I sometimes receive emails from Solar Explorer users with suggestions for alternate planet and moon surface images to be used in the application. These images are higher resolution than the ones I have, but they aren’t suitable for creating the illusion of a solid 3D object in the app, without a great deal of manipulation. It’s difficult to explain why in an email, because it’s a complicated process, so I’ve decided to write an article which I hope will give people who aren’t 3D artists or software developers an idea of what’s involved, and the reasons why I am grateful for the interest that’s shown, but the images will never make it into the app.
To create the illusion of a solid 3D planet or moon, an image of the surface has to be provided in a format that is suitable for a 3D engine to draw onto a sphere. The way this is done is using a mercator projection, which is basically a distorted 2D image of the body that has been flattened out into a rectangle.
Most of the mercator maps that are used in Solar Explorer were partially created by NASA, who patched them together from many different photos taken by various space probes. One photo isn’t enough because the camera can only see a part of the planet or moon when it is close, and the only sections of the image that aren’t distorted by the curve of the planet’s surface are the parts that directly face the camera. For the best results, shots of the body would need to exist that show all sides in full sunlight, with the Sun almost directly behind the camera in every shot, to minimise shadows being cast by surface features which would ruin the 3D effect.
Unfortunately, these photos don’t exist in many cases, particularly for the moons of Uranus and Neptune, which have only been visited by Voyager 1 and 2. These moons are tidally locked, meaning that one face is always turned towards the planet, so when the Voyager probes passed by, they only saw the side of the moon that was facing towards them. The planet would have been in the way of viewing the other side.
Nonetheless, NASA have done the best that they could and created maps of the parts of the moons that are visible. I’ve taken these partial images, and filled in as much detail as possible by extrapolating what is visible, then coloured them and adjusted them to make sure that they wrap around the 3D sphere model without showing a seam where the edges join.
I’m eternally grateful for the work that NASA has done creating the raw maps, because creating them myself from hundreds of photos of dozens of bodies would have taken me months.
There you have it, a basic overview of what’s involved in creating a surface map for a planet or moon. There is a lot more to it, such as dealing with hardware limitations on images sizes and creating additional specular and normal maps for adding details to the surface, but that’s topic, for another day.