如何在Python中为OpenCV minMaxLoc函数创建Mask矩阵变量？

Question

I am using OpenCV in Python.

I am trying to create a Mask Matrix variable to be used in this function: cv.minMaxLoc . The Matrix variable should have the same size like my template image, with type = CV_8UC1 .

My template image has a alpha channel which consists of only 0% or 100% transparency pixels. How can I extract the alpha channel data from my image and put it into the Mask Matrix with 0 = 100% transparency and 1 = 0% transparency ?

Answer 1

import numpy as np
import cv
from PIL import Image

# open the image
img = Image.open('./pic.png', 'r')

r,g,b, alpha_channel = img.split()

mask = np.array(alpha_channel)

# all elements in alpha_channel that have value 0 
# are set to 1 in the mask matrix
mask[alpha_channel==0] = 1

# all elements in alpha_channel that have value 100
# are set to 0 in the mask matrix
mask[alpha_channel==100] = 0

credit to other posts: How to get alpha value of a PNG image with PIL? Converting numpy array having image data to CvMat

To convert numpy array to cvmat do:

cv_arr = cv.fromarray(mask)

Check the dtype of mask. It should be dtype('uint8') When I convert it my cv_arr is cvmat(type=42424000 8UC1 rows=1245 cols=2400 step=2400 )

I am not an expert in opencv, but it seems to me that 8UC1 is automatically selected based on the fact that dtype is uint8 (I am guessing here because I couldn't find the documentation about that).