How to replace a block in an image with another image?

Question

I am trying to replace a block of an image with another image but I am getting System.ArgumentOutOfRangeException on Marshal.Copy(rgbValues, startIndx, desPtr, bytes) just when I go to the second row. I can't figure why as I'm new to this image manipulation. Here is my code:

public static void ReplaceImageBlock(Bitmap src, Bitmap des, int x, int y, int xParts, int yParts)
    {
        var srcRec = new Rectangle(0, 0, src.Width, src.Height);
        BitmapData srcData = src.LockBits(srcRec,
                                         ImageLockMode.ReadOnly,                                                       src.PixelFormat);

        var desRec = new Rectangle(x, y, src.Width, src.Height);
        BitmapData desData = des.LockBits(desRec, ImageLockMode.WriteOnly, des.PixelFormat);

        int srcStride = srcData.Stride;
        IntPtr srcPtr = srcData.Scan0;

        int desStride = desData.Stride;
        IntPtr desPtr = desData.Scan0;



        int bytes = 3 * xParts;
        var rgbIndex = 0;

        //actual pixel values
        int width = 3 * xParts; //3 bytes by xParts
        int height = yParts;

        var rgbValues = new byte[bytes];

        // Populate rgbValues with src image
        unsafe
        {
            byte* p = (byte*)(void*)srcPtr;

            for (int i = 0; i < height; i++)
            {
                for (int j = 0; j < width; j++)
                {
                    int index = (i * srcStride) + j;
                    rgbValues[rgbIndex++] = p[index];
                }
                rgbIndex = 0; // reset index
                var startIndx = (i * desStride) + x;


                Marshal.Copy(rgbValues, startIndx, desPtr, bytes);//copy to destination block
            }
        }
    }

Answer 1

Here's some code that accepts any format:

using System;
using System.Drawing;
using System.Drawing.Imaging;
using System.Windows.Forms;

namespace WindowsFormsApplication1
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
        }

        private void Form1_Load(object sender, EventArgs e)
        {
            var bitmap1 = new Bitmap(@"..\..\untitled.png");
            var bitmap2 = new Bitmap(256, 256, bitmap1.PixelFormat);
            CopyRegion(bitmap1, new Rectangle(0, 0, 100, 100), bitmap2, new Point(99, 99));
            bitmap2.Save("result.png");
        }

        private unsafe void CopyRegion(Bitmap src, Rectangle srcRect, Bitmap tgt, Point tgtPoint)
        {
            // TODO extra checks etc ...

            if (src == null) throw new ArgumentNullException(nameof(src));
            if (tgt == null) throw new ArgumentNullException(nameof(tgt));

            if (tgt.PixelFormat != src.PixelFormat)
                throw new ArgumentOutOfRangeException(nameof(tgt));

            var tgtRect = new Rectangle(Point.Empty, tgt.Size);
            var rect = new Rectangle(tgtPoint, srcRect.Size);
            if (!tgtRect.Contains(rect))
                throw new ArgumentOutOfRangeException(nameof(tgtPoint));

            var d1 = src.LockBits(srcRect, ImageLockMode.ReadOnly, src.PixelFormat);
            var d2 = tgt.LockBits(tgtRect, ImageLockMode.WriteOnly, tgt.PixelFormat);

            int bitsPerPixel;
            switch (src.PixelFormat)
            {
                case PixelFormat.Format24bppRgb:
                    bitsPerPixel = 24;
                    break;
                case PixelFormat.Format32bppPArgb:
                    bitsPerPixel = 32;
                    break;
                case PixelFormat.Format32bppRgb:
                    bitsPerPixel = 32;
                    break;
                default:
                    throw new ArgumentOutOfRangeException();
            }

            var cols = srcRect.Width;
            var rows = srcRect.Height;
            var bytesPerPixel = (bitsPerPixel + 7)/8;
            var bytes = cols*bytesPerPixel;
            var p1 = d1.Scan0;
            var p2 = d2.Scan0;
            var s1 = d1.Stride;
            var s2 = d2.Stride;
            var x1 = srcRect.X;
            var x2 = tgtPoint.X;
            var y1 = srcRect.Y;
            var y2 = tgtPoint.Y;
            for (var y = 0; y < rows; y++)
            {
                var b1 = (byte*) (p1 + (s1*(y1 + y) + x1*bytesPerPixel));
                var b2 = (byte*) (p2 + (s2*(y2 + y) + x2*bytesPerPixel));
                for (var x = 0; x < bytes; x++)
                {
                    *b2 = *b1;
                    b1++;
                    b2++;
                }
            }

            src.UnlockBits(d1);
            tgt.UnlockBits(d2);
        }
    }
}

Result:

EDIT

This turns out to be much simpler in the end :)

How to crop an image using C#?