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feat(core+app): alpha atlas loading (Phase 3c.2)

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Loads AC's terrain blending alpha masks into a second GL_TEXTURE_2D_ARRAY
alongside the existing terrain atlas. The alpha atlas is built but not
yet sampled by any shader — that wiring lands in Phase 3c.4.

SurfaceDecoder additions:
  - Handles PFID_A8 (generic single-byte-alpha) by replicating each
    alpha byte into all four RGBA channels
  - Same branch handles PFID_CUSTOM_LSCAPE_ALPHA (0xF4), AC's landscape-
    specific alpha format — the bit layout is identical, just a different
    format ID to distinguish the asset class in the dats. I only found
    this by adding a diagnostic in the first iteration (initial attempt
    returned Magenta for every alpha map because I only wired PFID_A8)
  - 3 new tests: 2x2 A8 round-trip, short-source fallback, and a
    CUSTOM_LSCAPE_ALPHA test verifying it's routed through the same path

TerrainAtlas additions:
  - New GlAlphaTexture property plus CornerAlphaLayers / SideAlphaLayers
    / RoadAlphaLayers index lists so the coming BuildSurface port can
    cite atlas layers by source category
  - BuildAlphaAtlas walks TexMerge.CornerTerrainMaps, SideTerrainMaps,
    RoadMaps and uploads each decoded mask as a layer in insertion
    order; categories carry their atlas-layer index in the respective
    list
  - Fallback handling (single-layer white) when TexMerge is missing or
    every map fails to decode
  - Alpha atlas uses ClampToEdge wrap so repeating tile sampling at
    mask boundaries doesn't produce seams
  - Dispose() now cleans up both textures

On Holtburg's region the log prints:
  TerrainAtlas: 33 terrain layers at 512x512
  AlphaAtlas:    8 layers at 512x512  (corners=4, sides=1, roads=3)

Tests: 61/61 passing. No visual change expected this commit (shader
still ignores Data0..3 and the alpha sampler).

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
This commit is contained in:
Erik 2026-04-11 13:45:40 +02:00
parent 8b940bd038
commit a8459eecbb
3 changed files with 324 additions and 19 deletions
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29
src/AcDream.Core/Textures/SurfaceDecoder.cs
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@ -38,6 +38,7 @@ public static class SurfaceDecoder
PixelFormat.PFID_DXT1 => DecodeBc(rs, CompressionFormat.Bc1),
PixelFormat.PFID_DXT3 => DecodeBc(rs, CompressionFormat.Bc2),
PixelFormat.PFID_DXT5 => DecodeBc(rs, CompressionFormat.Bc3),
PixelFormat.PFID_A8 or PixelFormat.PFID_CUSTOM_LSCAPE_ALPHA => DecodeA8(rs),
PixelFormat.PFID_INDEX16 when palette is not null => DecodeIndex16(rs, palette, isClipMap),
_ => DecodedTexture.Magenta,
};
@ -104,6 +105,34 @@ public static class SurfaceDecoder
Height: 1);
}
/// <summary>
/// Decode single-byte-per-pixel alpha (PFID_A8 / PFID_CUSTOM_LSCAPE_ALPHA)
/// into RGBA8 by replicating each alpha byte into all four channels. AC's
/// terrain blending alpha masks are stored as PFID_CUSTOM_LSCAPE_ALPHA and
/// other generic 8-bit alpha surfaces use PFID_A8; the bit layout is
/// identical so one decoder handles both. Replicating into all four
/// channels lets the fragment shader pull "the blend amount" from either
/// .a or .r without special-casing.
/// </summary>
private static DecodedTexture DecodeA8(RenderSurface rs)
{
int expected = rs.Width * rs.Height;
if (rs.SourceData.Length < expected)
return DecodedTexture.Magenta;
var rgba = new byte[expected * 4];
for (int i = 0; i < expected; i++)
{
byte a = rs.SourceData[i];
int d = i * 4;
rgba[d + 0] = a;
rgba[d + 1] = a;
rgba[d + 2] = a;
rgba[d + 3] = a;
}
return new DecodedTexture(rgba, rs.Width, rs.Height);
}
private static DecodedTexture DecodeA8R8G8B8(RenderSurface rs)
{
int expected = rs.Width * rs.Height * 4;