53 lines
2.7 KiB
C#
53 lines
2.7 KiB
C#
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using CultMath;
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using static CultMath.math;
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/// <summary>
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/// The global flow law: the one definition of the drift that stardust and nebula follow and that the sim knows.
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/// A pure function of the zone's FlowSettings, a position, a scroll and a downhill gradient; nothing here keeps
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/// a clock. Zone.Flow evaluates it at zone time. Assets/Shaders/Volumetric.cginc (globalFlow, slopeFlow) is its
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/// HLSL lowering, driven by VolumeSampling.LowerFlow and pinned to it by Aetheria.EditorTools.FlowFieldParityVerify.
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/// </summary>
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public static class FlowField
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{
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// The scroll offset of the noise field, in field units: speed times zone time and nothing else.
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public static float Scroll(FlowSettings settings, double zoneTime) => (float) (settings.GlobalScrollSpeed * zoneTime);
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// The procedural term (Volumetric.cginc globalFlow): a Tri3D field of position / scale, scrolled along y.
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// Skipped when its amplitude is zero, as the shader's FLOW_GLOBAL keyword skips it.
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public static float3 Global(FlowSettings settings, float3 position, float scroll)
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{
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if (settings.GlobalAmplitude == 0f) return float3.zero;
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var sample = Tri3D(position / settings.GlobalScale - float3(0f, scroll, 0f)) * settings.GlobalAmplitude;
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sample.y *= .5f;
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return sample;
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}
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// The slope term (Volumetric.cginc slopeFlow): swirl around and slide along the gravity gradient.
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// `downhill` is the DOWNHILL gradient, -grad(height) in world xz. The shader's texel gradient is that same
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// vector (getUV negates the world x and z axes), and Zone.GetNormal's xz is proportional to it.
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// Skipped when both amplitudes are zero (the FLOW_SLOPE keyword), and zero for a zero gradient.
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public static float3 Slope(FlowSettings settings, float2 downhill)
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{
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if (settings.SlopeAmplitude == 0f && settings.SwirlAmplitude == 0f) return float3.zero;
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if (dot(downhill, downhill) == 0f) return float3.zero;
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var direction = normalize(downhill);
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return float3(direction.y, 0f, -direction.x) * settings.SwirlAmplitude
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+ float3(direction.x, 0f, direction.y) * settings.SlopeAmplitude;
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}
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public static float3 At(FlowSettings settings, float3 position, double zoneTime, float2 downhill) =>
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Global(settings, position, Scroll(settings, zoneTime)) + Slope(settings, downhill);
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// Volumetric.cginc tri / tri3 / Tri3D, term for term.
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static float Tri(float x) => abs(frac(x) - .5f);
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static float3 Tri3(float3 p) =>
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float3(Tri(p.z + Tri(p.y)), Tri(p.z + Tri(p.x)), Tri(p.y + Tri(p.x)));
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static float3 Tri3D(float3 p)
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{
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var t1 = normalize(Tri3(p));
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var t2 = normalize(Tri3(p * 1.61803398875f));
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return cross(t1, t2);
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}
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}
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