使用 numpy 将数据从一个通道交换到另一个通道

Question

我正在研究如何将通道从 BGR 更改为 RGB，结果出现了这个问题。这可行，但我对这种语法感到困惑。这种类型的数据交换在 numpy 中到底是如何工作的？

要点代码：

rgb = bgr[...,::-1]

Answer 1

我不是 Numpy 及其操作名称的专家，但我可以向您展示如何使用各种切片（索引？）技术来进行一些图像处理。

一般来说，在 RGB 图像上，操作以逗号分隔，如下所示：

newImage = oldImage[ROWSTUFF, COLUMNSTUFF, CHANNELSTUFF]

其中 ROWSTUFF、COLUMNSTUFF 和 CHANNELSTUFF 分别由以下部分组成：

start:end:step

那么，让我们对这张图像进行一些处理：

# Load image with PIL/Pillow and make Numpy array - you can equally use OpenCV imread(), or other libraries
im = np.array(Image.open('start.png').convert('RGB'))                                           

# im.shape is (400, 400, 3)

# Now extract top half by ending ROWSTUFF at 200
tophalf = im[:200,:,:]

# Now extract bottom half by starting ROWSTUFF at 200
bottomhalf = im[200:,:,:] 

# Now extract left half by ending ROWSTUFF at 200
lefthalf = im[:,:200,:]

# Now extract right half by starting ROWSTUFF at 200
righthalf = im[:,200:,:]  

# Now scale the image by taking only every 4th row and every second column:
scaled = im[::4,::2,:]

# Now extract Red channel, by setting CHANNELSTUFF to 0
red = im[:,:,0]

# Now extract Green channel, by setting CHANNELSTUFF to 1
green = im[:,:,1] 

# Now flop the image top to bottom by striding backwards through ROWSTUFF
flop = im[::-1,:,:]

# Now flip the image left to right by striding backwards through COLUMNSTUFF
flip = im[:,::-1,:]  

# And finally, like the question, reverse the channels by striding through CHANNELSTUFF backwards, which will make RGB -> BGR, thereby leaving Green and black unchanged
OP = im[:,:,::-1]  

然后意识到这是“保留未指定的尺寸不变”...的简写，所以

[:,:,:,:, a:b:c] can be written as [..., a:b:c]

和

[a:b:c, :,:,:,:,:] can be written as [a:b:c, ...]

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