使用灰度图像用matlab创建RGB图像

Question

I have a grayscale image and I want this image as the three bands of an RGB image. In other words, each band of the new RGB image will be this grayscale image

That's what i tried so far:

gray_image=imread('image.pgm');
rgb_image=zeros(size(gray_image,1),size(gray_image,2),3);
rgb_image(:,:,1)=gray_image;
rgb_image(:,:,2)=gray_image;
rgb_image(:,:,3)=gray_image;


>> rgb_image(1,2,1)

 ans = 44

>> rgb_image(1,2,2)

 ans = 44

>> rgb_image(1,2,3)

ans = 44

You can see that the above code works, but when I do the following command:

imwrite(rgb_image,'rgb_image.ppm');

the image is all white. How to save the image with the right format?

Answer 1

Cause of problem:

gray_image is of type uint8 while rgb_image is of type double , but both has values in the range [0..255] . When matlab saves a double precision image it expects its values to be in range [0..1] all values greater than 1 are truncated to 1 - the brightest possible value. This is why saveing a double precision rgb_image results with a completely white image.

Correction:

Use uint8 type for rgb_imge as well:

rgb_image=zeros(size(gray_image,1),size(gray_image,2),3,'uint8');

Alternatively, you can convert gray_image to double

gray_image = im2double( gray_image );

Answer 2

I managed to create a RGB image in Matlab 2016a where the original image has mostly gray colors

imgGray = imread('Ceres-Bright-Spots-4_500-miles.jpg');  

[r g b]=size(imgGray);
rgb=zeros(r,g,3); 
rgb(:,:,1)=imgGray;
rgb(:,:,2)=rgb(:,:,1);
rgb(:,:,3)=rgb(:,:,1);
imgRGB=rgb/255; 
figure,imshow(imgRGB);

Doing Shai's commands were not sufficient. I prepared the data to complete all steps in the guide here about Image Processing Made Easy .