使用OpenCV裁剪黑色边缘
image-processing
opencv
6
0

我认为这应该是一个非常简单的问题，但是我找不到解决方案或有效的搜索关键字。

我只有这张图片。

原始图像

黑色边缘是无用的，因此我想剪切它们，只留下Windows图标（和蓝色背景）。

我不想计算Windows图标的坐标和大小。 GIMP和Photoshop具有自动裁剪功能。 OpenCV没有一个？

参考资料：
Stack Overflow
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共 4 个回答
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好的，为了完整起见，我实现了上述每个建议，添加了递归算法的迭代版本（一旦更正），并进行了一组性能测试。

TLDR：对于一般情况，递归可能是最好的（但请使用以下示例-OP有几个错误），而自动裁剪对于希望几乎为空的图像是最佳的。

总体发现：1.上面的递归算法中有一些偏离1的错误。更正的版本如下。 2. cv2.findContours函数对于非矩形图像有问题，实际上在各种情况下甚至会修剪掉一些图像。我添加了一个使用cv2.CHAIN_APPROX_NONE的版本，以查看是否有帮助（无济于事）。 3. autocrop实现对于稀疏图像非常有用，但对于密集图像则较差，这是递归/迭代算法的逆过程。 

import numpy as np
import cv2

def trim_recursive(frame):
  if frame.shape[0] == 0:
    return np.zeros((0,0,3))

  # crop top
  if not np.sum(frame[0]):
    return trim_recursive(frame[1:])
  # crop bottom
  elif not np.sum(frame[-1]):
    return trim_recursive(frame[:-1])
  # crop left
  elif not np.sum(frame[:, 0]):
    return trim_recursive(frame[:, 1:])
    # crop right
  elif not np.sum(frame[:, -1]):
    return trim_recursive(frame[:, :-1])
  return frame

def trim_contours(frame):
  gray = cv2.cvtColor(frame,cv2.COLOR_BGR2GRAY)
  _,thresh = cv2.threshold(gray,1,255,cv2.THRESH_BINARY)
  _, contours, hierarchy = cv2.findContours(thresh, cv2.RETR_EXTERNAL, cv2.CHAIN_APPROX_SIMPLE)
  if len(contours) == 0:
    return np.zeros((0,0,3))
  cnt = contours[0]
  x, y, w, h = cv2.boundingRect(cnt)
  crop = frame[y:y + h, x:x + w]
  return crop

def trim_contours_exact(frame):
  gray = cv2.cvtColor(frame,cv2.COLOR_BGR2GRAY)
  _,thresh = cv2.threshold(gray,1,255,cv2.THRESH_BINARY)
  _, contours, hierarchy = cv2.findContours(thresh, cv2.RETR_EXTERNAL, cv2.CHAIN_APPROX_NONE)
  if len(contours) == 0:
    return np.zeros((0,0,3))
  cnt = contours[0]
  x, y, w, h = cv2.boundingRect(cnt)
  crop = frame[y:y + h, x:x + w]
  return crop

def trim_iterative(frame):
  for start_y in range(1, frame.shape[0]):
    if np.sum(frame[:start_y]) > 0:
      start_y -= 1
      break
  if start_y == frame.shape[0]:
    if len(frame.shape) == 2:
      return np.zeros((0,0))
    else:
      return np.zeros((0,0,0))
  for trim_bottom in range(1, frame.shape[0]):
    if np.sum(frame[-trim_bottom:]) > 0:
      break

  for start_x in range(1, frame.shape[1]):
    if np.sum(frame[:, :start_x]) > 0:
      start_x -= 1
      break
  for trim_right in range(1, frame.shape[1]):
    if np.sum(frame[:, -trim_right:]) > 0:
      break

  end_y = frame.shape[0] - trim_bottom + 1
  end_x = frame.shape[1] - trim_right + 1

  # print('iterative cropping x:{}, w:{}, y:{}, h:{}'.format(start_x, end_x - start_x, start_y, end_y - start_y))
  return frame[start_y:end_y, start_x:end_x]

def autocrop(image, threshold=0):
  """Crops any edges below or equal to threshold

  Crops blank image to 1x1.

  Returns cropped image.

  """
  if len(image.shape) == 3:
    flatImage = np.max(image, 2)
  else:
    flatImage = image
  assert len(flatImage.shape) == 2

  rows = np.where(np.max(flatImage, 0) > threshold)[0]
  if rows.size:
    cols = np.where(np.max(flatImage, 1) > threshold)[0]
    image = image[cols[0]: cols[-1] + 1, rows[0]: rows[-1] + 1]
  else:
    image = image[:1, :1]

  return image

然后进行测试，我做了一个简单的功能： 

import datetime
import numpy as np
import random

ITERATIONS = 10000

def test_image(img):
  orig_shape = img.shape
  print ('original shape: {}'.format(orig_shape))
  start_time = datetime.datetime.now()
  for i in range(ITERATIONS):
    recursive_img = trim_recursive(img)
  print ('recursive shape: {}, took {} seconds'.format(recursive_img.shape, (datetime.datetime.now()-start_time).total_seconds()))
  start_time = datetime.datetime.now()
  for i in range(ITERATIONS):
    contour_img = trim_contours(img)
  print ('contour shape: {}, took {} seconds'.format(contour_img.shape, (datetime.datetime.now()-start_time).total_seconds()))
  start_time = datetime.datetime.now()
  for i in range(ITERATIONS):
    exact_contour_img = trim_contours(img)
  print ('exact contour shape: {}, took {} seconds'.format(exact_contour_img.shape, (datetime.datetime.now()-start_time).total_seconds()))
  start_time = datetime.datetime.now()
  for i in range(ITERATIONS):
    iterative_img = trim_iterative(img)
  print ('iterative shape: {}, took {} seconds'.format(iterative_img.shape, (datetime.datetime.now()-start_time).total_seconds()))
  start_time = datetime.datetime.now()
  for i in range(ITERATIONS):
    auto_img = autocrop(img)
  print ('autocrop shape: {}, took {} seconds'.format(auto_img.shape, (datetime.datetime.now()-start_time).total_seconds()))


def main():
  orig_shape = (10,10,3)

  print('Empty image--should be 0x0x3')
  zero_img = np.zeros(orig_shape, dtype='uint8')
  test_image(zero_img)

  print('Small image--should be 1x1x3')
  small_img = np.zeros(orig_shape, dtype='uint8')
  small_img[3,3] = 1
  test_image(small_img)

  print('Medium image--should be 3x7x3')
  med_img = np.zeros(orig_shape, dtype='uint8')
  med_img[5:8, 2:9] = 1
  test_image(med_img)

  print('Random image--should be full image: 100x100')
  lg_img = np.zeros((100,100,3), dtype='uint8')
  for y in range (100):
    for x in range(100):
      lg_img[y,x, 0] = random.randint(0,255)
      lg_img[y, x, 1] = random.randint(0, 255)
      lg_img[y, x, 2] = random.randint(0, 255)
  test_image(lg_img)

main()

...结果

Empty image--should be 0x0x3
original shape: (10, 10, 3)
recursive shape: (0, 0, 3), took 0.295851 seconds
contour shape: (0, 0, 3), took 0.048656 seconds
exact contour shape: (0, 0, 3), took 0.046273 seconds
iterative shape: (0, 0, 3), took 1.742498 seconds
autocrop shape: (1, 1, 3), took 0.093347 seconds
Small image--should be 1x1x3
original shape: (10, 10, 3)
recursive shape: (1, 1, 3), took 1.342977 seconds
contour shape: (0, 0, 3), took 0.048919 seconds
exact contour shape: (0, 0, 3), took 0.04683 seconds
iterative shape: (1, 1, 3), took 1.084258 seconds
autocrop shape: (1, 1, 3), took 0.140886 seconds
Medium image--should be 3x7x3
original shape: (10, 10, 3)
recursive shape: (3, 7, 3), took 0.610821 seconds
contour shape: (0, 0, 3), took 0.047263 seconds
exact contour shape: (0, 0, 3), took 0.046342 seconds
iterative shape: (3, 7, 3), took 0.696778 seconds
autocrop shape: (3, 7, 3), took 0.14493 seconds
Random image--should be full image: 100x100
original shape: (100, 100, 3)
recursive shape: (100, 100, 3), took 0.131619 seconds
contour shape: (98, 98, 3), took 0.285515 seconds
exact contour shape: (98, 98, 3), took 0.288365 seconds
iterative shape: (100, 100, 3), took 0.251708 seconds
autocrop shape: (100, 100, 3), took 1.280476 seconds
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我不确定您所有的图像是否都是这样。但是对于这张图片，下面是一个简单的python-opencv代码来裁剪它。

首次导入库： 

import cv2
import numpy as np

读取图像，将其转换为灰度，然后将二进制图像制成阈值1。 

img = cv2.imread('sofwin.png')
gray = cv2.cvtColor(img,cv2.COLOR_BGR2GRAY)
_,thresh = cv2.threshold(gray,1,255,cv2.THRESH_BINARY)

现在在其中找到轮廓。将只有一个对象，因此请为其找到边界矩形。 

contours,hierarchy = cv2.findContours(thresh,cv2.RETR_EXTERNAL,cv2.CHAIN_APPROX_SIMPLE)
cnt = contours[0]
x,y,w,h = cv2.boundingRect(cnt)

现在裁剪图像，并将其保存到另一个文件中。 

crop = img[y:y+h,x:x+w]
cv2.imwrite('sofwinres.png',crop)

结果如下：

在此处输入图片说明

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import numpy as np

def autocrop(image, threshold=0):
    """Crops any edges below or equal to threshold

    Crops blank image to 1x1.

    Returns cropped image.

    """
    if len(image.shape) == 3:
        flatImage = np.max(image, 2)
    else:
        flatImage = image
    assert len(flatImage.shape) == 2

    rows = np.where(np.max(flatImage, 0) > threshold)[0]
    if rows.size:
        cols = np.where(np.max(flatImage, 1) > threshold)[0]
        image = image[cols[0]: cols[-1] + 1, rows[0]: rows[-1] + 1]
    else:
        image = image[:1, :1]

    return image
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我认为这个答案要简洁得多： 

def crop(image):
    y_nonzero, x_nonzero, _ = np.nonzero(image)
    return image[np.min(y_nonzero):np.max(y_nonzero), np.min(x_nonzero):np.max(x_nonzero)]
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