What changed
- More black, when the genes say so. Bekko spots used to be one size range for every fish. Now the koi's existing coverage gene scales them, so a Bekko carries between its old amount of sumi and twice that. No new random draws are involved, so every spot stays where it was and only grows. The visible part of each spot is what doubles, not its radius: the soft edge fades out a fixed rim on every spot, and scaling the full radius gave small spots up to 2.4 times the black. Measured over 20 Bekko, the most-covered version of each fish has 1.73 to 2.02 times the black of the least.
- Stripes in the fins, sometimes. A new gene, `finStripes`, gives four in ten Bekko black stripes along the fin rays: thin, uneven in strength, with some rays left clear. The tail fin always sampled the last column of the pattern, so the stripes are drawn there, across its width. On a striped koi the pectoral fins sample that column too, from leading edge to trailing edge, so their stripes run from root to tip. The gene is rolled after every other gene, so older seeds keep all of theirs, and it does nothing on other varieties. The Koi Designer has a toggle for it.
- Fins that don't kink. The first try had the stripes zig-zagging down the tail. The tail and pectoral fins are strips of trapezoids, and the GPU interpolates texture coordinates linearly across each triangle, so anything that changes across a fin bends at every triangle's diagonal. Nothing changed across a fin before, so it never showed. The fin vertices now carry projective UVs: each vertex's UV is multiplied by a weight in proportion to the width of its edge, and the shader divides it back out per pixel. Lines across a fin stay straight and fan out with the rays. The body keeps a weight of 1, so it maps exactly as before. On other koi the only visible effect is the flank shading on the tail, which now lands slightly differently: at most 8/255 on most fish, 13/255 on a few Asagi pixels.
Why
A r/Koi judge said Bekko can carry up to double the sumi ours had, and that black stripes in the fins are fine on a Bekko. Both make the variety less samey: some Bekko are clean with a few stepping-stone spots, and some are heavy with striped fins.
