處理圖片中的多角形

原本的需求是想知道圖片中一個4角形的內容值,剛好看到一個實用的範例。似乎 OpenCV 和 numpy 變成必學的項目了,好多專案都在使用。


How to get pixel values inside of a rectangle within an image
https://stackoverflow.com/questions/58790535/how-to-get-pixel-values-inside-of-a-rectangle-within-an-image

You can do that with Python/OpenCV by first drawing a white filled polygon on black background as a mask from your four points. The use np.where to locate and then print all the points in the image corresponding to the white pixels in the mask.

Input:

import cv2
import numpy as np

# read image
image = cv2.imread('lena.png')

# create mask with zeros
mask = np.zeros((image.shape), dtype=np.uint8)

# define points (as small diamond shape)
pts = np.array( [[[25,20],[30,25],[25,30],[20,25]]], dtype=np.int32 )
cv2.fillPoly(mask, pts, (255,255,255) )

# get color values
values = image[np.where((mask == (255,255,255)).all(axis=2))]
print(values)

# save mask
cv2.imwrite('diamond_mask.png', mask)

cv2.imshow('image', image)
cv2.imshow('mask', mask)
cv2.waitKey()


Mask:

Results:

 [[108 137 232]
 [104 134 232]
 [108 136 231]
 [106 134 231]
 [109 133 228]
 [108 136 229]
 [109 137 230]
 [110 135 232]
 [103 126 230]
 [112 134 228]
 [114 136 228]
 [111 138 230]
 [110 137 233]
 [103 135 234]
 [103 126 230]
 [101 120 226]
 [108 137 230]
 [112 133 228]
 [114 136 227]
 [115 139 232]
 [112 137 232]
 [105 134 233]
 [102 128 232]
 [ 98 119 226]
 [ 93 105 220]
 [108 139 230]
 [110 137 230]
 [112 135 230]
 [113 135 230]
 [111 138 231]
 [112 139 232]
 [109 134 233]
 [101 128 232]
 [100 120 224]
 [ 90 104 221]
 [ 87  95 211]
 [111 138 229]
 [109 135 231]
 [109 136 230]
 [113 141 233]
 [110 139 233]
 [105 136 234]
 [101 127 232]
 [ 95 117 225]
 [ 90 107 220]
 [110 137 231]
 [110 138 231]
 [107 140 236]
 [110 139 233]
 [104 135 234]
 [105 130 231]
 [ 92 116 227]
 [114 141 234]
 [112 142 235]
 [111 140 235]
 [111 138 234]
 [110 132 232]
 [114 140 234]
 [108 140 233]
 [107 134 233]
 [107 140 235]]

換成 PIL 寫法:

How to read an image file as ndarray

from PIL import Image
import numpy as np

im = np.array(Image.open('data/src/lena_square.png'))

print(im.dtype)
# uint8

print(im.ndim)
# 3

print(im.shape)
# (512, 512, 3)

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