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Convolution of Bitmaps

, 29 Mar 2006
Article discussing how to convolve images, specificially the convolution of bitmaps.
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<LI><A href="http://www.codeproject.com/bitmap/ImageConvolution/ImageConvolution.zip">Download demo project - 24.2 
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<H2>Introduction</H2>
<P>Image convolution, plays an important role in computer graphics applications. 
Convolution of images allows for image alterations that enable the creation of 
new images from old ones. Convolution also allows for important features, such 
as edge detection, with many widespread uses. The convolution of an image is a 
simple process by which the pixel of an image is multiplied by a kernel or 
masked to create a new pixel value. Convolution is commonly referred to as 
filtering.</P>
<H2>Details</H2>
<P>First, for a given pixel (x,y), we give a weight to each of the surrounding 
pixels. It may be thought of as giving a number telling how important the pixel 
is to us. The number may be any integer or floating point number, though I 
usually stick to floating point since floats will accept integers as well. The 
kernel or mask, that contains the filter may actually be any size (3x3, 5x5, 
7x7), however 3x3 is very common. Since the process for each is the same, I will 
concentrate only on the 3x3 kernels.</P>
<P>Second, the actual process of convolution involves getting each pixel near a 
given pixel (x,y), and multiplying each of the pixel's channels by the weighted 
kernel value. This means that for a 3x3 kernel, we would multiply the pixels 
like so:</P><PRE lang=text>(x-1,y-1)       *       kernel_value[row0][col0]
(x  ,y-1)       *       kernel_value[row0][col1]
(x+1,y-1)       *       kernel_value[row0][col2]
(x-1,y  )       *       kernel_value[row1][col0]
(x  ,y  )       *       kernel_value[row1][col1]
(x+1,y  )       *       kernel_value[row1][col2]
(x-1,y+1)       *       kernel_value[row2][col0]
(x  ,y+1)       *       kernel_value[row2][col1]
(x+1,y+1)       *       kernel_value[row2][col2]</PRE>
<P>The process is repeated for each channel of the image. This means that the 
red, green and blue color channels (if working in RGB colorspace) must each be 
multiplied by the kernel values. The kernel position is related to the pixel 
position it is multiplied by. Simply put, the kernel is allocated in 
<CODE>kernel[rows][cols]</CODE> which would be <CODE>kernel[3][3]</CODE> in this 
case. The 3x3 (5x5 or 7x7 if using a larger kernel) area around pixel (x,y) is 
then multiplied by the kernel to get the total sum. If we were working with a 
100x100 image, allocated as <CODE>image[100][100]</CODE>, and we wanted the 
value for pixel (10,10), the process for each channel would look like:</P><PRE>float fTotalSum =
Pixel(10-1,10-1)       *       kernel_value[row0][col0] +
Pixel(10  ,10-1)       *       kernel_value[row0][col1] +
Pixel(10+1,10-1)       *       kernel_value[row0][col2] +
Pixel(10-1,10  )       *       kernel_value[row1][col0] +
Pixel(10  ,10  )       *       kernel_value[row1][col1] +
Pixel(10+1,10  )       *       kernel_value[row1][col2] +
Pixel(10-1,10+1)       *       kernel_value[row2][col0] +
Pixel(10  ,10+1)       *       kernel_value[row2][col1] +
Pixel(10+1,10+1)       *       kernel_value[row2][col2] +</PRE>
<P>Finally, each value is added to the total sum, which is then divided by the 
total weight of the kernel. The kernel's weight is given by adding each value 
contained in the kernel. If the value is zero or less, then a weight of 1 is 
given to avoid a division by zero.</P>
<P>The actual code to convolve an image is:</P><PRE>for (int i=0; i &lt;= 2; i++)//loop through rows
{
    for (int j=0; j &lt;= 2; j++)//loop through columns
    {
        //get pixel near source pixel
        /*
        if x,y is source pixel then we loop through and
        get pixels at coordinates:
        x-1,y-1
        x-1,y
        x-1,y+1
        x,y-1
        x,y
        x,y+1
        x+1,y-1
        x+1,y
        x+1,y+1
        */
        COLORREF tmpPixel = pDC-&gt;GetPixel(sourcex+(i-(2&gt;&gt;1)),
                                              sourcey+(j-(2&gt;&gt;1)));
        //get kernel value
        float fKernel = kernel[i][j];
        //multiply each channel by kernel value, and add to sum
        //notice how each channel is treated separately
        rSum += (GetRValue(tmpPixel)*fKernel);
        gSum += (GetGValue(tmpPixel)*fKernel);
        bSum += (GetBValue(tmpPixel)*fKernel);
        //add the kernel value to the kernel sum
        kSum += fKernel;
    }
}
//if kernel sum is less than 0, reset to 1 to avoid divide by zero
if (kSum &lt;= 0)
    kSum = 1;
//divide each channel by kernel sum
rSum/=kSum;
gSum/=kSum;
bSum/=kSum;</PRE>
<P>The source code included performs some common image convolutions. Also 
included is a Convolve Image menu option that allows users to enter their own 
kernel. Common 3x3 kernels include:</P><PRE>gaussianBlur[3][3] = {0.045, 0.122, 0.045, 0.122, 
  0.332, 0.122, 0.045, 0.122, 0.045};
gaussianBlur2[3][3] = {1, 2, 1, 2, 4, 2, 1, 2, 1};
gaussianBlur3[3][3] = {0, 1, 0, 1, 1, 1, 0, 1, 0};
unsharpen[3][3] = {-1, -1, -1, -1, 9, -1, -1, -1, -1};
sharpness[3][3] = {0,-1,0,-1,5,-1,0,-1,0};
sharpen[3][3] = {-1, -1, -1, -1, 16, -1, -1, -1, -1};
edgeDetect[3][3] = {-0.125, -0.125, -0.125, -0.125, 
  1, -0.125, -0.125, -0.125, -0.125};
edgeDetect2[3][3] = {-1, -1, -1, -1, 8, -1, -1, -1, -1};
edgeDetect3[3][3] = {-5, 0, 0, 0, 0, 0, 0, 0, 5};
edgeDetect4[3][3] = {-1, -1, -1, 0, 0, 0, 1, 1, 1};
edgeDetect5[3][3] = {-1, -1, -1, 2, 2, 2, -1, -1, -1};
edgeDetect6[3][3] = {-5, -5, -5, -5, 39, -5, -5, -5, -5};
sobelHorizontal[3][3] = {1, 2, 1, 0, 0, 0, -1, -2, -1 };
sobelVertical[3][3] = {1, 0, -1, 2, 0, -2, 1, 0, -1 };
previtHorizontal[3][3] = {1, 1, 1, 0, 0, 0, -1, -1, -1 };
previtVertical[3][3] = {1, 0, -1, 1, 0, -1, 1, 0, -1};
boxBlur[3][3] = {0.111f, 0.111f, 0.111f, 0.111f, 
  0.111f, 0.111f, 0.111f, 0.111f, 0.111f};
triangleBlur[3][3] = { 0.0625, 0.125, 0.0625, 
  0.125, 0.25, 0.125, 0.0625, 0.125, 0.0625};</PRE>
<P>Last but not least is the ability to show a convoluted image as a grayscale 
result. In order to display a filtered image as grayscale, we just add a couple 
lines to the bottom of the <CODE>Convolve </CODE>function: <PRE>//return new pixel value
    if (bGrayscale)
    {
        int grayscale=0.299*rSum + 0.587*gSum + 0.114*bSum;
        rSum=grayscale;
        gSum=grayscale;
        bSum=grayscale;
    }

    clrReturn = RGB(rSum,gSum,bSum);</PRE>
<P>This means that the entire <CODE>Convolve </CODE>function now looks like:</O> 
<PRE>COLORREF CImageConvolutionView::Convolve(CDC* pDC, int sourcex, 
    int sourcey, float kernel[3][3], int nBias,BOOL bGrayscale)
{
    float rSum = 0, gSum = 0, bSum = 0, kSum = 0;
    COLORREF clrReturn = RGB(0,0,0);
    for (int i=0; i &lt;= 2; i++)//loop through rows
    {
        for (int j=0; j &lt;= 2; j++)//loop through columns
        {
            //get pixel near source pixel
            /*
            if x,y is source pixel then we loop 
            through and get pixels at coordinates:
            x-1,y-1
            x-1,y
            x-1,y+1
            x,y-1
            x,y
            x,y+1
            x+1,y-1
            x+1,y
            x+1,y+1
            */
            COLORREF tmpPixel = pDC-&gt;GetPixel(sourcex+(i-(2&gt;&gt;1)),
                sourcey+(j-(2&gt;&gt;1)));
            //get kernel value
            float fKernel = kernel[i][j];
            //multiply each channel by kernel value, and add to sum
            //notice how each channel is treated separately
            rSum += (GetRValue(tmpPixel)*fKernel);
            gSum += (GetGValue(tmpPixel)*fKernel);
            bSum += (GetBValue(tmpPixel)*fKernel);
            //add the kernel value to the kernel sum
            kSum += fKernel;
        }
    }
    //if kernel sum is less than 0, reset to 1 to avoid divide by zero
    if (kSum &lt;= 0)
        kSum = 1;
    //divide each channel by kernel sum
    rSum/=kSum;
    gSum/=kSum;
    bSum/=kSum;
    //add bias if desired
    rSum += nBias;
    gSum += nBias;
    bSum += nBias;
    //prevent channel overflow by clamping to 0..255
    if (rSum &gt; 255)
        rSum = 255;
    else if (rSum &lt; 0)
        rSum = 0;
    if (gSum &gt; 255)
        gSum = 255;
    else if (gSum &lt; 0)
        gSum = 0;
    if (bSum &gt; 255)
        bSum = 255;
    else if (bSum &lt; 0)
        bSum = 0;
    //return new pixel value
    if (bGrayscale)
    {
        int grayscale=0.299*rSum + 0.587*gSum + 0.114*bSum;
        rSum=grayscale;
        gSum=grayscale;
        bSum=grayscale;
    }

    clrReturn = RGB(rSum,gSum,bSum);
    return clrReturn;
}
</PRE>
<P>Last but not least I did a little tweaking to get the program to load a 
default image from a resource (<CODE>IDB_BITMAP1</CODE>). Then I added the 
ability to convolve this default image. The program will still load image from a 
file, the only difference is that it will now show a default image at 
startup.</P>
<P>Please note that this article is by no means, an example of fast processing of pixels. It is merely meant to show how convolution CAN be done on images. If you would like a more advanced image processor, then feel free to email me with the subject "WANT CODE:ImageEdit Please". That is an unreleased image processor I have done, though parts are not implemented yet due to lack of time, that contains much more functionality using CxImage library as its basis for reading and saving images.</P>


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About the Author

Fred Ackers
Web Developer
United States United States
Programming using MFC and ATL for almost 12 years now. Currently studying Operating System implementation as well as Image processing. Previously worked on DSP and the use of FFT for audio application. Programmed using ADO, ODBC, ATL, COM, MFC for shell interfacing, databasing tasks, Internet items, and customization programs.

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