Convolution using C# by asif


Convolution is a simple mathematical operation which is fundamental to many common image processing operators. Convolution provides a way of `multiplying together’ two arrays of numbers, generally of different sizes, but of the same dimensionality, to produce a third array of numbers of the same dimensionality. This can be used in image processing to implement operators whose output pixel values are simple linear combinations of certain input pixel values.

In an image processing context, one of the input arrays is normally just a gray level image. The second array is usually much smaller, and is also two-dimensional (although it may be just a single pixel thick), and is known as the kernel. The following image shows an example image and kernel that we will use to illustrate convolution.

The convolution is performed by sliding the kernel over the image, generally starting at the top left corner, so as to move the kernel through all the positions where the kernel fits entirely within the boundaries of the image. (Note that implementations differ in what they do at the edges of images, as explained below.) Each kernel position corresponds to a single output pixel, the value of which is calculated by multiplying together the kernel value and the underlying image pixel value for each of the cells in the kernel, and then adding all these numbers together.

So, in our example, the value of the bottom right pixel in the output image will be given by:

If the image has M rows and N columns, and the kernel has m rows and n columns, then the size of the output image will have M – m + 1 rows, and N – n + 1 columns.

Mathematically we can write the convolution as:

where i runs from 1 to M – m + 1 and j runs from 1 to N – n + 1.

Note that many implementations of convolution produce a larger output image than this because they relax the constraint that the kernel can only be moved to positions where it fits entirely within the image. Instead, these implementations typically slide the kernel to all positions where just the top left corner of the kernel is within the image. Therefore the kernel `overlaps’ the image on the bottom and right edges. One advantage of this approach is that the output image is the same size as the input image. Unfortunately, in order to calculate the output pixel values for the bottom and right edges of the image, it is necessary to invent input pixel values for places where the kernel extends off the end of the image. Typically pixel values of zero are chosen for regions outside the true image, but this can often distort the output image at these places. Therefore in general if you are using a convolution implementation that does this, it is better to clip the image to remove these spurious regions. Removing n – 1 pixels from the right hand side and m – 1 pixels from the bottom will fix things.

How It Works:

The effect of a simple Mask:

On the Image:

The result after convolution is as:

 

Sample Project:

For understanding Convolution and its concepts, lets make a Class of CSharpMask First as:

public class CSharpMask
{
   public int TopLeft=0,TopMid=0,TopRight=0;
   public int MidLeft=0,Pixel=1,MidRight=0;
   public int BottomLeft=0,BottomMid=0,BottomRight=0;
   public int Factor=1;
   public int Offset=0;<
   public void setAll(int nVal)
   {
     TopLeft=TopMid=TopRight=MidLeft=Pixel=MidRight=BottomLeft=BottomMid=BottomRight=nVal;
   }

   public CSharpMask()
   {
   //
   }
}
//// TODO: Add constructor logic here

This class would be used in all of the articles we will be writing after this simple one in order to illustrate the basics of every operator.

See Line #: 55 in the class ConvOperation

nPixel=(((pOutPut[2]*m.TopLeft)+(pOutPut[5]*m.TopMiddle)+(pOutPut[8]*m.TopRight)+

(pOutPut[stride+2]*m.MiddleLeft)+(pOutPut[stride+5]*m.Pixel)+(pOutPut[stride+8]*m.MiddleRight)

+(pOutPut[stride2+2]*m.BottomLeft)+(pOutPut[stride2+5]*m.BottomMiddle)+(pOutPut[stride2+8]*m.BottomRight))/m.Factor)+m.Offset;

This is the most important thing to understand here. We are now applying the mask on the pixel. This is the Red Pixel. Actully at the end of the RGB series as it is stored BGR. It is shown as :

pOutPut[2] pOutPut[5] pOutPut[8]
pOutPut[stride+2] pOutPut[stride+5] pOutPut[stride+8]
pOutPut[stride2+2] pOutPut[stride2+5] pOutPut[stride2+8]

The Mask being Applied is also given as:

TopLeft TopMiddle TopRight
MiddleLeft Pixel MiddleRight
BottomLeft BottomMiddle BottomRight

The GUI of the whole Project is as:

The project is a part of the series of the image processing articles written just for the prosperity and help for the students searching for Image Processing free stuff.

Attachments:

Project Files: Convolution_Example.zip