python - 如何在 BGR 图像中添加特定值的 Alpha channel

Question

我尝试了下面的代码，它没有显示任何错误并且运行正常，但是更改 alpha channel 的值，并没有显示图像的任何变化

img3 = cv2.cvtColor(img2, cv2.COLOR_BGR2BGRA)
img3[:,:,3] = 100
cv2.imshow('img1',img2)    
cv2.imshow('img',img3)     
cv2.waitKey(0)

工作正常，但两个图像的输出是相同的，应用 alpha channel 后没有可见的变化

我已经尝试过下面的代码

Answer 1

您的代码实际上是正确的。

简单的答案是 OpenCV 的 imshow()忽略透明度，因此如果您想查看其效果，请将您的图像保存为 PNG/TIFF(两者都支持透明度)并使用不同的查看器查看它 - 例如 GIMP , Photoshop 或 feh .

作为替代方案，我为 OpenCV 的 imshow() 制作了一个包装器/装饰器。像 Photoshop 一样显示覆盖在棋盘上的透明图像。所以，从这张 RGBA Paddington 图像和这张 gray+alpha Paddington 图像开始:

#!/usr/bin/env python3

import cv2
import numpy as np

def imshow(title,im):
    """Decorator for OpenCV "imshow()" to handle images with transparency"""

    # Check we got np.uint8, 2-channel (grey + alpha) or 4-channel RGBA image
    if (im.dtype == np.uint8) and (len(im.shape)==3) and (im.shape[2] in set([2,4])):

       # Pick up the alpha channel and delete from original
       alpha = im[...,-1]/255.0
       im = np.delete(im, -1, -1)

       # Promote greyscale image to RGB to make coding simpler
       if len(im.shape) == 2:
          im = np.stack((im,im,im))

       h, w, _ = im.shape

       # Make a checkerboard background image same size, dark squares are grey(102), light squares are grey(152)
       f = lambda i, j: 102 + 50*((i+j)%2)
       bg = np.fromfunction(np.vectorize(f), (16,16)).astype(np.uint8)

       # Resize to square same length as longer side (so squares stay square), then trim
       if h>w:
          longer = h
       else:
          longer = w
       bg = cv2.resize(bg, (longer,longer), interpolation=cv2.INTER_NEAREST)
       # Trim to correct size
       bg = bg[:h,:w]

       # Blend, using result = alpha*overlay + (1-alpha)*background
       im = (alpha[...,None] * im + (1.0-alpha[...,None])*bg[...,None]).astype(np.uint8)

    cv2.imshow(title,im)


if __name__ == "__main__":

    # Open RGBA image
    im = cv2.imread('paddington.png',cv2.IMREAD_UNCHANGED)

    imshow("Paddington (RGBA)",im)
    key = cv2.waitKey(0)
    cv2.destroyAllWindows()

    # Open Grey + alpha  image
    im = cv2.imread('paddington-ga.png',cv2.IMREAD_UNCHANGED)

    imshow("Paddington (grey + alpha)",im)
    key = cv2.waitKey(0)
    cv2.destroyAllWindows()

你会得到这个:



还有这个:



关键词 :图像、图像处理、Python、alpha channel 、透明度、叠加、棋盘、棋盘、混合、混合。 OpenCV、imshow、cv2.imshow。