Aetheria/Assets/Shaders/Compute/ShieldField/ShieldSim.compute

// ShieldSim.compute
//
// Stress propagates as a wave on the dual graph of the tiling rather than as a
// diffusion. The whole spall effect rests on one convention:
//
//   u > 0 is TENSION, u < 0 is COMPRESSION. An impact injects compression.
//   Material fails on tension only.
//
// A free surface must hold stress at zero, which is a Dirichlet condition, and
// Dirichlet boundaries reflect with INVERTED sign. So the compressive pulse
// returns from the rim (and from the faces of already-broken cells) as tension,
// and that reflected tension is what spalls. Imposing Neumann here instead --
// which is what you get by default if you simply omit missing neighbours from
// the Laplacian -- reflects compression as compression and nothing ever breaks.

#pragma kernel KInit
#pragma kernel KInject
#pragma kernel KWaveStep
#pragma kernel KEcho
#pragma kernel KFracture
#pragma kernel KUpdate

#include "ShieldCommon.hlsl"

#define ECHO_SLOTS 16

StructuredBuffer<CellStatic>  _Cells;
StructuredBuffer<DualEdge>    _Edges;
RWStructuredBuffer<CellState> _State;

// wave field, ping-ponged by the host
StructuredBuffer<float>       _UIn;
RWStructuredBuffer<float>     _UOut;
RWStructuredBuffer<float>     _V;         // du/dt
RWStructuredBuffer<float>     _Echo;      // [cell * ECHO_SLOTS + slot] through-thickness delay line
RWStructuredBuffer<float>     _EdgeFlow;  // per-arc |gradient|, for conduit rendering
AppendStructuredBuffer<uint>  _Visible;
RWStructuredBuffer<uint>      _BreakBudget; // [0]=count this frame (atomic), [1]=cap; caps cascade rate per frame

uint  _CellCount;
float _Time;
float _Dt;

// --- wave
float _WaveSpeed2;      // c^2
float _Damping;
float _StressClamp;

// --- through-thickness echo
uint  _EchoSlot;        // current write slot, advanced by the host each substep
uint  _EchoDelay;       // slots to wait ~ 2 * thickness / c
float _EchoReflect;     // negative: free back face inverts the pulse

// --- hit
float2 _HitPos;
float2 _HitDir;         // in-plane component of the incoming direction
float  _HitEnergy;
float  _HitRadius;
float  _HitObliquity;   // 0 head-on, 1 grazing
uint   _Pattern;
float  _StarArms;
float  _PatternFocus;
float  _HitSpin;
float  _SpiralTwist;

// --- material
float _TensileStrength;
float _TemperInit;        // surface compression at full charge
float _TemperErosion;     // how fast sub-critical tension wears the temper down
float _StoredEnergyGain;  // fraction of temper released back into the field on break
float _DamageGain;
float _BreakAt;
float _CrackStart;

// --- shards
float _RadialKick;
float _LateralKick;
float _NormalKick;
float _SpinKick;
float _ShardLife;
float _ShardDrag;

// --- growth
float _PanelSpawnTime;
float _GrowDuration;
float _GlowDecay;

// ==================================================================
[numthreads(64,1,1)]
void KInit(uint3 id : SV_DispatchThreadID)
{
    uint i = id.x; if (i >= _CellCount) return;

    CellState s = (CellState)0;
    s.breakTime   = -1.0;
    s.temper      = _TemperInit;
    _State[i] = s;

    _UOut[i] = 0.0;
    _V[i]    = 0.0;
    for (uint k = 0; k < ECHO_SLOTS; k++) _Echo[i * ECHO_SLOTS + k] = 0.0;
}

// ==================================================================
// Stamp the initial pulse. The pattern shapes the pulse; the material's
// own anisotropy (baked into the arc weights) decides how it spreads.
// ==================================================================
float InjectionProfile(float2 rel, float r)
{
    float2 rad = (r > 1e-5) ? rel / r : float2(1, 0);
    float falloff = saturate(1.0 - r / max(_HitRadius, 1e-4));
    falloff = falloff * falloff;

    float shape = 1.0;
    switch (_Pattern)
    {
    case PAT_RADIAL:
        break;

    case PAT_STAR:
    {
        float ang   = atan2(rad.y, rad.x) + _HitSpin;
        float spoke = abs(frac(ang * _StarArms / TAU + 0.5) - 0.5) * 2.0;  // 0 on a spoke
        shape = lerp(1.0, _PatternFocus, spoke);
        break;
    }
    case PAT_LANCE:
    {
        float a = saturate(dot(rad, _HitDir) * 0.5 + 0.5);
        shape = lerp(_PatternFocus, 1.0, a * a);
        break;
    }
    case PAT_SPIRAL:
    {
        float ang    = atan2(rad.y, rad.x);
        float target = _SpiralTwist * r;
        float d      = abs(frac((ang - target) / TAU * _StarArms + 0.5) - 0.5) * 2.0;
        shape = lerp(1.0, _PatternFocus, d);
        break;
    }
    case PAT_CRYSTAL:
        // leave the pulse round; the grain in the arc weights does the shaping
        break;

    case PAT_FIZZLE:
        shape = lerp(0.35, 1.0, Hash01(asuint(r * 991.0) ^ asuint(rad.x * 577.0)));
        break;
    }

    // A grazing hit couples less energy in and skews it downrange.
    float oblique = lerp(1.0, saturate(dot(rad, _HitDir) * 0.5 + 0.5) * 1.6, _HitObliquity);
    return falloff * shape * oblique;
}

[numthreads(64,1,1)]
void KInject(uint3 id : SV_DispatchThreadID)
{
    uint i = id.x; if (i >= _CellCount) return;
    CellState s = _State[i];
    if (s.breakTime >= 0.0 || s.growth < 0.6) return;

    CellStatic c = _Cells[i];
    float2 rel = c.center - _HitPos;
    float  r   = length(rel);
    if (r > _HitRadius) return;

    float amp = InjectionProfile(rel, r) * _HitEnergy;

    // Negative: the strike puts the material into compression. Injecting into
    // velocity rather than displacement gives a propagating pulse instead of a
    // standing bump.
    _V[i] -= amp / max(c.area, 1e-5);

    // Seed the through-thickness delay line at the current slot.
    _Echo[i * ECHO_SLOTS + (_EchoSlot % ECHO_SLOTS)] -= amp;
}

// ==================================================================
// One substep of u_tt = c^2 * lap(u) - damping * u_t
// ==================================================================
[numthreads(64,1,1)]
void KWaveStep(uint3 id : SV_DispatchThreadID)
{
    uint i = id.x; if (i >= _CellCount) return;

    CellStatic c  = _Cells[i];
    CellState  st = _State[i];
    float ui = _UIn[i];

    // A broken cell carries no stress and reflects everything back at its neighbours.
    if (st.breakTime >= 0.0 || st.growth < 0.6)
    {
        _UOut[i] = 0.0;
        _V[i]    = 0.0;
        return;
    }

    float lap = 0.0;
    float flowMax = 0.0;

    for (uint k = 0; k < c.nbrCount; k++)
    {
        uint  ei = c.nbrStart + k;
        DualEdge e = _Edges[ei];
        CellState ns = _State[e.nbr];

        // Ghost value: an intact neighbour contributes its own stress; a broken
        // one (or an ungrown one) is a free surface, so the ghost is -ui, which
        // pins the face to zero and inverts the reflected pulse.
        bool solid = (ns.breakTime < 0.0) && (ns.growth >= 0.6);
        float uj = solid ? _UIn[e.nbr] : -ui;

        lap += e.w * (uj - ui);

        float f = abs(uj - ui) * e.w;
        _EdgeFlow[ei] = f;
        flowMax = max(flowMax, f);
    }

    // Rim edges have no arc at all, so their ghost term was precomputed.
    lap += c.openW * (-2.0 * ui);

    lap /= max(c.area, 1e-5);

    float v = _V[i];
    v += _WaveSpeed2 * lap * _Dt;
    v *= exp(-_Damping * _Dt);

    float u = ui + v * _Dt;
    u = clamp(u, -_StressClamp, _StressClamp);

    _UOut[i] = u;
    _V[i]    = v;
}

// ==================================================================
// Through-thickness reflection: the compressive pulse that entered the
// plate returns from the back face, inverted, one round trip later.
// This is the delayed tensile spike co-located with the impact, which is
// why the far face erupts a beat after the near face was struck.
// ==================================================================
[numthreads(64,1,1)]
void KEcho(uint3 id : SV_DispatchThreadID)
{
    uint i = id.x; if (i >= _CellCount) return;
    CellState s = _State[i];
    if (s.breakTime >= 0.0) return;

    uint base = i * ECHO_SLOTS;
    uint read = (_EchoSlot + ECHO_SLOTS - (_EchoDelay % ECHO_SLOTS)) % ECHO_SLOTS;

    float pulse = _Echo[base + read];
    _Echo[base + read] = 0.0;
    if (abs(pulse) < 1e-6) return;

    _V[i] += _EchoReflect * pulse / max(_Cells[i].area, 1e-5);
}

// ==================================================================
// Fracture. Tension is the only thing that breaks anything.
//
//   effective tension = u - temper
//
// temper is the tempered-glass surface compression: it must be cancelled
// before a crack can start. Sub-critical tension erodes it, which is what
// lets a sequence of weak hits eventually produce a break that no single
// one of them could. When a cell finally goes, its stored core energy is
// dumped back into the wave field, and that is the dicing cascade.
// ==================================================================
[numthreads(64,1,1)]
void KFracture(uint3 id : SV_DispatchThreadID)
{
    uint i = id.x; if (i >= _CellCount) return;

    CellState s = _State[i];
    if (s.breakTime >= 0.0 || s.growth < 0.6) return;

    CellStatic c = _Cells[i];
    float u = _UIn[i];
    if (u <= 0.0) return;                       // compression does nothing

    s.peakTension = max(s.peakTension, u);

    float strength = _TensileStrength * c.toughness;
    float net      = u - s.temper;               // must out-pull the surface compression

    if (net <= 0.0)
    {
        // Sub-critical: wears the temper down. This is the multi-hit budget.
        s.temper = max(0.0, s.temper - u * _TemperErosion * _Dt);
        s.glow   = max(s.glow, saturate(u / max(strength, 1e-4)) * 0.35);
        _State[i] = s;
        return;
    }

    s.damage = saturate(s.damage + (net / max(strength, 1e-4)) * _DamageGain * _Dt);
    s.glow   = max(s.glow, saturate(net / max(strength, 1e-4)));
    s.temper = max(0.0, s.temper - u * _TemperErosion * _Dt * 3.0);

    if (s.damage >= _BreakAt && net >= strength)
    {
        uint slot;
        InterlockedAdd(_BreakBudget[0], 1u, slot);
        if (slot < _BreakBudget[1])              // budget caps the cascade rate per frame
        {
            s.breakTime = _Time;

            // Stored core energy, released. Sets fragment fineness: a panel that
            // was still heavily charged dices finely, a depleted one breaks into
            // a few large pieces.
            s.released = saturate((s.temper + net) * _StoredEnergyGain);

            float2 rel = c.center - _HitPos;
            float  r   = max(length(rel), 1e-4);
            float2 rad = rel / r;

            float2 lateral = rad * _RadialKick + _HitDir * _LateralKick * _HitObliquity;
            float2 j       = (Hash2(c.splitSeed) - 0.5) * 0.6;

            s.vel  = float3(lateral + j, _NormalKick * (0.4 + net));
            s.vel *= (0.5 + saturate(net / max(strength, 1e-4)));
            s.spin = (Hash01(c.splitSeed ^ 0x51ed2701u) - 0.5) * _SpinKick * (0.5 + net);

            // Dump the released energy back into the neighbours as tension. This
            // is the feedback loop that makes the panel dice rather than chip.
            for (uint k = 0; k < c.nbrCount; k++)
            {
                DualEdge e = _Edges[c.nbrStart + k];
                _V[e.nbr] += s.released * e.w / max(_Cells[e.nbr].area, 1e-5);
            }
        }
    }

    _State[i] = s;
}

// ==================================================================
[numthreads(64,1,1)]
void KUpdate(uint3 id : SV_DispatchThreadID)
{
    uint i = id.x; if (i >= _CellCount) return;

    CellStatic c = _Cells[i];
    CellState  s = _State[i];

    s.growth = saturate((_Time - _PanelSpawnTime - c.spawnDelay) / max(_GrowDuration, 1e-3));
    s.glow  *= exp(-_GlowDecay * _Dt);

    if (s.breakTime >= 0.0)
    {
        float age = _Time - s.breakTime;
        if (age > _ShardLife) return;            // dead: not appended, not drawn

        s.pos  += s.vel * _Dt;
        s.vel  *= exp(-_ShardDrag * _Dt);
        s.vel.z -= 0.6 * _Dt;                    // gentle settle, not real gravity
        s.rot  += s.spin * _Dt;
    }
    else if (s.growth <= 0.0)
    {
        _State[i] = s;
        return;
    }

    _State[i] = s;
    _Visible.Append(i);
}