Add rotation around point

samples/pong/Game.cs

@@ -20,7 +20,7 @@ public override void Init()
     public override void Start()
     {
         World world = new();
-        Entity player = new(new Vector3(456, 228, 1));
+        Entity player = new(new Vector3(400, 200, 1));
         Entity enemy = new();
 
         Sprite spr = new(456, 456, andrei);

samples/pong/Soldier.cs

@@ -17,6 +17,7 @@ public override void Start()
     public override void Update(double deltaTime)
     {
         base.Update(deltaTime);
+
     }
 
     public override void FrameUpdate(double deltaTime)
@@ -25,7 +26,7 @@ public override void FrameUpdate(double deltaTime)
 
         if (Input.IsKeyPressed(Keys.V))
         {
-            Log.Info("V pressed");
+            Entity.Transform.RotateFromPoint(new Vector2(800, 200), 90);
         }
 
         PhysicsBody? body = Entity?.GetComponent<PhysicsBody>();

src/rasdaq/Transformations/Transform.cs

@@ -26,6 +26,34 @@ public class Transform(Vector3 spawnPosition)
     /// </summary>
     public float rotation = 0.0f;
 
+    /// <summary>
+    /// Rotate the entity from a point in the world space.
+    ///
+    /// Note that this does not rotate the entity itself.
+    /// Instead, it moves the entity in a circular arc, with `point` being the centre of the arc.
+    ///
+    /// </summary>
+    /// <param name="point">centre of rotation</param>
+    /// <param name="rotation">rotation in degrees</param>
+    public void RotateFromPoint(Vector2 point, float rotation)
+    {
+        // get radius
+        Vector2 entityPos = new(position.X, position.Y);
+        Vector2 radius = (OVector2)entityPos - (OVector2)point;
+
+        // get trig
+        var sin = Math.Sin(MathHelper.DegreesToRadians(rotation));
+        var cos = Math.Cos(MathHelper.DegreesToRadians(rotation));
+
+        // get x,y from origin
+        var xFromOrigin = radius.X * cos - radius.Y * sin;
+        var yFromOrigin = radius.X * sin + radius.Y * cos;
+
+        // set position and round to 4 dp
+        position.X = Math.Round(point.X + xFromOrigin, 4);
+        position.Y = Math.Round(point.Y + yFromOrigin, 4);
+    }
+
     internal void CoordUpdate(OVector2 delta)
     {
         position.X += delta.X;
@@ -65,5 +93,6 @@ internal Matrix4 GetRenderedTransform(float imageWidth, float imageHeight)
         var scale = ScaleFromNDC(imageWidth, imageHeight);
 
         return scale * rotation * translation;
+        // return scale * rotation * translation;
     }
 }

tests/TransformTests.cs

@@ -140,4 +140,30 @@ public void GetRenderedTransform_SetPositionScale_ScaleThenTranslate()
         // need to transpose back since openTK uses row major
         AssertMatrixEqual(transform.GetRenderedTransform(1, 1), correctMatrix.Transposed());
     }
+
+    [Test]
+    public void RotateFromPoint_At90CCW_Rotate90CCW()
+    {
+        // (x: 2, y: 0)
+        transform.position = new Vector3(2, 0, 0);
+
+        Vector2 center = new(0, 0);
+        transform.RotateFromPoint(center, 90);
+        var actual = new Vector3(0, 2, 0);
+
+        Assert.That(transform.position, Is.EqualTo(actual));
+    }
+
+    [Test]
+    public void RotateFromPoint_AtMinus90WithCenterOnRight_Rotate90CW()
+    {
+        // (x: 2, y: 0)
+        transform.position = new Vector3(-2, 0, 0);
+
+        Vector2 center = new(0, 0);
+        transform.RotateFromPoint(center, -90);
+        var actual = new Vector3(0, 2, 0);
+
+        Assert.That(transform.position, Is.EqualTo(actual));
+    }
 }