nx_texpaint_lib.nx

nx_texpaint_lib.nx -- TEXTURE PAINTING: a brush stroke placed on the SURFACE that lands in the correct texel of the baked PBR map set, and that does not TEAR where the UV parameterization is cut. /compare/dcc DC7, and the board says why it matters: texture-painting is the ONE axis on that board where all four rivals are Best -- Blender paints directly onto the mesh, ZBrush PolyPaint needs no UVs assigned in advance, Maya has the 3D Paint Tool, and ArmorPaint exists for precisely this. CHECK-BEFORE-BUILD, MEASURED NOT ASSUMED. nx_capsearch over 6892 organs (1375 tools + 5517 libs, corpus_complete=1) returned a glyph STROKE PEN, four 2D FRAMEBUFFER painters and one mesh SCULPT brush -- nothing that writes a texel of a texture. nx_absent then returned tp_stroke ABSENT-PROVEN at coverage_complete=1 corpus_complete=1 over buildroot/runtime. The estate BAKES maps (ntb_bake) and GENERATES a whole PBR set per genome (ntb_bake_pbr); it has never PAINTED one. THE ATLAS CONTRACT IS READ OUT OF THE INCUMBENTS, NEVER INVENTED. Two files own it and both were read: nx_nxa_texc_lib.nx -- the TEXC section. Payload words [nv, stride=3, grid g, reserved] then per vertex [u q16 0..65535][v q16 0..65535][owning joint]. Joint j owns ONE TILE of a g x g grid, tile width tw = 65536/g in UV units, so charts from different bones are DISJOINT BY CONSTRUCTION. The unwrap is CYLINDRICAL around the bone: u is the angle (a full circle mapped onto the tile) and v is the clamped position along the parent->joint segment. nx_nxa_texbake_lib.nx -- the atlas itself: res*res*3 u8 RGB row-major, the TM_IMG contract, and a map set of albedo, specular, gloss and tangent-space normal. EVERYTHING BELOW FOLLOWS FROM THOSE TWO FACTS. In particular the map COUNT is a parameter of the map set rather than a constant in this file: a second copy of NTB_NMAP here would be a duplicate ruler that drifts the day the PBR set grows a fifth map. WHERE THE SEAM IS, AND WHY THE NAIVE PAINTER TEARS. Because the unwrap is cylindrical, EVERY chart carries its own wrap seam: the texel column at the left edge of a tile and the column at the right edge are the SAME PLACE ON THE BODY, cut apart so the cylinder could be laid flat. A brush disc centred near that cut covers surface on both sides of it, but in the atlas the far side is a whole tile-width away. A painter that clips the disc to the tile rectangle -- which is what a painter must do, or paint leaks onto another bone -- writes one side and drops the other, and the asset renders with a hard-edged half-stroke: THE TEAR. This is the defect that makes naive 3D painting unusable and it is the reason DC7 exists. THE FIX IS A METRIC, NOT A HEURISTIC. tp_stroke_core paints with the PLANAR metric and therefore tears -- it is shipped, callable and named, because a negative control you can still run is worth more than a paragraph saying you once deleted the fix and watched a test fail. tp_seam_bleed then writes exactly the texels the planar pass could not reach: those whose CYLINDRICAL distance to the centre is inside the brush while their planar distance is not. The two sets are disjoint by that definition, so no texel is ever written twice, and their union is the full disc. tp_stroke is the composition of the two. u WRAPS AND v DOES NOT, and that asymmetry is the geometry rather than a preference: u is an angle around