image - How to convert a grayscale matrix to an RGB matrix in MATLAB?

Question

rgbImage = grayImage / max(max(grayImage));

or

rgbImage = grayImage / 255;

Which of the above is right,and reason?

Answer 1

要将灰度图像转换为RGB 图像，您必须解决两个问题：

• 灰度图像是2-D，而RGB 图像是 3-D，因此您必须将灰度图像数据复制三次，然后将三个副本沿三维连接。
• 图像数据可以存储在许多不同的数据类型中，因此您必须对它们进行相应的转换。作为double数据类型存储时，图像像素值应为 0 到 1 范围内的浮点数。作为uint8数据类型存储时，图像像素值应为 0 到 255 范围内的整数。您可以查看使用函数的图像矩阵的数据类型class。

以下是您可能会遇到的 3 种典型情况：

• 要将灰度图像uint8或double灰度图像转换为相同数据类型的 RGB 图像，可以使用函数repmat或cat：

  rgbImage = repmat(grayImage,[1 1 3]);
  rgbImage = cat(3,grayImage,grayImage,grayImage);
  

• 要将uint8灰度图像转换为doubleRGB 图像，应double先转换为，然后按 255 缩放：

  rgbImage = repmat(double(grayImage)./255,[1 1 3]);
  

• 要将double灰度图像转换为uint8RGB 图像，应先缩放 255，然后转换为uint8：

  rgbImage = repmat(uint8(255.*grayImage),[1 1 3]);
  

Answer 2

By definition, an RGB image has 3 channels, which implies you need a three-dimensional matrix to represent the image. So, the right answer is:

rgbImage = repmat(255*grayImage/max(grayImage(:)),[1 1 3]);

Be careful when normalizing grayImage. If grayImage is uint8 then you will lose some precision in the 255*grayImage/max(grayImage(:)) operation.

Also, normalizing grayImage depends on the data. In your question, you used two methods:

rgbImage = grayImage / max(max(grayImage));

which normalizes the grayscale image such that the maximum value in the image is 1 and

rgbImage = grayImage / 255;

which only makes sense if the values in grayImage lie in the 0-255 range.

So it really depends on what you want to do. But, if you want an RGB image you need to convert your single-channel matrix to a 3-channel matrix.