fix letterboxing calculation & use new UiGeometry

Program.cs

@@ -6,6 +6,7 @@ using OpenTK.Windowing.GraphicsLibraryFramework;
 // https://docs.sixlabors.com/api/ImageSharp/SixLabors.ImageSharp.Image.html
 using SixLabors.ImageSharp;
 using SixLabors.ImageSharp.Metadata.Profiles.Exif;
+using System;
 using System.Runtime.CompilerServices;
 using Image = SixLabors.ImageSharp.Image;
 
@@ -133,6 +134,7 @@ void main() {
 
 public class Texture : IDisposable {
   public int Handle;
+  // FIXME: use a Vector2i for dimensions.
   public int Width;
   public int Height;
 
@@ -188,6 +190,7 @@ public class UiGeometry {
 
   public readonly Vector2i WindowSize;
   public readonly List<Box2i> ThumbnailBoxes = new();
+  public readonly Box2i PhotoBox;
 
   public UiGeometry() : this(MIN_WINDOW_SIZE) {}
 
@@ -201,6 +204,8 @@ public class UiGeometry {
       Box2i box = Util.makeBox(WindowSize.X - thumbnailWidth, i * thumbnailHeight, thumbnailWidth, thumbnailHeight);
       ThumbnailBoxes.Add(box);
     }
+
+    PhotoBox = new(0, 0, WindowSize.X - thumbnailWidth, WindowSize.Y);
   }
 }
 
@@ -312,14 +317,16 @@ public class Game : GameWindow {
     GL.BindBuffer(BufferTarget.ArrayBuffer, VertexBufferObject);
     GL.ActiveTexture(TextureUnit.Texture0);
 
-    int maxPhotoWidth = geometry.WindowSize.X - geometry.ThumbnailBoxes[0].Size.X;  // FIXME
-
     Texture active = textures[textureIndex];
-    // TODO: handle the case where we need to letterbox vertically instead.
-    int photoWidth = (int) (1.0 * geometry.WindowSize.Y / active.Height * active.Width);
-    int letterboxWidth = (maxPhotoWidth - photoWidth) / 2;
 
-    DrawTexture(active, Util.makeBox(letterboxWidth, 0, photoWidth, geometry.WindowSize.Y));
+    // FIXME: make a function for scaling & centering one box on another.
+    double scaleX = 1.0 * geometry.PhotoBox.Size.X / active.Width;
+    double scaleY = 1.0 * geometry.PhotoBox.Size.Y / active.Height;
+    double scale = Math.Min(scaleX, scaleY);
+    int renderWidth = (int) (active.Width * scale);
+    int renderHeight = (int) (active.Height * scale);
+    Box2i photoBox = Util.makeBox((int) geometry.PhotoBox.Center.X - renderWidth / 2, (int) geometry.PhotoBox.Center.Y - renderHeight / 2, renderWidth, renderHeight);
+    DrawTexture(active, photoBox); 
     for (int i = 0; i < textures.Count; i++) {
       Box2i box = geometry.ThumbnailBoxes[i];
       DrawTexture(textures[i], box);