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cvInRangeS method for HSV colorspace is used like:

cvInRangeS(imgHSV, cvScalar( hMin,sMin,vMin),cvScalar(hMax,sMax, vMax),imgThresh);

For hue value we generally use a range between two numbers, a pair of small and big one like $(10-50)$, which means $10,11,12...48,49,50$ .

Sometimes I need to use a reverse range for red colors. By "reverse range", I mean a range between a big and a small number like $(170-10)$ which loops at the value of $180$: $170,171,172 ... 179,180,0,1,2,3 ... 8,9,10$).

How do I adjust this reverse range to cvInRangeS method? Can anyone explain how to make this work?

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1 Answer

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You can generate two different threshold images, one with hue in the range from 0 to 10 and the other with hue from 170 to 180. To get an image with hue in range both from 0 to 10 and 170 to 180, just add the two threshold images together.

In C++ code this would look something like:

inRange(imgHSV, Scalar(0, sMin, vMin), Scalar(10, sMax, vMax), imgThresh);
inRange(imgHSV, Scalar(170, sMin, vMin), Scalar(180, sMax, vMax), imgThresh1);

Mat imgThreshCombined = imgThresh + imgThresh1;

You could probably do the same thing by finding the threshold image with hue in the range from 10 to 170 and then just subtracting it from a white image.

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Thank you, i have just used this method. It works perfect, but i'm worried because i think using double inRange methods per frame effects my program's efficiency in bad way. Because of this, I am looking for new solutions. – gurcan15 Nov 28 '12 at 14:05
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Did you try the other method where you threshold the image with hMin = 10 and hMax = 170 and then subtract it from a white image? It might be faster, I'm not sure. – ppalasek Nov 28 '12 at 14:15
@gurcan15 Did you measure the slow-down? I wouldn't imagine it being so slow as you say. Threshold with (10, 170) and then subtraction should be the fastest you can get with built-in inRange method. One image pass for inRange and one for subtraction (vs. 2 for inRange and 1 for addition). The only faster alternative I can think of is to implement your own version of inRange or inRangeHSV which will loop ranges on it's own. – penelope Nov 28 '12 at 14:40
@penelope I didn't measure the slow-down, I just think, it should be slower. Subtraction is alternative, I will try it. And I think it will be faster like you said. – gurcan15 Nov 28 '12 at 14:46
@gurcan15 I think the slowest part of this approach is traversing the image pixels. For a normal (10-70) range you just do it once. Proposed way does it trice, while the way with the subtraction would traverse the image twice. BUT, if going from trice to twice offers a substantial speedup (for your needs), you should seriously consider making your own implementation: the speedup from twice to once should be substantial if that's the case. – penelope Nov 28 '12 at 14:56

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