straighten-and-crop kinda works

Program.cs

@@ -290,6 +290,36 @@ public class StraightenTool : ITool {
     }
 
     if (input.IsKeyPressed(Keys.Enter)) {
+      Matrix2 rotation = Matrix2.CreateRotation(MathHelper.DegreesToRadians(photo.RotationDegreeHundredths / 100f));
+      Vector2 center = new(photo.Size.X / 2f, photo.Size.Y / 2f);
+
+      float[] xCoords = new float[4];
+      float[] yCoords = new float[4];
+      List<Vector2i> corners = Util.RectangleCorners(new Rectangle(0, 0, photo.Size.X, photo.Size.Y));
+      for (int i = 0; i < 4; i++) {
+        Vector2i corner = corners[i];
+        Vector2 rotated = Util.RotateAboutCenter(corner, center, rotation);
+        xCoords[i] = rotated.X;
+        yCoords[i] = rotated.Y;
+      }
+      Array.Sort(xCoords);
+      Array.Sort(yCoords);
+      for (int i = 3; i >= 0; i--) {
+        xCoords[i] -= xCoords[0];
+        yCoords[i] -= yCoords[0];
+      }
+      // FIXME: we can get a bigger crop using a better algorithm -- this is
+      // too conservative. Also, preserve aspect ratio? Maybe we need to
+      // commit this as a separate crop immediately so that further crops are
+      // working on an image that isn't weirdly rotated. Or keep the
+      // straighten-crop as a separate operation from an ordinary crop.
+      int left = (int) Math.Ceiling(xCoords[1]);
+      int right = (int) Math.Floor(xCoords[2]);
+      int top = (int) Math.Ceiling(yCoords[1]);
+      int bottom = (int) Math.Floor(yCoords[2]);
+
+      photo.CropRectangle = Rectangle.FromLTRB(left, top, right, bottom);
+
       return ToolStatus.Done;
     }
 
@@ -377,6 +407,12 @@ public static class Util {
     return result;
   }
 
+  public static Vector2 RotateAboutCenter(Vector2 vec, Vector2 center, Matrix2 transform) {
+    Vector2 centerRelative = vec - center;
+    centerRelative *= transform;
+    return centerRelative + center;
+  }
+
   public static Image<Rgba32> MakeImage(float width, float height) {
     return new((int) Math.Ceiling(width), (int) Math.Ceiling(height));
   }
@@ -1158,20 +1194,14 @@ public class Game : GameWindow {
     GL.Viewport(0, 0, e.Width, e.Height);
   }
 
-  private Vector2 RotateAboutCenter(Vector2 vec, Vector2 center, Matrix2 transform) {
-    Vector2 centerRelative = vec - center;
-    centerRelative *= transform;
-    return centerRelative + center;
-  }
-
   private void SetVertices(float left, float top, float width, float height, float rotationDegrees) {
     Matrix2 transform = Matrix2.CreateRotation(MathHelper.DegreesToRadians(rotationDegrees));
 
     Vector2 center = new(left + width / 2, top + height / 2);
-    Vector2 topLeft = RotateAboutCenter(new Vector2(left, top), center, transform);
-    Vector2 topRight = RotateAboutCenter(new Vector2(left + width, top), center, transform);
-    Vector2 bottomRight = RotateAboutCenter(new Vector2(left + width, top + height), center, transform);
-    Vector2 bottomLeft = RotateAboutCenter(new Vector2(left, top + height), center, transform);
+    Vector2 topLeft = Util.RotateAboutCenter(new Vector2(left, top), center, transform);
+    Vector2 topRight = Util.RotateAboutCenter(new Vector2(left + width, top), center, transform);
+    Vector2 bottomRight = Util.RotateAboutCenter(new Vector2(left + width, top + height), center, transform);
+    Vector2 bottomLeft = Util.RotateAboutCenter(new Vector2(left, top + height), center, transform);
 
     // top left
     vertices[0] = topLeft.X;