Method used to project MNIST images on the sphere. The image is aligned so that the center of the pixel in the middle (marked with a ×) is on the Equatorial plane. A number of rays are fired, originating from the center of the sphere, targeting each of the pixels in the image, and the intersection with each pixel on the sphere is computed. In order to cover all the pixels on the sphere, the angular separation among rays is 1/2 of the angular resolution of the HEALPix map.