Conversion of RGB images into Lab color space

Hello, I tried to convert the RGB image into Lab color space using following two codes in MATLAB2014b(first code use RGB while second code use sRGB). For both code, output Lab images are different. I cannot get any use able data using these images. Also, my images are totally different from few images, I found on internet of Lab color space (I searched Lab image on internet to know how Lab images look like).

Codes which I used are below:

first code

 A = imread('C:\Users\Naseeb\Desktop\images\1.jpg');
lab = rgb2lab(A);
imshow(lab);

Second Code:

A = imread('C:\Users\Naseeb\Desktop\images\1.jpg');
colorTransform = makecform('srgb2lab');
lab = applycform(A, colorTransform);
imshow(lab);

Also, I observe that image obtained from second code is same as image I get by converting RGB into YCrCb image. How can I get correct Lab image.

I'm attaching my images with this. Original image (RGB image which used to convert into Lab); 1st code (Lab image from 1st code); 2nd code (Lab image from 2nd code); internet image (which I found on internet to know how Lab image look like)

[Adam]

imshow will interpret an n*m*3 image as being RGB so your conversion to Lab colour space will still get interpreted as RGB for displaying on the screen rather than as Lab. Is that what you want?

It doesn't explain why two results would be different though that should return the same answer, but plotting is not the best way to check that two results are identical anyway.

[Naseeb Gill]

Hello Adam,

Thanks for your time. But I want Lab color space image to store so that I can further processed it. What I should do for that?

[Adam]

For storage and processing it is fine, just that when you visualise any results from that processing you will need to convert back to RGB in order to plot the images properly. I'm not aware of any plotting function that will take data in any other form than RGB.

[Naseeb Gill]

Dear Adam,

For storing and processing also it is creating a problem. For example when I changed RGB into HSV, at that instance using point cursor in matlab window all values H,S and V are below one but when I saved image as jpg it's HSV value get changed. But I want the real color space values which is between 0-1 after saving image. I'm attaching both images for your reference. I want these point to plot on graph.

[Andrew Lee]

may I know at the end of this, which code did you use for the conversion. And how did you store the La*b* values which ranged from 0to100 for L, and -128to127 for a* & b* so that you can furthere process with it?

Answer 1

[Image Analyst]

NASEEB: See Steve's blog for lots and lots of info: http://www.mathworks.com/search/site_search.html?q=color+space&c[]=blogs

I use rgb2lab() to convert to CIE LAB color space whenever I need relative, quick and dirty, kind of color space conversion. Never use JPG images for image analysis. It's no wonder the values are different. JPG will destroy the image to various degrees. Use PNG instead.

All of those are "book" formulas. They will not give you the "true" LAB like you'd get from a spectrophotometer. Most of the time I use calibrated digital color imaging because I need the "true" LAB values out, not some arbitrary ones. But that's a much higher level of sophistication that you probably want to do.

For a color toolbox by one of the worlds leading color researchers, you can get Professor Westland's toolbox here: http://www.mathworks.com/matlabcentral/fileexchange/40640-computational-colour-science-using-matlab-2e He literally wrote the book on color analysis with MATLAB. http://www.mathworks.com/matlabcentral/fileexchange/28790-colorspace-transformations

Professor Westland's Book