How to change colortype truecolor to grayscale

I am converting rgb image to grayscale using rgb2gray but at the image info its showing true color instead of grayscale. what should i do ? i'm new to matlab .

Answer 1

[Image Analyst]

You have to display the gray scale image with imshow() after you do the conversion.

grayImage = rgb2gray(rgbImage);
imshow(grayImage, []);  % You probably forgot to do this step.

[Iqra Rizvi]

it is still the same. Even after conversion it shows its a 3 channel rgb image instead of 1 channel for grayscale

this is the image i'm using and applying the changes as you said

rgb_img = imread('mri image.jpg'); grayImage = rgb2gray(rgb_img); imshow(grayImage, []);

I'm getting same imfinfo

[Iqra Rizvi]

In my project i have to find rms , ambe and entropy values of different histogram equalization. The image i'm getting looks like gray but its not gray as per the imfinfo . The image should be of values 0s n 1s so then i can calculate mean gray values and apply them in my functions . With above image i'm getting 'zero' as answer .

[Image Analyst]

imfinfo() operates on a disk file, not a variable in memory. So unless you saved the gray scale image out with imwrite() and then checked with imfinfo(), you're doing it on the original RGB image. Do this and see what it says in the command window:

>> whos grayImage

[Iqra Rizvi]

this is what i get .. its a uint8 class

[Image Analyst]

OK, so you've proved that it is actually a gray scale image, not still an RGB color image like you asserted. You can use mean2() or masking to determine mean gray level in some region. So, are we done now?

[Iqra Rizvi]

no .. it is still an rgb image .. when i'm checking imfinfo

[Image Analyst]

Show me all your code, including the lines where you save grayImage (not A like you showed already) with imwrite(), and where you call imfinfo() on the newly saved gray scale image file (not the "A" image file which we don't care about.) Here's demo that I know for a fact works:

clc;    % Clear the command window.
close all;  % Close all figures (except those of imtool.)
clear;  % Erase all existing variables. Or clearvars if you want.
workspace;  % Make sure the workspace panel is showing.
format long g;
format compact;
fontSize = 15;

rgbImage = imread('peppers.png');
% Get the dimensions of the image.  
% numberOfColorChannels should be = 1 for a gray scale image, and 3 for an RGB color image.
[rows, columns, numberOfColorChannels] = size(rgbImage);
if numberOfColorChannels > 1
  % It's not really gray scale like we expected - it's color.
  % Use weighted sum of ALL channels to create a gray scale image.
  grayImage = rgb2gray(rgbImage); 
end
% Display the color image.
subplot(2, 1, 1);
imshow(rgbImage, []);
title('Original Color Image', 'FontSize', fontSize, 'Interpreter', 'None');
% Display the gray scale image.
subplot(2, 1, 2);
imshow(grayImage, []);
title('Grayscale Image', 'FontSize', fontSize, 'Interpreter', 'None');
hp = impixelinfo();

% Save it out
fullFileName = fullfile(pwd, 'Gray Scale Image.bmp');
imwrite(grayImage, fullFileName);
fileInfo = imfinfo(fullFileName)
uiwait(helpdlg('Check out the command window'));

In the command window you will see the proof that it is a gray scale image because it's an 8 bit (BitDepth=8), single plane (NumPlanes=1) image:

fileInfo = 
  struct with fields:

                Filename: 'C:\Users\Mark\Documents\MATLAB\work\Tests\Gray Scale Image.bmp'
             FileModDate: '17-Mar-2017 11:53:48'
                FileSize: 197686
                  Format: 'bmp'
           FormatVersion: 'Version 3 (Microsoft Windows 3.x)'
                   Width: 512
                  Height: 384
                BitDepth: 8
               ColorType: 'indexed'
         FormatSignature: 'BM'
      NumColormapEntries: 256
                Colormap: [256×3 double]
                 RedMask: []
               GreenMask: []
                BlueMask: []
         ImageDataOffset: 1078
        BitmapHeaderSize: 40
               NumPlanes: 1
         CompressionType: 'none'
              BitmapSize: 196608
          HorzResolution: 0
          VertResolution: 0
           NumColorsUsed: 256
      NumImportantColors: 0

So now you need to basically do the same thing and show me how when you do it, it says NumPlanes = 3 or BitDepth = 24.