colors - Shifting HSV pixel values in python using Numpy

Question

I'm trying to convert (shift) the values of every pixel in an HSV image (taken from a frame of a video).

The idea is to invert yellow and red colours into blue colour (to avoid using three threshold later in the program, when I can use just one) by inverting the red and yellow values into blue values using following equation.

(Hue + 90) % 180 (in OpenCV 3 Hue is in range [0,180])

Here's what I came up with:

hsv = cv2.cvtColor(frame, cv2.COLOR_BGR2HSV);
H = hsv[:,:,0]
mask= [H<75 and H>128]
print("orig",hsv[mask])
hsv[mask] = ((hsv[mask]+90) % 180)

Unfortunately It doesn't work as by this approach Im selecting the whole hue channel not its pixel values

Answer 1

There's two different possibilities here, and I'm not sure which you want, but they're both trivial to implement. You can invert (reverse may be a better word) the hue rainbow, which you can just do by using 180 - hue. Or you can shift the color by 180 degrees by using (hue + 90) % 180 like you mention.

Reversing the colors:

hsv = cv2.cvtColor(img, cv2.COLOR_BGR2HSV)
h, s, v = cv2.split(hsv)
rev_h = 180 - h
rev_hsv = cv2.merge([rev_h, s, v])
rev_img = cv2.cvtColor(rev_hsv, cv2.COLOR_HSV2BGR)

Shifting the colors:

hsv = cv2.cvtColor(img, cv2.COLOR_BGR2HSV)
h, s, v = cv2.split(hsv)
shift_h = (h + 90) % 180
shift_hsv = cv2.merge([shift_h, s, v])
shift_img = cv2.cvtColor(shift_hsv, cv2.COLOR_HSV2BGR)

Those are the idiomatic ways to do it in OpenCV.

Now you want to do the same thing as above but only for some masked subset of pixels that meet a condition. This is not too hard to do; if you want to shift some masked pixels:

hsv = cv2.cvtColor(img, cv2.COLOR_BGR2HSV)
h, s, v = cv2.split(hsv)
h_mask = (h < 75) | (h > 128)
h[h_mask] = (h[h_mask] + 90) % 180
shift_hsv = cv2.merge([h, s, v])
shift_img = cv2.cvtColor(shift_hsv, cv2.COLOR_HSV2BGR)