Added additional tests and some small fixes and improvements

HPPH/Images/Image.cs

@@ -205,7 +205,7 @@ public sealed class Image<T> : IImage<T>
         if (_buffer.Length == 0)
             return ref Unsafe.NullRef<byte>();
 
-        return ref MemoryMarshal.GetReference(new ReadOnlySpan<byte>(_buffer)[((_y * _stride) + (_x * ColorFormat.BytesPerPixel))..]);
+        return ref Unsafe.Add(ref MemoryMarshal.GetReference(_buffer.AsSpan()), (_y * _stride) + (_x * ColorFormat.BytesPerPixel));
     }
 
     public IEnumerator<T> GetEnumerator()

HPPH/Images/ImageColumn.cs

@@ -60,10 +60,10 @@ public readonly ref struct ImageColumn<T>
             _buffer.Slice(_start, SizeInBytes).CopyTo(destination);
         else
         {
-            ReadOnlySpan<T> data = MemoryMarshal.Cast<byte, T>(_buffer[_start..]);
+            ref byte dataRef = ref Unsafe.Add(ref MemoryMarshal.GetReference(_buffer), _start);
             Span<T> target = MemoryMarshal.Cast<byte, T>(destination);
             for (int i = 0; i < Length; i++)
-                target[i] = data[i * _step];
+                target[i] = Unsafe.As<byte, T>(ref Unsafe.Add(ref dataRef, i * _step));
         }
     }
 
@@ -143,7 +143,7 @@ internal class IColorImageColumn<T> : IImageColumn
         {
             if ((y < 0) || (y >= _length)) throw new IndexOutOfRangeException();
 
-            return MemoryMarshal.Cast<byte, T>(_buffer.AsSpan()[(_start + (y * _step))..])[0];
+            return Unsafe.As<byte, T>(ref Unsafe.Add(ref MemoryMarshal.GetReference(_buffer.AsSpan()), _start + (y * _step)));
         }
     }
 
@@ -181,10 +181,10 @@ internal class IColorImageColumn<T> : IImageColumn
             _buffer.AsSpan().Slice(_start, SizeInBytes).CopyTo(destination);
         else
         {
-            ReadOnlySpan<T> data = MemoryMarshal.Cast<byte, T>(_buffer.AsSpan()[_start..]);
+            ref byte dataRef = ref Unsafe.Add(ref MemoryMarshal.GetReference(_buffer.AsSpan()), _start);
             Span<T> target = MemoryMarshal.Cast<byte, T>(destination);
             for (int i = 0; i < Length; i++)
-                target[i] = data[i * _step];
+                target[i] = Unsafe.As<byte, T>(ref Unsafe.Add(ref dataRef, i * _step));
         }
     }
 

HPPH/Images/ImageRow.cs

@@ -131,7 +131,7 @@ internal class IColorImageRow<T> : IImageRow
         {
             if ((x < 0) || (x >= _length)) throw new IndexOutOfRangeException();
 
-            return MemoryMarshal.Cast<byte, T>(_buffer.AsSpan()[_start..])[x];
+            return Unsafe.Add(ref Unsafe.As<byte, T>(ref Unsafe.Add(ref MemoryMarshal.GetReference(_buffer.AsSpan()), (nint)(uint)_start)), (nint)(uint)x);
         }
     }
 

HPPH/Images/RefImage.cs

@@ -126,8 +126,7 @@ public readonly ref struct RefImage<T>
         if (_data.Length == 0)
             return ref Unsafe.NullRef<byte>();
 
-        int offset = (_y * RawStride) + (_x * T.ColorFormat.BytesPerPixel);
-        return ref MemoryMarshal.GetReference(_data[offset..]);
+        return ref Unsafe.Add(ref MemoryMarshal.GetReference(_data), (_y * RawStride) + (_x * T.ColorFormat.BytesPerPixel));
     }
 
     /// <inheritdoc cref="System.Collections.IEnumerable.GetEnumerator"/>

HPPH/PixelHelper.Average.cs

@@ -39,7 +39,7 @@ public static partial class PixelHelper
         try
         {
             image.CopyTo(buffer);
-            return Average<T>(buffer);
+            return Average(buffer);
         }
         finally
         {
@@ -47,6 +47,19 @@ public static partial class PixelHelper
         }
     }
 
+    public static T Average<T>(this Span<T> colors)
+        where T : struct, IColor
+    {
+        if (colors == null) throw new ArgumentNullException(nameof(colors));
+
+        return T.ColorFormat.BytesPerPixel switch
+        {
+            3 => Unsafe.BitCast<Generic3ByteData, T>(Average(MemoryMarshal.Cast<T, Generic3ByteData>(colors))),
+            4 => Unsafe.BitCast<Generic4ByteData, T>(Average(MemoryMarshal.Cast<T, Generic4ByteData>(colors))),
+            _ => throw new NotSupportedException("Data is not of a supported valid color-type.")
+        };
+    }
+
     public static T Average<T>(this ReadOnlySpan<T> colors)
         where T : struct, IColor
     {

HPPH/PixelHelper.Quantize.cs

@@ -86,7 +86,7 @@ public static partial class PixelHelper
 
         T[] result = new T[cubes.Length];
         for (int i = 0; i < cubes.Length; i++)
-            result[i] = Average<T>(cubes[i].Slice(colors));
+            result[i] = Average(cubes[i].Slice(colors));
 
         return result;
     }

HPPH/PixelHelper.Sum.cs

@@ -41,7 +41,7 @@ public static unsafe partial class PixelHelper
         try
         {
             image.CopyTo(buffer);
-            return Sum<T>(buffer);
+            return Sum(buffer);
         }
         finally
         {
@@ -53,6 +53,10 @@ public static unsafe partial class PixelHelper
         where T : struct, IColor
         => T.ColorFormat.Sum(MemoryMarshal.AsBytes(colors));
 
+    public static ISum Sum<T>(this Span<T> colors)
+        where T : struct, IColor
+        => T.ColorFormat.Sum(MemoryMarshal.AsBytes(colors));
+
     internal static ISum Sum<T, TSum>(ReadOnlySpan<T> colors)
         where T : struct, IColor
         where TSum : struct, ISum