add ability to draw a simple crop box

Program.cs

@@ -522,6 +522,10 @@ public class UiGeometry {
 public static class Util {
   public const float PI = (float) Math.PI;
 
+  public static int Lerp(int start, int end, double fraction) {
+    return start + (int) ((end - start) * fraction);
+  }
+
   public static Box2i MakeBox(int left, int top, int width, int height) {
     return new Box2i(left, top, left + width, top + height);
   }
@@ -633,6 +637,9 @@ public class Game : GameWindow {
   readonly object loadedImagesLock = new();
   int photoIndex = 0;
   int ribbonIndex = 0;
+  Vector2i mouseDragStart;
+  Vector2i mouseDragEnd;
+  bool mouseDragging;
   Shader shader = new();
   Matrix4 projection;
   float zoomLevel = 0f;
@@ -671,6 +678,21 @@ public class Game : GameWindow {
           photoIndex = ribbonIndex + i;
         }
       }
+
+      if (geometry.PhotoBox.ContainsInclusive(click)) {
+        mouseDragging = true;
+        mouseDragStart = click;
+      }
+    }
+
+
+    if (MouseState.IsButtonDown(MouseButton.Button1)) {
+      // FIXME: really this should be clipped to the active photo's drawable area, not the whole photobox.
+      mouseDragEnd = new Vector2i(
+          Math.Clamp((int) MouseState.Position.X, geometry.PhotoBox.Min.X, geometry.PhotoBox.Max.X),
+          Math.Clamp((int) MouseState.Position.Y, geometry.PhotoBox.Min.Y, geometry.PhotoBox.Max.Y));
+    } else {
+      mouseDragging = false;
     }
 
     if (MouseState.IsButtonPressed(MouseButton.Button4)) {
@@ -958,9 +980,28 @@ public class Game : GameWindow {
       DrawText("No photos found.", 10, 10);
     }
 
+    if (mouseDragging) {
+      DrawCropBox();
+    }
+
     SwapBuffers();
   }
 
+  void DrawCropBox() {
+    int left = Math.Min(mouseDragStart.X, mouseDragEnd.X);
+    int right = Math.Max(mouseDragStart.X, mouseDragEnd.X);
+    int top = Math.Min(mouseDragStart.Y, mouseDragEnd.Y);
+    int bottom = Math.Max(mouseDragStart.Y, mouseDragEnd.Y);
+
+    DrawBox(new Box2i(left, top, right, bottom), 1, Color4.White);
+    DrawBox(new Box2i(left - 1, top - 1 , right + 1, bottom + 1), 1, Color4.Black);
+    DrawBox(new Box2i(left - 2, top - 2 , right + 2, bottom + 2), 1, Color4.White);
+    DrawHorizontalLine(left, Util.Lerp(top, bottom, 1.0 / 3), right, Color4.White);
+    DrawHorizontalLine(left, Util.Lerp(top, bottom, 2.0 / 3), right, Color4.White);
+    DrawVerticalLine(Util.Lerp(left, right, 1.0 / 3), top, bottom, Color4.White);
+    DrawVerticalLine(Util.Lerp(left, right, 2.0 / 3), top, bottom, Color4.White);
+  }
+
   void DrawPhotos() {
     Photo activePhoto = photos[photoIndex];
     Texture active = activePhoto.Texture();
@@ -1029,6 +1070,14 @@ public class Game : GameWindow {
     GL.DrawElements(PrimitiveType.Triangles, indices.Length, DrawElementsType.UnsignedInt, 0);
   }
 
+  void DrawHorizontalLine(int left, int top, int right, Color4 color) {
+    DrawTexture(TEXTURE_WHITE, Util.MakeBox(left, top, right - left, 1), color);
+  }
+
+  void DrawVerticalLine(int left, int top, int bottom, Color4 color) {
+    DrawTexture(TEXTURE_WHITE, Util.MakeBox(left, top, 1, bottom - top), color);
+  }
+
   void DrawBox(Box2i box, int thickness, Color4 color) {
     DrawTexture(TEXTURE_WHITE, Util.MakeBox(box.Min.X, box.Min.Y, box.Size.X, thickness), color);
     DrawTexture(TEXTURE_WHITE, Util.MakeBox(box.Min.X, box.Min.Y, thickness, box.Size.Y), color);