handle different directions of crop

Program.cs

@@ -186,15 +186,22 @@ public class CropTool : ITool {
 
   // left, right, top, bottom
   (int, int, int, int) GetCrop() {
-    // FIXME: this expects the start point in the top left and the end point
-    // in the bottom right; some sign flipping needs to occur to make anchors
-    // in other directions work well.
     Vector2i start = mouseDragStart;
     Vector2i end = mouseDragEnd;
+    int width = Math.Abs(end.X - start.X);
+    int height = Math.Abs(end.Y - start.Y);
+    int heightChange = Math.Min(height, (int) (width / CameraInfo.AspectRatio));
+    int widthChange = (int) (heightChange * CameraInfo.AspectRatio);
+    if (end.X < start.X) {
+      widthChange *= -1;
+    }
+    if (end.Y < start.Y) {
+      heightChange *= -1;
+    }
     // FIXME: choose the aspect ratio based on the original image aspect ratio.
     // FIXME: allow for unconstrained crop, 1:1, etc.
-    end.Y = Math.Min(end.Y, (int) (start.Y + (end.X - start.X) / CameraInfo.AspectRatio));
-    end.X = (int) (start.X + (end.Y - start.Y) * CameraInfo.AspectRatio);
+    end.Y = start.Y + heightChange;
+    end.X = start.X + widthChange;
     int left = Math.Min(start.X, end.X);
     int right = Math.Max(start.X, end.X);
     int top = Math.Min(start.Y, end.Y);