations were done with ImageMagick's convert/mogrify utilities.
Code: Select all
convert ../../earth8k.png ../../earth8k.ppm
project gnomonic -scale 2 -lat 0 -long 90 -w 2048 -h 2048 -grid -gridcolor 255:0:0 -f ../../earth8k.ppm > real_earth03.ppm
project gnomonic -scale 2 -lat 0 -long 270 -w 2048 -h 2048 -grid -gridcolor 255:0:0 -f ../../earth8k.ppm >real_earth02.ppm
project gnomonic -scale 2 -lat 90 -long 0 -w 2048 -h 2048 -grid -gridcolor 255:0:0 -f ../../earth8k.ppm >real_earth04.ppm
project gnomonic -scale 2 -lat -90 -long 0 -w 2048 -h 2048 -grid -gridcolor 255:0:0 -f ../../earth8k.ppm >real_earth05.ppm
project gnomonic -scale 2 -lat 0 -long 0 -w 2048 -h 2048 -grid -gridcolor 255:0:0 -f ../../earth8k.ppm >real_earth00.ppm
project gnomonic -scale 2 -lat 0 -long 180 -w 2048 -h 2048 -grid -gridcolor 255:0:0 -f ../../earth8k.ppm >real_earth01.ppm
mogrify -rotate 180 -format bmp real_earth00.ppm
mogrify -format bmp real_earth01.ppm
mogrify -rotate 90 -format bmp real_earth02.ppm
mogrify -rotate -90 -format bmp real_earth03.ppm
mogrify -format bmp real_earth04.ppm
mogrify -rotate 180 -format bmp real_earth05.ppm
Now, let's do a careful comparison:
Seen from a distance the resulting cubemap rendering from cyber_space_doc's code looks quite nice:
Note the purple square! It indicates one of the critical regions that I was pointing out all along. The point, where 3 cubemap faces are meeting!
Let's zoom into the purple square:
In the following, magnified image of cyber_space_doc's cubemap rendering of my 8k hires texture you can see the three lines with a discontinuouls rendering where three cubemap faces are meeting! It's where the red grid-line ends, towards the center of the image. What is most disturbing is that the surface patterns change direction abruptly across the discontinuity lines!
No doubt, this is NOT nice and illustrates the kind of cubemap discontinuities I expected to appear all along. Definitely a serious loss of image quality/detail along extended regions of the texture rendering!
For comparison, here is the analogous rendering of the original 8k simple cylindrical texture via Celestia in the same region. Everything perfect, of course.
In summary: I conclude that at the hires level, the price for getting rid of the two polar pinches with cubemaps is HIGH.