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feat(render3d): the grass knows a body is standing in it

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You walked through a meadow and every blade ignored you, which is the most noticeable
thing missing from every step the game asks you to take.

u_push (x, z, radius) bends blades away from a body and DOWN - a trodden stem is shorter
as well as leaning, and leaving the height alone made them splay outward like a fan
instead of being walked through. Applied after the blade's own yaw has put it into world
axes and before it is tipped onto the ground normal, so the push is a world direction
rather than something in the blade's private frame.

Radius 0 means nobody is there, so nothing is paid for when it does not apply. Measured:
GL 6.9 s, VK 7.3 s over 400 frames, unchanged.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
This commit is contained in:
Orkun ÇAKILKAYA 2026-09-19 18:40:08 +03:00
parent 9dff8f21bd
commit 6169cd05b9
5 changed files with 45 additions and 17 deletions
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18
packages/ludic.render3d/shaders/grass.vert
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@ -24,6 +24,7 @@ uniform float u_sea_level; // the sea (terrain_sea); the lake's level when th
uniform vec4 u_lake; // the carved lake: centre x/z, half extents (z = 0: none)
uniform float u_snow_line;
uniform float u_wind;
uniform vec3 u_push; // x, z, radius: a body standing in the grass
uniform vec2 u_tile; // world xz of this tile's corner
uniform int u_tile_cells; // 16 m cells per tile side
uniform int u_per_cell; // indices drawn per cell in this tile
@ -157,6 +158,23 @@ void main() {
p.x += sway * hgt * hgt * 0.35;
p.z += sway * hgt * hgt * 0.15 * cos(ph * 0.7);
p = vec3(c_ * p.x + s * p.z, p.y, -s * p.x + c_ * p.z);
// A BODY STANDING IN IT. Applied here, after the blade's own yaw has put p into world axes
// and before it is tipped onto the ground normal, so the push is a world direction rather
// than something in the blade's private frame. The blade bends AWAY and DOWN - a trodden
// stem is shorter as well as leaning, and leaving the height alone made them splay outward
// like a fan instead of being walked through.
if (u_push.z > 0.0) {
vec2 away = xz - u_push.xy;
float pd = length(away);
float push = smoothstep(u_push.z, u_push.z * 0.2, pd);
if (push > 0.0) {
vec2 pdir = (pd > 1e-3) ? away / pd : vec2(1.0, 0.0);
float bh = max(p.y, 0.0);
p.x += pdir.x * push * bh * 1.05;
p.z += pdir.y * push * bh * 1.05;
p.y -= push * bh * 0.40;
}
}
n = normalize(vec3(c_ * n.x + s * n.z, n.y, -s * n.x + c_ * n.z));
// stand on the ground: rotate the blade's frame from world-up to the surface normal
{