python - NumPy/OpenCV 2：如何裁剪非矩形区域？

Question

我有一组形成形状的点（闭合折线）。现在我想从这个形状内的某个图像中复制/裁剪所有像素，其余部分为黑色/透明。我该怎么做呢？

例如，我有这个：

我想得到这个：

Answer 1

*edit - 更新为使用具有 Alpha 通道的图像。

这对我有用：

• 制作一个全黑的蒙版（全部蒙版）
• 用 ROI 形状的白色填充多边形
• 将蒙版和您的图像结合起来，在其他地方获得黑色的 ROI

对于接受掩码的函数，您可能只想将图像和掩码分开。但是，我相信这可以满足您的具体要求：

import cv2
import numpy as np

# original image
# -1 loads as-is so if it will be 3 or 4 channel as the original
image = cv2.imread('image.png', -1)
# mask defaulting to black for 3-channel and transparent for 4-channel
# (of course replace corners with yours)
mask = np.zeros(image.shape, dtype=np.uint8)
roi_corners = np.array([[(10,10), (300,300), (10,300)]], dtype=np.int32)
# fill the ROI so it doesn't get wiped out when the mask is applied
channel_count = image.shape[2]  # i.e. 3 or 4 depending on your image
ignore_mask_color = (255,)*channel_count
cv2.fillPoly(mask, roi_corners, ignore_mask_color)
# from Masterfool: use cv2.fillConvexPoly if you know it's convex

# apply the mask
masked_image = cv2.bitwise_and(image, mask)

# save the result
cv2.imwrite('image_masked.png', masked_image)

Answer 2

以下代码有助于裁剪图像并将它们置于白色背景中。

import cv2
import numpy as np

# load the image
image_path = 'input image path'
image = cv2.imread(image_path)

# create a mask with white pixels
mask = np.ones(image.shape, dtype=np.uint8)
mask.fill(255)

# points to be cropped
roi_corners = np.array([[(0, 300), (1880, 300), (1880, 400), (0, 400)]], dtype=np.int32)
# fill the ROI into the mask
cv2.fillPoly(mask, roi_corners, 0)

# The mask image
cv2.imwrite('image_masked.png', mask)

# applying th mask to original image
masked_image = cv2.bitwise_or(image, mask)

# The resultant image
cv2.imwrite('new_masked_image.png', masked_image)

输入图像：

蒙版图片：

结果输出图像：