C# - RGB Buffer from Bitmap different from C++

Question

I'm using a third party DLL which has as parameter a RGB buffer.

I have used the following code to read RGB buffer from Bitmap:

private byte[] GetBGRValues(Bitmap bmp)
    {

        // Lock the bitmap's bits. 
        Rectangle rect = new Rectangle(0, 0, bmp.Width, bmp.Height);
        System.Drawing.Imaging.BitmapData bmpData = bmp.LockBits(rect, System.Drawing.Imaging.ImageLockMode.ReadOnly, PixelFormat.Format24bppRgb);

        // Get the address of the first line.
        IntPtr ptr = bmpData.Scan0;
        // Declare an array to hold the bytes of the bitmap.
        int bytes = Math.Abs(bmpData.Stride) * bmp.Height;
        byte[] rgbValues = new byte[bytes];


        // Copy the RGB values into the array.
        System.Runtime.InteropServices.Marshal.Copy(ptr, rgbValues, 0, bytes); 
        bmp.UnlockBits(bmpData);

        return rgbValues;
    }

The problem is that the generated RGB buffer is not correct. If I open this buffer in IrfanView, supplying correct parameters, the generated image is not correct (looks like it is shifted).

If a get a buffer that I read using C++ code it works.

I have noticed that bmpData.Stride is 1 unity greater than what I was expecting (width * channels). (I know that .NET uses 4 bytes alignment).

The question is: why is the RGB buffer not correct?

Answer 1

You noticed right - you need to take Stride into account. In general you cannot simply copy image in one Copy call. Stride include both row length and padding and could be greater then row length. So you need to copy only bytes you need from each row, ignore padding bytes and advance to next row by adding Stride.

I guess this is what you see with your code:

- original image and expected result

- invalid result without stride

Here is working code:

public static byte[] GetBGRValues(Bitmap bmp)
{
    var rect = new Rectangle(0, 0, bmp.Width, bmp.Height);
    var bmpData = bmp.LockBits(rect, System.Drawing.Imaging.ImageLockMode.ReadOnly, bmp.PixelFormat);

    var rowBytes = bmpData.Width * Image.GetPixelFormatSize(bmp.PixelFormat) / 8;
    var imgBytes = bmp.Height * rowBytes;
    byte[] rgbValues = new byte[imgBytes];

    var ptr = bmpData.Scan0;
    for (var i = 0; i < bmp.Height; i++)
    {
        Marshal.Copy(ptr, rgbValues, i * rowBytes, rowBytes);
        ptr += bmpData.Stride; // next row
    }

    bmp.UnlockBits(bmpData);

    return rgbValues;
}

More details you can read in this answer: Byte Array to Image Conversion

Also maybe this image will help you to understand Stride purpose:

You need to skip white area at the right when you getting bytes from Bitmap.