Draw prop models with a single atlas material

Each generated prop used one material per part (body, accent, metal...). The
model pipeline now packs a prop's palette into one power-of-two colour atlas plus a
matching metallic/roughness atlas and emits a single material:

- atlas_for(): 8x8 cells per palette role, colours in sRGB, roughness in the G and
  metallic in the B channel of the second atlas; NEAREST sampling keeps cells flat.
- build_glb(): every part merges into one primitive with constant per-vertex atlas
  UVs; the material uses baseColorTexture + metallicRoughnessTexture with factors 1.0.
- audit_glb(): fails unless a prop has exactly one material, both atlas textures,
  two images, and TEXCOORD_0 on every primitive.
- Removed the now-unused sRGB->linear factor helpers; colour travels in the texture.

Verification: headless 181 -> 182 (new "one material per prop scene" test), Python
tooling 53/53 (atlas determinism/size/UV/palette-override tests replace the old
factor tests), model audit OK for all six props, screenshots regenerated and
screen_designs.py compare still 6 good / 0 bad. Version 0.3.1.

tools/model_pipeline.py

@@ -475,99 +475,196 @@ def generate_concepts(only="", model=DEFAULT_CONCEPT_MODEL, size=1024,
 # glTF binary writer.
 # ---------------------------------------------------------------------------
 
+ATLAS_CELL = 8
+
+
+def _next_pow2(value: int) -> int:
+    size = 1
+    while size < value:
+        size *= 2
+    return size
+
+
+def _png_bytes(width: int, height: int, rgba: bytearray) -> bytes:
+    """Encode raw RGBA rows as a PNG without requiring Pillow."""
+    import zlib
+    rows = bytearray()
+    stride = width * 4
+    for y in range(height):
+        rows.append(0)  # filter type 0
+        rows.extend(rgba[y * stride:(y + 1) * stride])
+
+    def chunk(tag: bytes, payload: bytes) -> bytes:
+        return (struct.pack(">I", len(payload)) + tag + payload +
+                struct.pack(">I", zlib.crc32(tag + payload) & 0xFFFFFFFF))
+
+    header = struct.pack(">IIBBBBB", width, height, 8, 6, 0, 0, 0)
+    return (b"\x89PNG\r\n\x1a\n" + chunk(b"IHDR", header) +
+            chunk(b"IDAT", zlib.compress(bytes(rows), 9)) + chunk(b"IEND", b""))
+
+
+def atlas_for(prop: dict, palette: dict = None) -> dict:
+    """Pack every colour this prop uses into one tiny texture atlas.
+
+    Each part gets a constant UV pointing at its palette cell, so all parts can
+    share a single material instead of one material per part. The same layout
+    is written into a metallic/roughness atlas (glTF reads roughness from G and
+    metallic from B), keeping the per-role surface response.
+    """
+    palette = palette or PALETTE
+    slots = {}
+    for part in prop["parts"]:
+        role = part.get("role", "body")
+        colour = tuple(round(float(c), 5)
+                       for c in palette.get(role, PALETTE[role])[:3])
+        metallic, roughness = PBR.get(role, PBR["body"])
+        slots[role] = (colour, metallic, roughness)
+    roles = sorted(slots.keys())
+    grid = max(1, int(math.ceil(math.sqrt(len(roles)))))
+    size = _next_pow2(grid * ATLAS_CELL)
+    base = bytearray(size * size * 4)
+    metal_rough = bytearray(size * size * 4)
+    role_uv = {}
+    for index, role in enumerate(roles):
+        colour, metallic, roughness = slots[role]
+        col = index % grid
+        row = index // grid
+        x0 = col * ATLAS_CELL
+        y0 = row * ATLAS_CELL
+        rgb = bytes((int(round(colour[0] * 255)), int(round(colour[1] * 255)),
+                     int(round(colour[2] * 255)), 255))
+        mr = bytes((0, int(round(roughness * 255)), int(round(metallic * 255)), 255))
+        for y in range(ATLAS_CELL):
+            for x in range(ATLAS_CELL):
+                offset = ((y0 + y) * size + (x0 + x)) * 4
+                base[offset:offset + 4] = rgb
+                metal_rough[offset:offset + 4] = mr
+        role_uv[role] = ((x0 + ATLAS_CELL / 2.0) / size,
+                         (y0 + ATLAS_CELL / 2.0) / size)
+    return {
+        "size": size,
+        "roles": roles,
+        "role_uv": role_uv,
+        "colours": {role: slots[role][0] for role in roles},
+        "base_png": _png_bytes(size, size, base),
+        "metal_rough_png": _png_bytes(size, size, metal_rough),
+    }
+
+
 def _pad(data: bytes, fill: bytes) -> bytes:
     remainder = len(data) % 4
     return data if remainder == 0 else data + fill * (4 - remainder)
 
 
-def srgb_to_linear(channel: float) -> float:
-    """Convert one 0..1 sRGB channel to linear.
+def build_glb(prop: dict, palette: dict = None, atlas: dict = None) -> bytes:
+    """Serialise one prop as a self-contained .glb (glTF 2.0) file.
 
-    glTF stores `baseColorFactor` in linear space, while the catalogue palettes
-    (and the colours sampled from a concept PNG) are sRGB. Writing sRGB numbers
-    straight into the factor makes every prop render several shades too light,
-    which is why the first build's bins and newspaper boxes looked pastel.
+    All parts share one material backed by a colour atlas and a
+    metallic/roughness atlas, so the prop draws with a single material.
     """
-    if channel <= 0.04045:
-        return channel / 12.92
-    return ((channel + 0.055) / 1.055) ** 2.4
-
-
-def _linear_rgba(color) -> list:
-    return [round(srgb_to_linear(float(c)), 5) for c in color[:3]] + [1.0]
-
-
-def build_glb(prop: dict, palette: dict = None) -> bytes:
-    """Serialise one prop as a self-contained .glb (glTF 2.0) file."""
     palette = palette or PALETTE
+    atlas = atlas or atlas_for(prop, palette)
+
+    positions = []
+    normals = []
+    uvs = []
+    indices = []
+    for part in prop["parts"]:
+        mesh = build_part_mesh(part)
+        base = len(positions)
+        uv = atlas["role_uv"][part.get("role", "body")]
+        for position, normal in zip(mesh.positions, mesh.normals):
+            positions.append(position)
+            normals.append(normal)
+            uvs.append(uv)
+        indices.extend(index + base for index in mesh.indices)
 
     binary = bytearray()
     buffer_views = []
     accessors = []
-    materials = []
-    primitives = []
 
-    def add_view(payload: bytes, target: int) -> int:
-        buffer_views.append({
+    def add_view(payload: bytes, target=None) -> int:
+        view = {
             "buffer": 0,
             "byteOffset": len(binary),
             "byteLength": len(payload),
-            "target": target,
-        })
+        }
+        if target is not None:
+            view["target"] = target
+        buffer_views.append(view)
         binary.extend(_pad(payload, b"\x00"))
         return len(buffer_views) - 1
 
-    # One primitive per part: the parts share a mesh but need separate
-    # materials, and a glTF primitive is the unit of material assignment.
-    for part in prop["parts"]:
-        mesh = build_part_mesh(part)
-        positions = b"".join(struct.pack("<3f", *p) for p in mesh.positions)
-        normals = b"".join(struct.pack("<3f", *n) for n in mesh.normals)
-        indices = b"".join(struct.pack("<H", i) for i in mesh.indices)
-        pos_view = add_view(positions, 34962)
-        nrm_view = add_view(normals, 34962)
-        idx_view = add_view(indices, 34963)
+    pos_payload = b"".join(struct.pack("<3f", *p) for p in positions)
+    nrm_payload = b"".join(struct.pack("<3f", *n) for n in normals)
+    uv_payload = b"".join(struct.pack("<2f", *uv) for uv in uvs)
+    idx_payload = b"".join(struct.pack("<H", i) for i in indices)
 
-        xs = [p[0] for p in mesh.positions] or [0.0]
-        ys = [p[1] for p in mesh.positions] or [0.0]
-        zs = [p[2] for p in mesh.positions] or [0.0]
-        pos_accessor = len(accessors)
-        accessors.append({"bufferView": pos_view, "componentType": 5126,
-                          "count": len(mesh.positions), "type": "VEC3",
-                          "min": [min(xs), min(ys), min(zs)],
-                          "max": [max(xs), max(ys), max(zs)]})
-        nrm_accessor = len(accessors)
-        accessors.append({"bufferView": nrm_view, "componentType": 5126,
-                          "count": len(mesh.normals), "type": "VEC3"})
-        idx_accessor = len(accessors)
-        accessors.append({"bufferView": idx_view, "componentType": 5123,
-                          "count": len(mesh.indices), "type": "SCALAR"})
+    pos_view = add_view(pos_payload, 34962)
+    nrm_view = add_view(nrm_payload, 34962)
+    uv_view = add_view(uv_payload, 34962)
+    idx_view = add_view(idx_payload, 34963)
 
-        role = part.get("role", "body")
-        color = palette.get(role, PALETTE[role])
-        metallic, roughness = PBR.get(role, PBR["body"])
-        materials.append({
-            "name": role,
-            "doubleSided": False,
-            "pbrMetallicRoughness": {
-                "baseColorFactor": _linear_rgba(color),
-                "metallicFactor": metallic,
-                "roughnessFactor": roughness,
-            },
-        })
-        primitives.append({
-            "attributes": {"POSITION": pos_accessor, "NORMAL": nrm_accessor},
-            "indices": idx_accessor,
-            "material": len(materials) - 1,
-        })
+    xs = [p[0] for p in positions] or [0.0]
+    ys = [p[1] for p in positions] or [0.0]
+    zs = [p[2] for p in positions] or [0.0]
+    pos_accessor = len(accessors)
+    accessors.append({"bufferView": pos_view, "componentType": 5126,
+                      "count": len(positions), "type": "VEC3",
+                      "min": [min(xs), min(ys), min(zs)],
+                      "max": [max(xs), max(ys), max(zs)]})
+    nrm_accessor = len(accessors)
+    accessors.append({"bufferView": nrm_view, "componentType": 5126,
+                      "count": len(normals), "type": "VEC3"})
+    uv_accessor = len(accessors)
+    accessors.append({"bufferView": uv_view, "componentType": 5126,
+                      "count": len(uvs), "type": "VEC2"})
+    idx_accessor = len(accessors)
+    accessors.append({"bufferView": idx_view, "componentType": 5123,
+                      "count": len(indices), "type": "SCALAR"})
+
+    base_view = add_view(atlas["base_png"])
+    mr_view = add_view(atlas["metal_rough_png"])
 
     gltf = {
         "asset": {"version": "2.0", "generator": "step-line model_pipeline"},
         "scene": 0,
         "scenes": [{"name": prop["name"], "nodes": [0]}],
         "nodes": [{"name": prop["name"], "mesh": 0}],
-        "meshes": [{"name": prop["name"], "primitives": primitives}],
-        "materials": materials,
+        "meshes": [{
+            "name": prop["name"],
+            "primitives": [{
+                "attributes": {"POSITION": pos_accessor,
+                               "NORMAL": nrm_accessor,
+                               "TEXCOORD_0": uv_accessor},
+                "indices": idx_accessor,
+                "material": 0,
+            }],
+        }],
+        "materials": [{
+            "name": "prop_palette",
+            "doubleSided": False,
+            "pbrMetallicRoughness": {
+                "baseColorTexture": {"index": 0},
+                "metallicRoughnessTexture": {"index": 1},
+                "metallicFactor": 1.0,
+                "roughnessFactor": 1.0,
+            },
+        }],
+        "textures": [
+            {"sampler": 0, "source": 0},
+            {"sampler": 0, "source": 1},
+        ],
+        "samplers": [{
+            "magFilter": 9728,   # NEAREST, so a palette cell stays a flat colour
+            "minFilter": 9728,
+            "wrapS": 33071,      # CLAMP_TO_EDGE
+            "wrapT": 33071,
+        }],
+        "images": [
+            {"bufferView": base_view, "mimeType": "image/png"},
+            {"bufferView": mr_view, "mimeType": "image/png"},
+        ],
         "accessors": accessors,
         "bufferViews": buffer_views,
         "buffers": [{"byteLength": len(binary)}],
@@ -646,8 +743,25 @@ def audit_glb(path: Path) -> dict:
                 issues.append("primitive references a missing accessor")
     if triangles <= 0:
         issues.append("no triangles")
-    if not gltf.get("materials"):
+    materials = gltf.get("materials", [])
+    if not materials:
         issues.append("no materials")
+    elif len(materials) != 1:
+        issues.append(f"expected 1 shared material, found {len(materials)}")
+    else:
+        pbr = materials[0].get("pbrMetallicRoughness", {})
+        if "baseColorTexture" not in pbr:
+            issues.append("shared material has no baseColorTexture")
+        if "metallicRoughnessTexture" not in pbr:
+            issues.append("shared material has no metallicRoughnessTexture")
+    if len(gltf.get("textures", [])) < 2:
+        issues.append("expected colour and metallic/roughness textures")
+    if len(gltf.get("images", [])) < 2:
+        issues.append("expected colour and metallic/roughness atlas images")
+    for mesh in gltf.get("meshes", []):
+        for primitive in mesh.get("primitives", []):
+            if "TEXCOORD_0" not in primitive.get("attributes", {}):
+                issues.append("primitive has no atlas UVs (TEXCOORD_0)")
     for buffer in gltf.get("buffers", []):
         if int(buffer.get("byteLength", 0)) > len(data):
             issues.append("buffer larger than the file")