What changed
Motoguro. Every Showa now has black at the base of its pectoral fins: a semicircle on the fin where it meets the body. It reaches 45% to 70% of the way down the fin, more on a koi with more sumi.
Fins carry their own coordinates. The pectoral fins are 4-vertex quads that sample one point of the pattern texture, so a shape on them can't come from the texture. The mesh now carries two more channels on the fin vertices: where a point is in the fin (UV1: along, from root to tip, and across, from leading to trailing edge) and the fin-base colour and reach (UV2). A small shared shader include, `KoiFin.hlsl`, draws the semicircle from them in both the lit and unlit shaders. It's all in the mesh, so the game, the capture tools and the Koi Designer pick it up with no material settings. Asagi's red motoaka can use the same channels.
Fins rooted at the body edge. Three things kept the black hidden at first, each fixed in turn:
The fin root sat 15% of the half-width inside the body, with the trailing half running almost along it, so most of the joint was under the body. Fins are now rooted at 96%, just inside the edge. This moves every koi's pectoral fins out by about 3% of the body width.
The along-fin coordinate was projective like the across one, which squeezed it toward the root. It's now plain, so a reach measures true distance down the fin. Across stays projective so lines along the rays stay straight.
The colour went into the mesh as sRGB, while the shader works in linear space, so black showed as mid-grey. It's now converted to linear, the way the pattern texture is.
The art-pass drawing templates follow the new fin root.
Why
A r/Koi judge: Showa should have motoguro, black where the pectoral fins meet the body, the same semicircle as the Asagi's red but in black. It's one of the first things that tells a Showa from a Sanke at a glance.
