3

I'm trying to understand the .houghcircles() function and I'm not fully understanding how dp affects the results.

Looking up http://docs.opencv.org/modules/imgproc/doc/feature_detection.html?highlight=houghcircles#houghcircles, it says:

dp – Inverse ratio of the accumulator resolution to the image resolution.
For example,
if dp=1 , the accumulator has the same resolution as the input image.
If dp=2 , the accumulator has half as big width and height.

Say you have a 1000x1000 image. You then set dp to 3, so the accumulator sees only 1/3 the image? Or is it something like, the image size remains the same but the number of pixels seen is less? i.e. ever 3 pixels in the original image correlate to 1 pixel that the accumulator sees, like the original image is "blurred"?

I've managed to detect accurately the number of red apples in a bowl by understanding the other parameters and then fiddling with dp.

Perhaps my understanding of the accumulator is incorrect as well since the two are correlated? From what I understand, it is the number of times a region is "hit", but I am not sure how it calculates a "hit region"

My code takes a bowl of apples, converts to HSV, grabs the reddish hues, converts it to either black or white (not greyscale format) using bitWiseOr, then do some .erode() / .dilate() to reduce noise, then .houghcircles().

My resulting image looks like this enter image description herehttp://i.imgur.com/iGyr7TG.jpg

Thanks in advance.

user3666197
  • 1
  • 6
  • 50
  • 92
PGT
  • 1,468
  • 20
  • 34

1 Answers1

2

Verba docent,
Exempla trahunt.

Best seen on an interactive GUI-Demo

A configurable UI-panel allows to adjust parameter(s) by moving a few sliders

import sys
import cv2
import math
import numpy
from scipy.ndimage import label
pi_4 = 4*math.pi

def nothing_asCallback(x):
    pass

def GUI_openCV_circles():
    # --------------------------------------------------------------------------------GUI-<image>
    frame = cv2.imread(  "openCV_CircleDetection_IMG_LASSO_AREA.JPG" )
    demo  = frame[:800,:800,:]
    # --------------------------------------------------------------------------------GUI-<window>-s
    cv2.namedWindow( "DEMO.IN",             cv2.cv.CV_WINDOW_AUTOSIZE )
    cv2.namedWindow( "DEMO.Canny",          cv2.cv.CV_WINDOW_AUTOSIZE )
    cv2.namedWindow( "DEMO.Canny.Circles",  cv2.cv.CV_WINDOW_AUTOSIZE )
    # --------------------------------------------------------------------------------GUI-<state>-initial-value(s)
    aKeyPRESSED                                     = None              # .init

    aCanny_LoTreshold                               = 127
    aCanny_LoTreshold_PREVIOUS                      =  -1
    aCanny_HiTreshold                               = 255
    aCanny_HiTreshold_PREVIOUS                      =  -1

    aHough_dp                                       =   1
    aHough_dp_PREVIOUS                              =  -1
    aHough_minDistance                              =  10
    aHough_minDistance_PREVIOUS                     =  -1
    aHough_param1_aCannyHiTreshold                  = 255
    aHough_param1_aCannyHiTreshold_PREVIOUS         =  -1
    aHough_param2_aCentreDetectTreshold             =  20
    aHough_param2_aCentreDetectTreshold_PREVIOUS    =  -1
    aHough_minRadius                                =  10
    aHough_minRadius_PREVIOUS                       =  -1
    aHough_maxRadius                                =  30
    aHough_maxRadius_PREVIOUS                       =  -1
    # --------------------------------------------------------------------------------GUI-<ACTOR>-s
    cv2.createTrackbar( "Lo_Treshold",          "DEMO.Canny",          aCanny_LoTreshold,                      255, nothing_asCallback )
    cv2.createTrackbar( "Hi_Treshold",          "DEMO.Canny",          aCanny_HiTreshold,                      255, nothing_asCallback )

    cv2.createTrackbar( "dp",                   "DEMO.Canny.Circles",  aHough_dp,                              255, nothing_asCallback )
    cv2.createTrackbar( "minDistance",          "DEMO.Canny.Circles",  aHough_minDistance,                     255, nothing_asCallback )
    cv2.createTrackbar( "param1_HiTreshold",    "DEMO.Canny.Circles",  aHough_param1_aCannyHiTreshold,         255, nothing_asCallback )
    cv2.createTrackbar( "param2_CentreDetect",  "DEMO.Canny.Circles",  aHough_param2_aCentreDetectTreshold,    255, nothing_asCallback )
    cv2.createTrackbar( "minRadius",            "DEMO.Canny.Circles",  aHough_minRadius,                       255, nothing_asCallback )
    cv2.createTrackbar( "maxRadius",            "DEMO.Canny.Circles",  aHough_maxRadius,                       255, nothing_asCallback )

    cv2.imshow( "DEMO.IN",          demo )                              # static ...
    # --------------------------------------------------------------------------------GUI-mainloop()
    print " --------------------------------------------------------------------------- press [ESC] to exit "
    while( True ):
        # --------------------------------------------------------------------------------GUI-[ESCAPE]?
        if aKeyPRESSED == 27:
            break
        # --------------------------------------------------------------------------------<vars>-DETECT-delta(s)
        aCanny_LoTreshold = cv2.getTrackbarPos( "Lo_Treshold", "DEMO.Canny" )
        aCanny_HiTreshold = cv2.getTrackbarPos( "Hi_Treshold", "DEMO.Canny" )

        if (    aCanny_LoTreshold      != aCanny_LoTreshold_PREVIOUS
            or  aCanny_HiTreshold      != aCanny_HiTreshold_PREVIOUS
            ):
            # --------------------------= FLAG
            aCannyRefreshFLAG           = True
            # --------------------------= RE-SYNC
            aCanny_LoTreshold_PREVIOUS  = aCanny_LoTreshold
            aCanny_HiTreshold_PREVIOUS  = aCanny_HiTreshold
        else:
            # --------------------------= Un-FLAG
            aCannyRefreshFLAG           = False

        aHough_dp                           = cv2.getTrackbarPos( "dp",                 "DEMO.Canny.Circles" )
        aHough_minDistance                  = cv2.getTrackbarPos( "minDistance",        "DEMO.Canny.Circles" )
        aHough_param1_aCannyHiTreshold      = cv2.getTrackbarPos( "param1_HiTreshold",  "DEMO.Canny.Circles" )
        aHough_param2_aCentreDetectTreshold = cv2.getTrackbarPos( "param2_CentreDetect","DEMO.Canny.Circles" )
        aHough_minRadius                    = cv2.getTrackbarPos( "minRadius",          "DEMO.Canny.Circles" )
        aHough_maxRadius                    = cv2.getTrackbarPos( "maxRadius",          "DEMO.Canny.Circles" )

        if (    aHough_dp                            != aHough_dp_PREVIOUS
            or  aHough_minDistance                   != aHough_minDistance_PREVIOUS
            or  aHough_param1_aCannyHiTreshold       != aHough_param1_aCannyHiTreshold_PREVIOUS
            or  aHough_param2_aCentreDetectTreshold  != aHough_param2_aCentreDetectTreshold_PREVIOUS    
            or  aHough_minRadius                     != aHough_minRadius_PREVIOUS
            or  aHough_maxRadius                     != aHough_maxRadius_PREVIOUS
            ):
            # --------------------------= FLAG
            aHoughRefreshFLAG           = True                  
            # ----------------------------------------------= RE-SYNC
            aHough_dp_PREVIOUS                              =  aHough_dp                          
            aHough_minDistance_PREVIOUS                     =  aHough_minDistance                 
            aHough_param1_aCannyHiTreshold_PREVIOUS         =  aHough_param1_aCannyHiTreshold     
            aHough_param2_aCentreDetectTreshold_PREVIOUS    =  aHough_param2_aCentreDetectTreshold
            aHough_minRadius_PREVIOUS                       =  aHough_minRadius                   
            aHough_maxRadius_PREVIOUS                       =  aHough_maxRadius                   
        else:
            # --------------------------= Un-FLAG
            aHoughRefreshFLAG           = False
        # --------------------------------------------------------------------------------REFRESH-process-pipe-line ( with recent <state> <vars> )
        if ( aCannyRefreshFLAG ):

            edges   = cv2.Canny(        demo,   aCanny_LoTreshold,
                                                aCanny_HiTreshold
                                        )
            # --------------------------------------------------------------------------------GUI-SHOW-Canny()-<edges>-onRefreshFLAG
            cv2.imshow( "DEMO.Canny",   edges )
            pass

        if ( aCannyRefreshFLAG or aHoughRefreshFLAG ):

            circles = cv2.HoughCircles( edges,  cv2.cv.CV_HOUGH_GRADIENT,
                                                aHough_dp,
                                                aHough_minDistance,
                                                param1      = aHough_param1_aCannyHiTreshold,
                                                param2      = aHough_param2_aCentreDetectTreshold,
                                                minRadius   = aHough_minRadius,
                                                maxRadius   = aHough_maxRadius
                                        )
            # --------------------------------------------------------------------------------GUI-SHOW-HoughCircles()-<edges>-onRefreshFLAG
            demoWithCircles = cv2.cvtColor( demo,            cv2.COLOR_BGR2RGB )                          # .re-init <<< src
            demoWithCircles = cv2.cvtColor( demoWithCircles, cv2.COLOR_RGB2BGR )

            for aCircle in circles[0]:
                cv2.circle( demoWithCircles,    ( int( aCircle[0] ), int( aCircle[1] ) ),
                                                aCircle[2],
                                                (0,255,0),
                                                1
                            )
                pass
            pass
            cv2.imshow( "DEMO.Canny.Circles", demoWithCircles )
        pass        
        # --------------------------------------------------------------------------------<vars>-UPDATE-<state>
        # ref. above in .onRefreshFLAG RE-SYNC sections
        # --------------------------------------------------------------------------------GUI-INPUT ? [ESCAPE]
        aKeyPRESSED = cv2.waitKey(1) & 0xFF
    pass
    # --------------------------------------------------------------------------------GUI-<window>-s / DESTROY
    cv2.destroyWindow( "DEMO.IN" )
    cv2.destroyWindow( "DEMO.Canny" )
    cv2.destroyWindow( "DEMO.Canny.Circles" )
    # --------------------------------------------------------------------------------GUI-<window>-s
    pass

def main():
    GUI_openCV_circles()
    return 0

if __name__ == '__main__':
    main()
user3666197
  • 1
  • 6
  • 50
  • 92
  • 1
    I'm going to accept this answer just because of that quote -- also because it's the only answer and probably will show me what dp is when I get to copying and pasting this code. – PGT Sep 03 '14 at 01:46
  • @PGT :o) nice -- *Seen once, makes more than hundred reads*. **OpenCV *RefMan*** says: `dp` – Inverse ratio of the accumulator resolution to the image resolution. For example, if `dp=1` , the accumulator has the same resolution as the input image. If `dp=2` , the accumulator has half as big width and height. – user3666197 Sep 03 '14 at 02:21
  • @user3666197 any source you know of that explains/details on the accumulator? – ibininja Sep 17 '16 at 05:46