Fixed issue with incorrect X offset in the PixelAccessor that was reported by Anton Firsov.

src/ImageSharp/Image/PixelAccessor.cs

@@ -297,7 +297,7 @@ namespace ImageSharp
             for (int y = 0; y < height; y++)
             {
                 byte* source = area.PixelBase + (y * area.RowByteCount);
-                byte* destination = this.GetRowPointer(targetY + y) + targetX;
+                byte* destination = this.GetRowPointer(targetX, targetY + y);
 
                 for (int x = 0; x < width; x++)
                 {
@@ -326,7 +326,7 @@ namespace ImageSharp
             for (int y = 0; y < height; y++)
             {
                 byte* source = area.PixelBase + (y * area.RowByteCount);
-                byte* destination = this.GetRowPointer(targetY + y) + targetX;
+                byte* destination = this.GetRowPointer(targetX, targetY + y);
 
                 for (int x = 0; x < width; x++)
                 {
@@ -355,7 +355,7 @@ namespace ImageSharp
             for (int y = 0; y < height; y++)
             {
                 byte* source = area.PixelBase + (y * area.RowByteCount);
-                byte* destination = this.GetRowPointer(targetY + y) + targetX;
+                byte* destination = this.GetRowPointer(targetX, targetY + y);
 
                 for (int x = 0; x < width; x++)
                 {
@@ -384,7 +384,7 @@ namespace ImageSharp
             for (int y = 0; y < height; y++)
             {
                 byte* source = area.PixelBase + (y * area.RowByteCount);
-                byte* destination = this.GetRowPointer(targetY + y) + targetX;
+                byte* destination = this.GetRowPointer(targetX, targetY + y);
 
                 for (int x = 0; x < width; x++)
                 {
@@ -484,13 +484,14 @@ namespace ImageSharp
         /// <summary>
         /// Gets the pointer at the specified row.
         /// </summary>
-        /// <param name="targetY">The target row index.</param>
+        /// <param name="x">The column index.</param>
+        /// <param name="y">The row index.</param>
         /// <returns>
         /// The <see cref="T:byte*"/>.
         /// </returns>
-        protected byte* GetRowPointer(int targetY)
+        protected byte* GetRowPointer(int x, int y)
         {
-            return this.pixelsBase + ((targetY * this.Width) * Unsafe.SizeOf<TColor>());
+            return this.pixelsBase + (((y * this.Width) + x) * Unsafe.SizeOf<TColor>());
         }
 
         [Conditional("DEBUG")]

src/ImageSharp/PixelAccessor.cs

@@ -29,7 +29,7 @@ namespace ImageSharp
             for (int y = 0; y < height; y++)
             {
                 byte* source = area.PixelBase + (y * area.RowByteCount);
-                byte* destination = this.GetRowPointer(targetY + y) + targetX;
+                byte* destination = this.GetRowPointer(targetX, targetY + y);
 
                 Unsafe.CopyBlock(destination, source, byteCount);
             }
@@ -41,7 +41,7 @@ namespace ImageSharp
             for (int y = 0; y < height; y++)
             {
                 byte* source = area.PixelBase + (y * area.RowByteCount);
-                byte* destination = this.GetRowPointer(targetY + y) + targetX;
+                byte* destination = this.GetRowPointer(targetX, targetY + y);
 
                 for (int x = 0; x < width; x++)
                 {
@@ -59,7 +59,7 @@ namespace ImageSharp
             for (int y = 0; y < height; y++)
             {
                 byte* source = area.PixelBase + (y * area.RowByteCount);
-                byte* destination = this.GetRowPointer(targetY + y) + targetX;
+                byte* destination = this.GetRowPointer(targetX, targetY + y);
 
                 for (int x = 0; x < width; x++)
                 {
@@ -77,7 +77,7 @@ namespace ImageSharp
             for (int y = 0; y < height; y++)
             {
                 byte* source = area.PixelBase + (y * area.RowByteCount);
-                byte* destination = this.GetRowPointer(targetY + y) + targetX;
+                byte* destination = this.GetRowPointer(targetX, targetY + y);
 
                 for (int x = 0; x < width; x++)
                 {
@@ -94,7 +94,7 @@ namespace ImageSharp
         {
             for (int y = 0; y < height; y++)
             {
-                byte* source = this.GetRowPointer(sourceY + y) + sourceX;
+                byte* source = this.GetRowPointer(sourceX, sourceY + y);
                 byte* destination = area.PixelBase + (y * area.RowByteCount);
 
                 for (int x = 0; x < width; x++)
@@ -114,7 +114,7 @@ namespace ImageSharp
         {
             for (int y = 0; y < height; y++)
             {
-                byte* source = this.GetRowPointer(sourceY + y) + sourceX;
+                byte* source = this.GetRowPointer(sourceX, sourceY + y);
                 byte* destination = area.PixelBase + (y * area.RowByteCount);
 
                 for (int x = 0; x < width; x++)