Here's a start: it looks at a horizontal row of pixels (defined by mouseY), and finds edges (defined as an alternation between a light pixel and a dark, so make sure the image is high contrast).
Dan

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PImage screwImage;
void setup(){
 size(640,480);
 screwImage=loadImage("screwphoto.jpg");
 frameRate(10);
}
void draw(){
 image(screwImage,0,0);
 int ridgeYPos = mouseY;//y position of horizontal line that crosses the threads
 int threadCount=0;
 float currBrightness, lastBrightness=0;
  stroke(0,255,0);
 for(int i=0;i<screwImage.width;i++){//for each pixel of the image's width
   currBrightness=brightness(screwImage.pixels[ridgeYPos*screwImage.width+i]);//get the brigtness of current pixel
   if(currBrightness<128&&lastBrightness>128){//detect edge between light and dark: if current pixel is dark and previous was light
     threadCount++;
     line(i,ridgeYPos-5,i,ridgeYPos+5);//draw little green line at edge
   }
   lastBrightness=currBrightness;
 }
 stroke(255,0,0);
 line(0,ridgeYPos,screwImage.width,ridgeYPos);//red line across
 print("\nY position:"+ridgeYPos+" threadCount "+threadCount);
}