Stride and buffersize when processing images

Question

On MSDN I found this for the definition of the image stride. In short it is the width in bytes of one line in the buffer.

Now I have an RGB image in a similar buffer and I have been given the stride and the image width in pixels, and I want to know how much padding bytes there have been added.

Is this simply stride - 3 * imagewidth since I have 3 bytes (RGB) per pixel?

unsafe private void setPrevFrame(IntPtr pBuffer)
    {
        prevFrame = new byte[m_videoWidth, m_videoHeight, 3];
        Byte* b = (byte*) pBuffer;
        for (int i = 0; i < m_videoWidth; i++)
        {
            for (int j = 0; j < m_videoHeight; j++)
            {
                for (int k = 0; k < 3; k++)
                {
                    prevFrame[i,j,k] = *b;
                    b++;
                }
            }
            b += (m_stride - 3 * m_videoHeight);
        }
    }

It's the last line of code I'm not sure about

Answer 1

Stride will be padded to a 4 byte boundary:

So if your image width is W and it is 3 bytes per pixel:

int strideWidth = ((W * 3) - 1) / 4 * 4 + 4; 
//or simply
int strideWidth = Math.Abs(bData.Stride);

//and so
int padding = strideWidth - W * 3;

If you are using the .Stride property of the BitmapData object, you must Abs that value because it may be negative.

Following your edit of the question:

There is no need for you to know the padding in order to iterate all the bytes of the image:

unsafe private void setPrevFrame(IntPtr pBuffer)
{
    for (int j = 0; j < m_videoHeight; j++)
    {
        b = scan0 + j * stride; //the first byte of the first pixel of the row
        for (int i = 0; i < m_videoWidth; i++)
        {
           ...
        }
    }
}

Your previous method will also fail on bottom-up images, as described in the MSDN link you posted.