Edge detection with Linear CCD

technovm said:

I have a 256 pixel linear ccd. I need to detect edges, i need a fast algorithm to for edge detection with minimum number of calculations. My processor is not powerfull enough.

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#define K 10
int Calc(int * d){
  int y0 = 0, y1 = 0, max = 0, pos = 0;
  for (int i = 1; i < 256; i++){
    y1 = y0 + (d[i] - d[i-1] - y0) / K;
    y0 = y1;
    if (y1 > max) {max = y1;pos = i;}
  }
return pos;  
}

technovm said:

I am getting a curved output with white background, cosine effect. How do I eliminate it?

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_pv said:

What is "not powerful enough" in terms of MIPS, MHz, anything?
What accuracy do you need?

Correct and slow way is to calculate correlation function of your signal and function you are looking for (step function in simple case). And then find the position of maximum of correlation function. This also could be done in different ways depending on required accuracy: just take position of maximum value, or approximate around maximum with some polynom(parabolic) and calculate maximum position analytically.
Let's say edge is smooth in area of about 30 pixels, then such calculation would require just: 256 * 30 = 8k multiplications with accumulation, and 256 compartions in order to find maximum.

Fast way is to calculate derivative, filter it a little bit, and then locate maximum of this derivative.
something like:

Code:

#define K 10
int Calc(int * d){
  int y0 = 0, y1 = 0, max = 0, pos = 0;
  for (int i = 1; i < 256; i++){
    y1 = y0 + (d[i] - d[i-1] - y0) / K;
    y0 = y1;
    if (y1 > max) {max = y1;pos = i;}
  }
return pos;  
}

Or combining both could improve accuracy with a little overhead: at first locate maximum by finding point with maximum derivative, then calculate correlation function in range of few pixels around in order to get more precise result.


Subtract it

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keith1200rs said:

Whenever I have done edge detection with line CCDs I have often done it using a comparator on the CCD output. You need good control of the illumination and/or exposure time and good optics to get a sharp edge. You can also digitise the data from an ADC and interpolate at the triggering point. Good optics are essential.

Keith

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technovm said:

Thanks for your reply.
I need atleast 12 bit accuracy.
I need atleast 150 readings per second of the whole picture.
I have the following questions:
1. How do i find the corelation function?
2. How to select value of K?
3. How to reduce the the cosine effect? Subtract what?

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technovm said:

Thanks, can you suggest ebooks, papers or anything for lighting and optics design for linear ccd cameras for edge detection.

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