Colorscale Conversion Gray to jet

Hi,

I save a 2d Matrix visualized in colormap 'jet'. When this is saved as an image, it is saved as RGB image.

Now, I want to convert the RGB image to gray. If I do RGB to gray, the image recovered is inaccurate.

How can I solve this?

Appreciate any response.

Thank you!

Answer 1

[Bhanu Prakash]

Hi Rashmi,

As per my understanding, the recovered image using the ‘rgb2gray’ function is not accurate and you want to improve its accuracy.

If you already have the RGB image, then you can load the image and the colormap using the ‘imread’ and ‘jet’ functions respectively as shown below:

% Load the RGB image 
rgb_img = imread('your_rgb_image.jpg');  
% Load the colormap 'jet' 
cmap = jet(256); 

To convert an RGB image to grey image, the colormap values are to be in the range [0,1]. You can normalize the colormap values to the required range in the following manner:

% Normalize the colormap to the range [0, 1] 
cmap_norm = cmap / max(cmap(:)); 

To preserve the image details during conversion, the RGB image is converted to an inverted image and then to a gray image using the ‘rgb2ind’ and ‘ind2gray’ functions as shown below:

% Convert the RGB image to indexed image using the colormap 'jet' 
indexed_img = rgb2ind(rgb_img, cmap); 
% Convert the indexed image to grayscale using the normalized grayscale colormap 
gray_img = ind2gray(indexed_img, cmap_norm); 

The above approach provides a better accuracy of the recovered image compared to the approach using ‘rgb2gray’ function.

For more information on the above-mentioned functions, refer to the following documentation:

1.For ‘imread’ function:

https://in.mathworks.com/help/matlab/ref/imread.html?s_tid=doc_ta

2.For ‘jet’ function:

https://in.mathworks.com/help/matlab/ref/jet.html

3.For ‘rgb2ind’ function:

https://in.mathworks.com/help/matlab/ref/rgb2ind.html?s_tid=doc_ta

4.For ‘ind2gray’ function:

https://in.mathworks.com/help/images/ref/ind2gray.html?s_tid=doc_ta