Im reading DIP 2nd edition by Gonzalez and Woods and try to my hands dirty with Laplacian mask (page 129&130) using wxImage.
float kernel [3][3]= {{1, 1, 1},{1,-8, 1},{1, 1, 1}};
here is the processing loops:
unsigned char r,g,b;
float rtotal, gtotal, btotal; rtotal = gtotal = btotal = 0.0;
//ignore the border pixel
for(int i = 1; i<imgWidth-1; i++)
{
for(int j = 1; j<imgHeight-1; j++)
{
rtotal = gtotal=btotal =0.0;
for(int y = -1; y<=1;y++)
{
for(int x = -1; x<=1;x++)
{
// get each channel pixel value
r = Image->GetRed(i+y,j+x);
g = Image->GetGreen(i+y,j+x);
b = Image->GetBlue(i+y,j+x);
// calculate each channel surrouding neighbour pixel value base
rtotal += r* kernel[y+1][x+1];
gtotal += g* kernel[y+1][x+1] ;
btotal += b* kernel[y+1][x+1];
}
}
//edit1: here is how to sharpen the image
// original pixel - (0.2 * the sum of pixel neighbour)
rtotal = loadedImage->GetRed(x,y) - 0.2*rtotal;
gtotal = loadedImage->GetGreen(x,y) - 0.2*gtotal;
btotal = loadedImage->GetBlue(x,y) - 0.2*btotal;
// range checking
if (rtotal >255) rtotal = 255;
else if (rtotal <0) rtotal = 0;
if(btotal>255) btotal = 255;
else if(btotal < 0) btotal = 0;
if(gtotal > 255) gtotal = 255;
else if (gtotal < 0 ) gtotal =0;
// commit new pixel value
Image->SetRGB(i,j, rtotal, gtotal, btotal);
I applied that to the North Pole picture (grey image) and all I get is a blob of black and white pixels!
Any ideas where may I have missed something in the for loops?
Edit1: Finally get the answer after looking around on google. This dsp stuff is definitely tricky! I added to the code above, it will sharpen the image.
Cheers
A Laplacian filter is an edge detector used to compute the second derivatives of an image, measuring the rate at which the first derivatives change. This determines if a change in adjacent pixel values is from an edge or continuous progression.
First, the result of convolving with a Laplacian can have negative values. Consider a pixel with a value of 1 surrounded by 0's. The result of the convolution at that pixel will be -8.
Second, the range of the result will be between [-8 * 255, 8 * 255], which definitely does not fit into 8 bits. Essentially, when you do your range checking, you are losing most of the information, and most of your resulting pixels will end up either being 0 or 255.
What you have to do is store the result in an array of a type that is signed and wide enough to handle the range. Then, if you wish to output an 8-bit image, you would need to rescale the values so that -8 * 255 maps to 0, and 8 * 255 maps to 255. Or you can rescale it so that the least value maps to 0 and the greatest value maps to 255.
Edit: in this specific case, you can do the following:
rtotal = (rtotal + 8 * 255) / (16 * 255) * 255;
which simplifies to
rtotal = (rtotal + 8 * 255) / 16;
This would map rtotal into a range between 0 and 255 without truncation. You should do the same for gtotal
and btotal
.
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