Cubemap from panoramic horizontally wrappable image

Question

I'm trying to write an algorithm to generate the "ceiling panel" from a horiontally wrappable panoramic image like the one above. Images 1 to 4 are a straight cut out for the walls of the cube but the ceiling will be more complicated as I assume it needs to be composited from parts 5a to 5d. Does anyone know the solution in pseudocode?

my guess is that we need to iterate over the coordinates of the ceiling tile i.e.

for y=0 to height
for x=0 to width
     colorofsomecoordinateonoriginalimage = some function (poloar coords?)
     set pixel(x,y) = colorofsomecoordinateonoriginalimage
next
next

Answer 1

You can't generate a panorama just by taking photos from a single location and stitch them. Well, you can for a single horizontal set, but it would look ugly (usually, you stitch many more than 4 photos to avoid distortions at the edges). Here, you have even more data in the y-direction, which means even more pictures, and some sort fancy projection to generate the final image.

If you look at the panorama you have closely, you'll notice that the boundary of the region in sunlight is not straight. That is because your panorama was projected on a cylinder, not a cube. So I don't think 1/2/3/4 would look right directly mapped to a cube.

Bottom line, you really can't consider those 8 chunks as 8 pictures taken from a fixed point (If you need convincing, try yourself to take 8 pictures like that and try to stitch them together. You'll see how fun it is for the upper row, and even though it is easy for the bottom row, how ugly it looks on the stitched regions).

Now, why you need cube maps changes drastically what your options are. If you're only looking for a cube map to do cheap environment mapping effects, then the simplest is to find an arbitrary function that maps the edges where you want them to be, and simply linearly interpolate in between. It's completely the wrong projection, but ought to give a picture that looks good enough for the intended goal.

If you're looking for something more accurate, then you need to know how the projection was generated, so that you can unproject it before re-projecting it on the cube.

All that said, it's also a lot easier to just photograph cube maps rather than process a panorama to generate them, but that might not be possible for you.

Answer 2

Hum... I remember doing something like that for computer vision class one time back in grad school. It's not impossible but a LOT of work needs to be done. One way would be to degrade the entire product's quality. That's the easiest starting point. Once you degraded it enough (depending on how much you need to stretch the edges), you can start applying nonlinear transformations to the image. This is probably best done approximating by maybe cutting out sections of the cylinder by degrees and then applying one of the age old projections used in making flat maps (like Mercator or CADRG or something)... but you have to remember to interpolate the pixels, make sure you at least do an averaging of the pixels to approximate. That's the best I can think of.