Correctly implemented image.ConverTo and added tests

HPPH/Images/Image.cs

@@ -51,7 +51,6 @@ public sealed class Image<T> : IImage<T>
         }
     }
 
-
     IImage IImage.this[int x, int y, int width, int height]
     {
         [MethodImpl(MethodImplOptions.AggressiveInlining)]
@@ -123,11 +122,23 @@ public sealed class Image<T> : IImage<T>
         return new Image<T>(data, 0, 0, width, height, stride);
     }
 
-    public void ConvertTo<TColor>()
+    public IImage<TColor> ConvertTo<TColor>()
         where TColor : struct, IColor
     {
-        for (int i = 0; i < Height; i++)
-            MemoryMarshal.Cast<byte, T>(_buffer.AsSpan().Slice(((i + _y) * _stride) + _x, Width)).ConvertInPlace<T, TColor>();
+        int targetBpp = TColor.ColorFormat.BytesPerPixel;
+        if (targetBpp == ColorFormat.BytesPerPixel)
+        {
+            byte[] data = ToRawArray();
+            MemoryMarshal.Cast<byte, T>(data.AsSpan()).ConvertInPlace<T, TColor>();
+            return new Image<TColor>(data, 0, 0, Width, Height, Width * targetBpp);
+        }
+        else
+        {
+            byte[] data = ToRawArray();
+            byte[] target = new byte[Width * Height * targetBpp];
+            MemoryMarshal.Cast<byte, T>(data.AsSpan()).Convert(MemoryMarshal.Cast<byte, TColor>(target));
+            return new Image<TColor>(target, 0, 0, Width, Height, Width * targetBpp);
+        }
     }
 
     public void CopyTo(Span<T> destination) => CopyTo(MemoryMarshal.AsBytes(destination));

HPPH/Images/Interfaces/IImage.cs

@@ -60,8 +60,8 @@ public interface IImage : IEnumerable<IColor>
     /// <returns>The <inheritdoc cref="RefImage{TColor}"/>.</returns>
     RefImage<TColor> AsRefImage<TColor>() where TColor : struct, IColor;
 
-    void ConvertTo<T>() where T : struct, IColor;
-
+    IImage<TColor> ConvertTo<TColor>() where TColor : struct, IColor;
+    
     /// <summary>
     /// Copies the contents of this <see cref="IImage"/> into a destination <see cref="Span{T}"/> instance.
     /// </summary>

HPPH/PixelHelper.Convert.cs

@@ -33,7 +33,7 @@ public static unsafe partial class PixelHelper
         if (colors == null) throw new ArgumentNullException(nameof(colors));
 
         TTarget[] buffer = new TTarget[colors.Length];
-        Convert<TSource, TTarget>(colors, buffer.AsSpan());
+        Convert(colors, buffer.AsSpan());
         return buffer;
     }
 
@@ -48,6 +48,17 @@ public static unsafe partial class PixelHelper
         return buffer;
     }
 
+    public static void Convert<TSource, TTarget>(this Span<TSource> source, Span<TTarget> target)
+        where TSource : struct, IColor
+        where TTarget : struct, IColor
+    {
+        if (source == null) throw new ArgumentNullException(nameof(source));
+        if (target == null) throw new ArgumentNullException(nameof(target));
+        if (target.Length < source.Length) throw new ArgumentException($"Target-buffer is not big enough. {target.Length} < {source.Length}", nameof(target));
+
+        Convert(MemoryMarshal.AsBytes(source), MemoryMarshal.AsBytes(target), TSource.ColorFormat, TTarget.ColorFormat);
+    }
+
     public static void Convert<TSource, TTarget>(this ReadOnlySpan<TSource> source, Span<TTarget> target)
         where TSource : struct, IColor
         where TTarget : struct, IColor