performance - C# LockBits perfomance (int[,] to byte[])

Question

Graphics g;
using (var bmp = new Bitmap(_frame, _height, PixelFormat.Format24bppRgb))
{
    var data = bmp.LockBits(new Rectangle(0, 0, _frame, _height), ImageLockMode.ReadWrite, PixelFormat.Format24bppRgb);
    var bmpWidth = data.Stride;
    var bytes = bmpWidth * _height;
    var rgb = new byte[bytes];
    var ptr = data.Scan0;
    Marshal.Copy(ptr, rgb, 0, bytes);

    for (var i = 0; i < _frame; i++)
    {
        var i3 = (i << 1) + i;
        for (var j = 0; j < _height; j++)
        {
            var ij = j * bmpWidth + i3;
            var val = (byte)(_values[i, j]);
            rgb[ij] = val;
            rgb[ij + 1] = val;
            rgb[ij + 2] = val;
        }
    }

    Marshal.Copy(rgb, 0, ptr, bytes);
    bmp.UnlockBits(data);

    g = _box.CreateGraphics();
    g.InterpolationMode = InterpolationMode.NearestNeighbor;
    g.DrawImage(bmp, 0, 0, _box.Width, _box.Height);
}
g.Dispose();

I use this code to convert an array of RGB values ??(grayscale) in the PictureBox, but it's slow. Please tell me my mistakes. At the moment, an array of 441 000 items handled for 35 ms. I need to handle an array of 4 million for the same time.

Answer 1

You can skip the first Array.Copy where you copy the data from the image to the array, as you will be overwriting all the data in the array anyway.

That will shave off something like 25% of time, but if you want it faster you will have to use an unsafe code block so that you can use pointers. That way you can get around the range checking when you access arrays, and you can write the data directly into the image data instead of copying it.