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C# Tutorial: How To Create Image Zero Padding


Andraz Krzisnik
C# Tutorial: How To Create Image Zero Padding

Zero padding an image is useful when we’re convolving it with a filter of certain size. How much padding
should we use depends on how big our filter is.

What is the purpose of zero padding?

When we put our kernel at the very edge of our image, some of it will hang off the image. So to prevent any errors we pad the image with zeros around it. The padding thickness is usually the distance from kernel center to its edge.

Obviously this process is not super necessary, because we could just convolve the image without it.
But the end result would yield a border that wouldn’t be processed. This is not noticable if our kernel is 3×3 or 5×5 and our image is of very large size. These example kernels would leave borders of 1 pixel and 2 pixel thick. Noticability of such borders depends also on what kind of kernel or filter are we using.

So to prevent any unwanted borders we use padding (sometimes reffered also as zero padding).
Padding size explained above is reffered to as minimal zero padding.
We can use larger padding of course. Common padding size is twice the each dimensions of the image we are processing (width, height).
Padding an image has its uses, but remember to crop the image when you finish processing it.

Code for padding function

private Bitmap Pad(Bitmap img, int border)
{
    int w = img.Width;
    int h = img.Height;
    int wp = w + 2 * border;
    int hp = h + 2 * border;
    Bitmap ri = new Bitmap(wp, hp);
    BitmapData rd = ri.LockBits(new Rectangle(0, 0, wp, hp),
        ImageLockMode.WriteOnly, PixelFormat.Format32bppArgb);
    BitmapData id = img.LockBits(new Rectangle(0, 0, w, h),
        ImageLockMode.ReadOnly, PixelFormat.Format32bppArgb);
    int imgb = id.Stride * id.Height;
    int borb = rd.Stride * rd.Height;
    byte[] imga = new byte[imgb];
    byte[] bora = new byte[borb];
    for (int i = 3; i < borb; i += 4)
    {
        bora[i] = 255;
    }
    Marshal.Copy(id.Scan0, imga, 0, imgb);
    img.UnlockBits(id);
    for (int y = 0; y < h; y++)
    {
        for (int x = 0; x < w; x++)
        {
            int ip = y * id.Stride + x * 4;
            int rp = y * rd.Stride + x * 4;
            for (int i = 0; i < 3; i++)
            {
                bora[(rd.Stride + 4) * border + rp + i] = imga[ip + i];
            }
        }
    }
    Marshal.Copy(bora, 0, rd.Scan0, borb);
    ri.UnlockBits(rd);
    return ri;
}

This post is acompanied by entire code for this project available for download here.

Download Project

Leave a comment below and let me know what do you think about this

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