On pixiv, hijiri links to a PSD file of this picture. I saved the PSD as a PNG (used PNGOUT to compress it).
Interesting things about the PSD file: 1. There is a layer of the original line sketch. 2. The PSD file has a layer for the reflections on the eyes and tongue. For some reason this was turned to invisible and I had to make it visible again in Photoshop. 3. In the PSD, the background isn't white, but transparent. I tried changing it to a different color (say, neon blue) and noticed that parts of Kaguya's and Tianzi's teeth also turned neon blue. I guess hijiri forgot to fill in those parts.
