Smooth terrain: subdivision / tessellation surface through the height samples #211

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opened 2026-09-30 21:16:57 +00:00 by sky_line · 2 comments

sky_line commented

2026-09-30 21:16:57 +00:00

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The Lightbox "Smooth terrain" checkbox (AlchemyRenderTerrainSmoothing) does nothing under Vulkan. Implement it as a GPU subdivision of the terrain patch.

Reference: upstream Alchemy GL (820dbaa0ab)

A tessellation evaluation stage evaluates a monotone bicubic (a Hermite patch per metre cell) through the same height samples, plus its analytic gradient, from 12 height texels.
Corner tangents are the mean of the neighbouring secants, limited Fritsch-Carlson style: at most 3x the smaller secant, and zero where the two secants disagree in sign. The value is held within the cell's four corners.
The result is C1, passes through every sample, and never overshoots. A cliff becomes an S-curve inside its own cell instead of a lip; Catmull-Rom was rejected because it grew metres of ridge on a 30 m cliff.
CPU mirror: ALTerrainSurfaceMaps::smoothHeight uses the same arithmetic, and LLSurface::resolveHeightRegion / resolveNormalGlobal switch to it, so picks, foot planting and parcel lines follow the drawn surface.
The simulator still collides against the linear surface. The two differ by less than one cell's rise.

Our plan

Subdivision: tessellation shader or mesh/compute pre-subdivision into the terrain VBO, with a distance-adaptive factor (full detail near the camera, none far away) so the cost stays bounded.
Surface: follow upstream's monotone bicubic; it has already solved the overshoot problem.
Consistency: keep the CPU mirror so picking and foot IK agree with what is drawn. Shadows, the depth pre-pass and the RT BLAS must all use the same surface.
Normals: analytic gradient into the terrain shading normal.
Region seams: neighbour height samples across region borders.

Refs the Lightbox cleanup (2375e1a183).

The Lightbox "Smooth terrain" checkbox (`AlchemyRenderTerrainSmoothing`) does nothing under Vulkan. Implement it as a GPU subdivision of the terrain patch. ## Reference: upstream Alchemy GL (820dbaa0ab) - A **tessellation evaluation stage** evaluates a **monotone bicubic** (a Hermite patch per metre cell) through the same height samples, plus its analytic gradient, from 12 height texels. - **Corner tangents** are the mean of the neighbouring secants, limited Fritsch-Carlson style: at most 3x the smaller secant, and zero where the two secants disagree in sign. The value is held within the cell's four corners. - The result is C1, passes through every sample, and never overshoots. A cliff becomes an S-curve inside its own cell instead of a lip; Catmull-Rom was rejected because it grew metres of ridge on a 30 m cliff. - **CPU mirror:** `ALTerrainSurfaceMaps::smoothHeight` uses the same arithmetic, and `LLSurface::resolveHeightRegion` / `resolveNormalGlobal` switch to it, so picks, foot planting and parcel lines follow the drawn surface. - The simulator still collides against the linear surface. The two differ by less than one cell's rise. ## Our plan - **Subdivision:** tessellation shader or mesh/compute pre-subdivision into the terrain VBO, with a distance-adaptive factor (full detail near the camera, none far away) so the cost stays bounded. - **Surface:** follow upstream's monotone bicubic; it has already solved the overshoot problem. - **Consistency:** keep the CPU mirror so picking and foot IK agree with what is drawn. Shadows, the depth pre-pass and the RT BLAS must all use the same surface. - **Normals:** analytic gradient into the terrain shading normal. - **Region seams:** neighbour height samples across region borders. Refs the Lightbox cleanup (2375e1a183).

sky_line referenced this issue from a commit

2026-09-30 21:33:37 +00:00

Lightbox shadow filter, avatar shadow detail and terrain hex tiling under Vulkan

sky_line commented

2026-10-02 13:38:31 +00:00

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Landed on branch terraintess (commit ba64bf4d4e, not yet merged/built).

Surface: upstream's monotone bicubic (820dbaa0ab) ported to llterrainsmooth.h, same arithmetic. It passes exactly through every sample and never overshoots. Normals are its analytic gradient. LLSurface::resolveHeightRegion / resolveNormalGlobal switch to it while the setting is on (picks, foot planting, parcel lines). The stencil reads one grid into neighbour regions; with no neighbour it clamps, which gives a zero tangent like upstream.

Geometry: static smooth meshes per patch LOD level, all in the existing mesh store / LOD group:

level 0: 1 m with smooth normals;
level 1: 0.5 m, out to ~120 m;
level 2: 0.25 m, out to ~56 m (the default finest);
level 3: 0.125 m, out to ~12 m.

Every view uses the same surface with no new pass: main, depth pre-pass, shadow cascades, mirror, probes (which draw level 0), the per-slot RT BLAS and the pick boxes. Fine levels stay resident only near the camera: they're built nearest first under a per-frame vertex budget and freed with hysteresis. Edits are caught by a content hash and rebuilt with force.

LOD:

The cull and the TLAS pass both choose the level from the main camera's distance to the patch box.
The TLAS references the nearest level whose BLAS is already built, so a patch never leaves the TLAS.
Geomorph: each vertex stores its height step to the coarser level in texcoord0.y, and the VS blends it in over a band that ends at the switch distance. Switches don't pop, and finer edges meet coarser neighbours.
Skirts: each level has two-sided skirts, as deep as the steepest edge cell's rise + 5 cm. They cover leftover cracks: very tall patches, levels still loading, a neighbour region not yet arrived.

Settings:

RenderVKTerrainSmooth (default on, live). It has a checkbox in Graphics > Advanced under "Hex-tile terrain textures", and the Lightbox "Smooth terrain" checkbox is rebound to it. The dead AlchemyRenderTerrainSmoothing is removed. Off gives today's faceted terrain exactly.
RenderVKTerrainSmoothDetail (0-3, default 2).
Both are in the feature tables (Low off, LowMid 0.5 m, Mid+ 0.25 m) and in graphics presets.

Known:

The simulator still collides against the flat surface. The two differ by less than one 1 m cell's rise.
Tall cliff patches (more than ~15-24 m of relief) can micro-pop at a level switch. The skirts cover any crack.
The RT BLAS is the unmorphed level, so inside a morph band on steep ground it differs from the raster by up to that band's step.

Landed on branch `terraintess` (commit ba64bf4d4e, not yet merged/built). **Surface:** upstream's monotone bicubic (820dbaa0ab) ported to `llterrainsmooth.h`, same arithmetic. It passes exactly through every sample and never overshoots. Normals are its analytic gradient. `LLSurface::resolveHeightRegion` / `resolveNormalGlobal` switch to it while the setting is on (picks, foot planting, parcel lines). The stencil reads one grid into neighbour regions; with no neighbour it clamps, which gives a zero tangent like upstream. **Geometry:** static smooth meshes per patch LOD level, all in the existing mesh store / LOD group: - level 0: 1 m with smooth normals; - level 1: 0.5 m, out to ~120 m; - level 2: 0.25 m, out to ~56 m (the default finest); - level 3: 0.125 m, out to ~12 m. Every view uses the same surface with no new pass: main, depth pre-pass, shadow cascades, mirror, probes (which draw level 0), the per-slot RT BLAS and the pick boxes. Fine levels stay resident only near the camera: they're built nearest first under a per-frame vertex budget and freed with hysteresis. Edits are caught by a content hash and rebuilt with force. **LOD:** - The cull and the TLAS pass both choose the level from the main camera's distance to the patch box. - The TLAS references the nearest level whose BLAS is already built, so a patch never leaves the TLAS. - **Geomorph:** each vertex stores its height step to the coarser level in texcoord0.y, and the VS blends it in over a band that ends at the switch distance. Switches don't pop, and finer edges meet coarser neighbours. - **Skirts:** each level has two-sided skirts, as deep as the steepest edge cell's rise + 5 cm. They cover leftover cracks: very tall patches, levels still loading, a neighbour region not yet arrived. **Settings:** - `RenderVKTerrainSmooth` (default on, live). It has a checkbox in Graphics > Advanced under "Hex-tile terrain textures", and the Lightbox "Smooth terrain" checkbox is rebound to it. The dead `AlchemyRenderTerrainSmoothing` is removed. Off gives today's faceted terrain exactly. - `RenderVKTerrainSmoothDetail` (0-3, default 2). - Both are in the feature tables (Low off, LowMid 0.5 m, Mid+ 0.25 m) and in graphics presets. **Known:** - The simulator still collides against the flat surface. The two differ by less than one 1 m cell's rise. - Tall cliff patches (more than ~15-24 m of relief) can micro-pop at a level switch. The skirts cover any crack. - The RT BLAS is the unmorphed level, so inside a morph band on steep ground it differs from the raster by up to that band's step.

sky_line referenced this issue from a commit

2026-10-03 01:50:56 +00:00

Smooth terrain (#211): monotone bicubic surface, distance LOD with geomorph, skirts

sky_line referenced this issue from a commit

2026-10-03 01:50:57 +00:00

RT terrain: BLAS follows the smooth-terrain geomorph; drawn level built same frame (#211)

sky_line commented

2026-10-04 02:45:44 +00:00

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Owner

Verified done: smooth terrain implemented under Vulkan: upstream's monotone bicubic surface, distance LOD with geomorph and skirts, CPU mirror for picks/foot planting, RT BLAS follows the morphed surface; RenderVKTerrainSmooth (Lightbox checkbox rebound). Commits: ba64bf4d4e, f7c3d9016c. Shipped in v1.0.0-beta.1.

sky_line closed this issue

2026-10-04 02:45:44 +00:00

sky_line referenced this issue from a commit

2026-10-15 17:33:35 +00:00

RT terrain (#211): morph/priority plan measured distances in the wrong frame

sky_line referenced this issue from a commit

2026-10-15 17:33:35 +00:00

RT shadow mask: re-anchor smooth-terrain receivers on the traced terrain (#211)

sky_line referenced this issue from a commit

2026-10-22 08:20:13 +00:00

#195 #85 #869 merged RT shadow pass: sun / moon trace in pass 15, SHADOW reads it