sneak/Shared/Geometry.cs

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C#
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using Microsoft.Xna.Framework;
using System;
// Design largely from https://noonat.github.io/intersect/.
namespace SemiColinGames {
public readonly struct Hit {
public readonly Aabb Collider;
public readonly Vector2 Position;
public readonly Vector2 Delta;
public readonly Vector2 Normal;
public readonly float Time; // ranges from [0, 1].
public Hit(Aabb collider, Vector2 position, Vector2 delta, Vector2 normal) :
this(collider, position, delta, normal, 0.0f) {
}
public Hit(Aabb collider, Vector2 position, Vector2 delta, Vector2 normal, float time) {
Collider = collider;
Position = position;
Delta = delta;
Normal = normal;
Time = time;
}
}
public readonly struct Sweep {
public readonly Hit? Hit;
public readonly Vector2 Position;
public readonly float Time;
public Sweep(Hit? hit, Vector2 position, float time) {
Hit = hit;
Position = position;
Time = time;
}
}
public readonly struct Aabb {
public readonly Vector2 Position; // centroid
public readonly Vector2 HalfSize;
static float Clamp(float value, float min, float max) {
if (value < min) {
return min;
} else if (value > max) {
return max;
} else {
return value;
}
}
public Aabb(Vector2 position, Vector2 halfSize) {
Position = position;
HalfSize = halfSize;
}
public Hit? Intersect(Aabb box) {
float dx = box.Position.X - Position.X;
float px = box.HalfSize.X + HalfSize.X - Math.Abs(dx);
if (px <= 0) {
return null;
}
float dy = box.Position.Y - Position.Y;
float py = box.HalfSize.Y + HalfSize.Y - Math.Abs(dy);
if (py <= 0) {
return null;
}
// TODO: which of delta/normal/hitPos do we actually care about?
if (px < py) {
int sign = Math.Sign(dx);
Vector2 delta = new Vector2(px * sign, 0);
Vector2 normal = new Vector2(sign, 0);
Vector2 hitPos = new Vector2(Position.X + HalfSize.X * sign, box.Position.Y);
return new Hit(box, hitPos, delta, normal);
} else {
int sign = Math.Sign(dy);
Vector2 delta = new Vector2(0, py * sign);
Vector2 normal = new Vector2(0, sign);
Vector2 hitPos = new Vector2(box.Position.X, Position.Y + HalfSize.Y * sign);
return new Hit(this, hitPos, delta, normal);
}
}
public Hit? IntersectSegment(Vector2 pos, Vector2 delta) {
return IntersectSegment(pos, delta, Vector2.Zero);
}
public Hit? IntersectSegment(Vector2 pos, Vector2 delta, Vector2 padding) {
float scaleX = 1.0f / delta.X;
float scaleY = 1.0f / delta.Y;
int signX = Math.Sign(scaleX);
int signY = Math.Sign(scaleY);
float nearTimeX = (Position.X - signX * (HalfSize.X + padding.X) - pos.X) * scaleX;
float nearTimeY = (Position.Y - signY * (HalfSize.Y + padding.Y) - pos.Y) * scaleY;
float farTimeX = (Position.X + signX * (HalfSize.X + padding.X) - pos.X) * scaleX;
float farTimeY = (Position.Y + signY * (HalfSize.Y + padding.Y) - pos.Y) * scaleY;
if (nearTimeX > farTimeY || nearTimeY > farTimeX) {
return null;
}
float nearTime = Math.Max(nearTimeX, nearTimeY);
float farTime = Math.Min(farTimeX, farTimeY);
if (nearTime >= 1 || farTime <= 0) {
return null;
}
// If we've gotten this far, a collision is happening. If the near time is greater than zero,
// the segment starts outside and is entering the box. Otherwise, the segment starts inside
// the box, so we set the hit time to zero.
float hitTime = Math.Max(0, nearTime);
Vector2 normal = nearTimeX > nearTimeY ?
new Vector2(-signX, 0) :
new Vector2(0, -signY);
// TODO: replace these with Vector2.Multiply (etc)
Vector2 hitDelta = new Vector2((1.0f - hitTime) * -delta.X, (1.0f - hitTime) * -delta.Y);
Vector2 hitPos = new Vector2(pos.X + delta.X * hitTime, pos.Y + delta.Y * hitTime);
return new Hit(this, hitPos, hitDelta, normal, hitTime);
}
public Sweep Sweep(Aabb box, Vector2 delta) {
// fast-path case if the other box is static
if (delta.X == 0 && delta.Y == 0) {
Hit? staticHit = Intersect(box);
// TODO: I don't understand the original source here, but I think this is correct.
return new Sweep(staticHit, box.Position, staticHit?.Time ?? 1);
}
Hit? maybeHit = IntersectSegment(box.Position, delta, box.HalfSize);
if (maybeHit == null) {
return new Sweep(null, Vector2.Add(box.Position, delta), 1);
}
Hit hit = (Hit) maybeHit;
Vector2 hitPos = new Vector2(
box.Position.X + delta.X * hit.Time,
box.Position.Y + delta.Y * hit.Time);
Vector2 direction = Vector2.Normalize(delta);
// TODO: why is this calculation made, and then thrown away?
Vector2 sweepHitPos = new Vector2(
Clamp(hit.Position.X + direction.X * box.HalfSize.X,
Position.X - HalfSize.X,
Position.X + HalfSize.X),
Clamp(hit.Position.Y + direction.Y * box.HalfSize.Y,
Position.Y - HalfSize.Y,
Position.Y + HalfSize.Y));
return new Sweep(hit, hitPos, hit.Time);
}
}
}