Generate colored histogram around contour

Hey OpenCV / Emgu guru,

I have an image that I am creating an outline for, see below. I am trying to create a color histogram based clipping of an image search space for search. How can I get a mask around just the selected outline of an object and block the rest. So I have 2 questions:

  • How do I "invert" an image outside the outline? FloodFill invert, don't? I am confused with all the options in OpenCV.

  • Second, how do I create a 1-dimensional color histogram from the outline of an object in this case a red car to exclude the black background and generate only a color histogram that includes the car.

How can I do this in OpenCV (preferably in Emgu / C # code)?

Source ImageContoured Image


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1 answer

Perhaps something like this? Done using Python bindings, but easy to translate methods into other bindings ...


import cv 
import colorsys

# get orginal image
orig = cv.LoadImage('car.jpg')

# show orginal 
cv.ShowImage("orig", orig)

# get mask image
maskimg = cv.LoadImage('carcontour.jpg')

# split original image into hue and value
hsv = cv.CreateImage(cv.GetSize(orig),8,3)
hue = cv.CreateImage(cv.GetSize(orig),8,1)
val = cv.CreateImage(cv.GetSize(orig),8,1)
cv.Split(hsv, hue, None, val, None)

# build mask from val image, select values NOT black
mask = cv.CreateImage(cv.GetSize(orig),8,1)

# show the mask
cv.ShowImage("mask", mask)

# calculate colour (hue) histgram of only masked area
hue_bins = 180 
hue_range = [0,180]
hist = cv.CreateHist([hue_bins], cv.CV_HIST_ARRAY, [hue_range], 1) 

# create the colour histogram 
(_, max_value, _, _) = cv.GetMinMaxHistValue(hist)
histimg = cv.CreateImage((hue_bins*2, 200), 8, 3) 
for h in range(hue_bins):
  bin_val = cv.QueryHistValue_1D(hist,h)
  norm_val = cv.Round((bin_val/max_value)*200)
  rgb_val = colorsys.hsv_to_rgb(float(h)/180.0,1.0,1.0) 
                ((h+1)*2-1, norm_val),

# wait for key press


This is a bit awkward mask detection - interesting, perhaps because of the JPEG compression artifacts in the image ... If you had the original path, simply "render" that mask instead.


An example of a histogram rendering function is also basic, but I think it shows the idea (and how the car is predominantly red!). Note that OpenCV's interpretation of Hue only varies from [0-180] degrees.


EDIT: if you want to use a mask to count colors in the original image - edit it like this from line 15 down:

# split original image into hue
hsv = cv.CreateImage(cv.GetSize(orig),8,3)
hue = cv.CreateImage(cv.GetSize(orig),8,1)
cv.Split(hsv, hue, None, None, None)

# split mask image into val
val = cv.CreateImage(cv.GetSize(orig),8,1)
cv.Split(hsv, None, None, val, None)


(I think this is more than what was intended, since then the mask is output separately and applied to a completely different image. The histogram is about the same in both cases ...)



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