Unity DDS Import Quirks (AC kn5 textures)

Assetto Corsa kn5 files embed DDS textures in layouts Unity's asset importer refuses. Every rejected file logs an importer LogError, and AssettoCar's headless build (AssettoHeadlessBuild.Run) fails on ANY error — so one quirky texture used to kill the whole conversion. All repairs live in AssettoCar/src-tauri/resources/unity-template/Assets/Editor/AssettoCarBuilder.cs (FixDdsForUnity + DecompressBcDds).

What Unity's DDS importer accepts: DXT1/3/5 (BC1/2/3) and BGRA-masked uncompressed, with mip chains either absent or complete down to 1×1.

AC cars routinely ship, and the builder now repairs:

AC layout	Repair
RGBA-masked uncompressed (R/B swapped)	swap R/B bytes, patch masks
L8 / A8L8 luminance	expand to A8R8G8B8
Truncated mip chains	drop to top level, clear mip flags
DXT1/3/5 with non-multiple-of-4 width/height (gauge faces, needles, badge plaques — e.g. 380×379, 14×35)	CPU-decode BC blocks → uncompressed A8R8G8B8 at the ORIGINAL size (added 2026-07-15; 11 such textures in the McLaren 765LT killed the build with 22 errors)

Gotchas:

• Do NOT "fix" odd-sized BC textures by relabeling the header to the padded block grid (e.g. 14×35 → 16×36): the physical data does cover full blocks, but UVs span the padded size afterwards — a 14-px-wide needle stretches ~13%. Decode instead.
• DXT1 with color0 <= color1 is 3-color + punch-through alpha mode; DXT3/5 color blocks are always 4-color regardless.
• Unknown FourCCs (BC4/BC5/BC7/DX10 headers) are still skipped with a warning — candidates for the same decode treatment if a car ever needs them.
• The failing-build tell in unity-work/Logs/headless.log: Invalid dimensions: ... must be a multiple of the formats (DXT5|BC3) block dimensions (4x4) followed by Unsupported Assets/Job/Textures/<x>.dds file.

DDS goes through IHVImageFormatImporter, not TextureImporter

.dds files are imported by IHVImageFormatImporter (2026-07-15, confirmed by the generated .meta files). Consequences:

• AssetPostprocessor.OnPreprocessTexture NEVER fires for them — the normal_maps.txt preprocessor in AssettoTexturePreprocessor.cs only affects PNG/JPG textures. DDS normal maps import as plain sRGB textures (Standard's UnpackNormalmapRGorAG still decodes RGB normals via the x*a trick, so lighting is roughly right, but they're not true NormalMap imports).
• Unity never (re)generates mipmaps for DDS: the file's own mip chain is all you get. Mip chains stripped by FixDdsForUnity stay mip-less in the bundle → shimmering/aliasing when minified.
• No aniso control: the importer API exposes only isReadable/filterMode/ wrapMode/sRGB. DDS textures ship with aniso: 1.
• Pixel data is used as-is (uncompressed stays uncompressed — good for AC's deliberately-uncompressed AO/seam skins like EXT_skin.dds).

Generated paint textures (__paint_baked.png, __paint_ao.png) are PNG precisely so they DO go through TextureImporter — the preprocessor gives all Job PNG/JPGs anisoLevel = 8 and gives the two paint bakes CompressedHQ (BC7): default-quality DXT1 turns thin dark panel-gap lines over flat paint into blocky crawl.

AC paint material picking: int_/ext_ twins

Cars commonly ship interior twins of the body paint material (int_carpaint alongside ext_carpaint — McLaren 765LT). Substring matching on "carpaint" can latch onto the interior one, silently wiring the whole dye/livery/bake system to the cabin (report.json said paintMaterial = int_carpaint; the factory bake came out as the INT_OCC interior AO atlas multiplied by black.dds). PickPaintMaterial now drops interior-named hits (int*/_int/interior/inner) when an exterior hit exists and breaks ties by triangle surface area. Check Build/report.json → paintMaterial/paintTexture when a conversion's paint looks wrong.

Smuggling an extra texture to the runtime via the detail slot

The paint material's _DetailAlbedoMap is set to the normalized AO bake WITHOUT enabling _DETAIL_MULX2 — the texture renders as nothing but is serialized into the bundle, and the runtime (AssettoPaint.NeutralAlbedo) fetches it with mat.GetTexture("_DetailAlbedoMap") to use as the albedo in solid-color mode (white albedo × _Color used to flatten all panel-seam AO).