[TimMasalme]

Description of the proposed changes

Adds a new terrain shader family, Terrain400, next to Terrain200. No existing technique changes, so no existing map changes.

The 2XX shaders lay the strata over each other in order, and splatLerp() compares each layer's height only with its own opacity. Terrain400 blends all layers at once by height instead, after Unity's HeightBasedSplatModify:

w_lower = saturate(1 - sum(w_i))          // masks are weights; a sum above 1 is scaled back to 1
s_i     = h_i * w_i                       // all 8 layers
t       = max(SpecularColor.r, 0.001)     // transition: small cuts hard, ~1.5 is close to a plain blend
w_i'    = (max(s_i + t - max(s), 0) + 0.01 * t) * w_i
w_i''   = w_i' / sum(w')                  // albedo, roughness and normals all blend with w''

ps_2_a decided the texture convention: holding eight albedos plus the atlas gradients does not fit in 22 temp registers, and ps_3_0 would lose the fixed-function terrain fog. So:

• each stratum carries its height in the alpha of both its albedo and its normal map, so each pass reads only its own textures and stays under 16 samplers
• the atlas in the upper slot only supplies the roughness, read at mip 0 (tex2Dbias(..., -16)) to avoid seams at tile borders
• the lower layer has a fixed height of 0.5, so its albedo alpha stays the macrotexture opacity of modes 1 and 2
• no 30° rotation and no far-scale height

Techniques: Terrain400, Terrain401, Terrain402 (last digit = macrotexture mode, as in Terrain20x) and Terrain400T, Terrain401T, Terrain402T, where Stratum2 and Stratum3 are sampled triplanar from the two strongest of the three projections (biplanar mapping as described by Inigo Quilez), chosen from the map-wide normal in Stratum7.

Pixel shader (ps_2_a)	Terrain200 / 200B	Terrain400 / 400T
Normals pass	440–473 slots	74 / 103 slots
Albedo pass	478–498 slots	229–232 / 256–259 slots

Testing done on the proposed changes

• The whole terrain.fx compiles with d3dx9_31, the compiler the game uses.
• The weight formula was checked on 20,000 random mask, height and transition combinations: no NaN, no negative weights, weights always sum to 1.
• Runestone (Terrain202B) converted to Terrain402T, compared in game against the original.
• Two test maps of my own, including a 20 km map with 4096² masks, an 8K map normal and Terrain402/402T.

Not tested yet: other GPUs than mine, and maps made from scratch for this convention by other mappers.

Additional context

Draft, following a short chat with @BlackYps. Open questions:

• Is the alpha convention (height in the albedo and normal alpha) acceptable? generate-pbr or Ozone could write it.
• Roughness at mip 0 can alias at long range. Keep it, or a single roughness per layer?
• The triplanar choice follows the map normal. A map normal that does not match the heightmap (for example one baked from a different Gaea build) picks wrong projections and stretches the triplanar strata; the selection reads the map normal two mip levels coarser to calm high-frequency detail. Worth documenting for mappers.
• The name: 400, or something else in your numbering?

Checklist

• New code is annotated and has comments where useful
• Changes are documented in a changelog snippet according to the guidelines.
• The feature is conceptually approved and the approval documented (see the explanation
• Request 2-3 reviewers from the list of reviewers and their areas of knowledge.

Approval

[ ] The feature is greenlighted through

• concluded discussion (add link)

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