Marshal.Copy() is not copying to bitmap

Question

I'm working on an image processing project, and I've read that the fastest way to manipulate a bitmap image is to copy it from a byte array using Marshal.Copy(). However, for whatever reason, nothing is being copied from my byte array to my Bitmap, and there's not a clear reason why. This is the code I'm using to copy into my Bitmap:

    public void UpdateImage()
    {
        var data = image.LockBits(
            new Rectangle(Point.Empty, image.Size),
            ImageLockMode.WriteOnly, 
            PixelFormat.Format32bppArgb);

        Marshal.Copy(backBuffer, 0, data.Scan0, backBuffer.Length);
        image.UnlockBits(data);
        Console.WriteLine("UpdateImage");//For debugging purposes
    }

I'm attempting to fill the image with complete black, and looking at the data of the backBuffer, it appears to be correct, and as expected, where as when I check any pixel of 'image' it is completely blank. I have no idea why nothing is happening. Any advice would be much appreciated!

Edit: I apologize, I'm a bit new around here, let me provide some more information. Specifically, I am working on some GPU accelerated image processing using Cloo/OpenCL. I wanted to fill the screen with black to make sure that I am doing things correctly, although I am evidently not. Here is the entire class file I'm using:

    public class RenderTarget
{
    public GraphicsDevice GraphicsDevice;
    private byte[] backBuffer;
    public Bitmap image;

    private ComputeKernel fillKernel;
    private ComputeProgram fillProgram;

    public void UpdateImage()
    {
        var data = image.LockBits(
            new Rectangle(Point.Empty, image.Size),
            ImageLockMode.WriteOnly, 
            PixelFormat.Format32bppArgb);

        Marshal.Copy(backBuffer, 0, data.Scan0, backBuffer.Length);
        image.UnlockBits(data);
        Console.WriteLine("UpdateImage");
    }

    //Test method ONLY
    public void FillScreen(Color color)
    {
        if (fillProgram == null)//temporary, all kernels should be compiled on start up. In fact, these probably should be static
        {
            string fillText = @"
                kernel void fillScreen(global uchar* data_out, int from, int to, uchar a, uchar r, uchar g, uchar b){
                    for (int i = from; i < to; i += 4){
                        data_out[i] = a;
                        data_out[i + 1] = r;
                        data_out[i + 2] = g;
                        data_out[i + 3] = b;
                    }
                }";

            fillProgram = new ComputeProgram(GraphicsDevice.context, fillText);

            fillProgram.Build(null, null, null, IntPtr.Zero);

            fillKernel = fillProgram.CreateKernel("fillScreen");
        }

        ComputeBuffer<byte> backBufferBuffer = new ComputeBuffer<byte>(GraphicsDevice.context, ComputeMemoryFlags.ReadOnly | ComputeMemoryFlags.UseHostPointer, backBuffer);

        fillKernel.SetMemoryArgument(0, backBufferBuffer);

        for (int i = 0; i < backBuffer.Length / 10000; i++)
        {
            fillKernel.SetValueArgument<int>(1, i * 10000);
            fillKernel.SetValueArgument<int>(2, (i + 1) * 10000);
            fillKernel.SetValueArgument<byte>(3, color.A);
            fillKernel.SetValueArgument<byte>(4, color.R);
            fillKernel.SetValueArgument<byte>(5, color.G);
            fillKernel.SetValueArgument<byte>(6, color.B);

            GraphicsDevice.queue.ExecuteTask(fillKernel, null);
        }

        GraphicsDevice.queue.ReadFromBuffer(backBufferBuffer, ref backBuffer, false, null);

        GraphicsDevice.queue.Finish();
    }

    public RenderTarget(int Width, int Height, GraphicsDevice device)
    {
        image = new Bitmap(Width, Height);
        backBuffer = new byte[4 * Width * Height];
        GraphicsDevice = device;

        //Fill the screen with black
        FillScreen(Color.Black);
        UpdateImage();
        Console.WriteLine(image.GetPixel(0, 0).A);
    }
}

I have checked to make absolutely sure that the backBuffer is correct. (The values I expected were 255, 0, 0, 0 for the first four elements of the backBuffer).

this question already answered in below link: https://stackoverflow.com/questions/41444923/copying-bytes-from-bitmap-to-byte-array-and-back-with-marshall-copy-doesnt-work — Mustafa Poya, Oct 23 '17 at 05:01
I'm not entirely sure how that answers my question? It seems like the OP in that post is having a different issue from mine. — A. Bearce, Oct 23 '17 at 05:08
If *all* you are trying to do is make the image completely black, you could always do `using (var g = Graphics.FromImage(image)) { g.Clear(Color.Black); }` Though I suspect that was probably just an example and not the *only* thing you are doing. — pinkfloydx33, Oct 23 '17 at 08:35
I assume from what you write there that your `backBuffer` is simply filled with 0 bytes. That does _not_ result in black. You forgot that one of the components of ARGB is 'Alpha', which means opacity. An opacity of zero means transparent pixels. — Nyerguds, Oct 23 '17 at 14:11
No, I have made absolutely sure the elements of the 'backBuffer' are what they should be. I've provided more information. — A. Bearce, Oct 23 '17 at 20:11

score 0 · Answer 1 · answered Oct 23 '17 at 22:58

0

Okay, I figured out what was going wrong. I had the format in the backBuffer wrong. I was expecting it to be ARGB when it should be ordered RGBA. So, it was a problem with my code in 'fillText'.

answered Oct 23 '17 at 22:58

A. Bearce

1
3

Actually, "ARGB" refers to a single pixel read as 32-bit integer. Since integers are little-endian, _the actual order of bytes inside such a value is reversed_, making the _byte_ order B, G, R, A. If you confuse that with RGBA you may end up with distorted colours since you switch blue and red. – Nyerguds Jan 05 '18 at 09:11
Also, an easy trick to be 100% sure of the internal pixel format is to paint your starting image on a new 32bpp argb image. – Nyerguds Jan 05 '18 at 09:16

Marshal.Copy() is not copying to bitmap

1 Answers1