Translate coordinates between coordinate systems opencv python

Question

The corner coordinates of square_1 = (0, 0, 1920, 1080). I then define square_2 as a smaller ROI within square one using numpy slicing like so roi = square_1[y1:y2, x1:x2]. I then resize square_1 using square_resize = cv2.resize(square_1, (960, 540), interpolation = cv2.INTER_AREA) . However, now my ROI is no longer accurate. I have a tool which tells me the screen coords of the mouse pos, which is how I find the dimensions of the ROI, but I need a function that translates the ROI coordinates I find, given the coordinates of square_1, in terms of the coordinates of square_resize.

EDIT: Solved using Panda50's answer. grab_screen() is my own custom function for getting screenshots. Here is my code if it helps anyone. It does not give 100% accurate coords but you can play around some and narrow it down.

from cv2 import cv2
import numpy as np

y1 = int(92 / 2)
y2 = int(491 / 2)
x1 = int(233 / 2)
x2 = int(858 / 2)

# grab screen and convert to RGB
screen = grab_screen(region = (0, 0, 1920, 1080))
screen = cv2.cvtColor(screen, cv2.COLOR_BGR2RGB)

# resize screen
screen = cv2.resize(screen, (960, 540), interpolation = cv2.INTER_AREA)

# define ROI
roi = screen[y1:y2, x1:x2].copy()

cv2.imshow('roi', roi)

cv2.waitKey()
cv2.destroyAllWindows()

Please see this recent answer from me. It's about adapting bounding boxes while resizing the parent image, but it's the same for adapting ROIs while resizing the parent image. — HansHirse
– HansHirse, Commented Jan 27, 2021 at 9:58

Panda50 · Accepted Answer · 2021-01-26 18:02:32Z

1

In python, = associate one variable with another. By changing square_1 you'll also change roi .

You have to use :

roi = square_1[y1:y2, x1:x2].copy()

answered Jan 26, 2021 at 18:02

Panda50

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