Convert and indexed image to RGB and back without losing data

Question

I am having trouble converting an indexed image to RGB and then back from RGB to an indexed image. For some reason, the result is different from the original. I am doing steganography so it can't work if the data is changed.

This is my code and this is the sample image:

[J map]=imread('expert.gif');
Jrgb=ind2rgb(J,map);
Jind=rgb2ind(Jrgb,map);
isequal(J,Jind)

Variables J and Jind are supposed to be equal. Why are they being detected as being different?

score 2 · Accepted Answer · edited May 23 '17 at 12:31

2

First of all, I'm certain that this is related to this question.

The issue is happening because if you actually look at the colormap of your loaded image:

map = 
       0         0         0
  0.6275    0.3216    0.1765
  0.4902    0.4902    0.4902
  0.8039    0.5216    0.2471
  0.7451    0.7451    0.7451
  0.8627    0.8627    0.8627
  0.9020    0.9020    0.9804
       0         0         0

You'll see that the color black (0,0,0) actually exists in there twice so both index = 0 and index = 7 will resolve to black in the RGB image.

When you do the conversion back to an indexed image, MATLAB is going to use the same index for both of those (because they are obviously the same color) even if the colormap that you pass to rgb2ind is the same colormap.

That explains why the differences that you're seeing are where the transparent pixels are (around the periphery).

As far as dealing with this, I think it's a little tricky. Unfortunately the transparency (3rd output) output of imread is an empty array.

You could potentially change the input colormap so that the first and last rows aren't the same (set the last row to 1's) and then you should get back something comparable.

map(end,:) = 1;
rgb = ind2rgb(J, map);
ind = rgb2ind(rgb, map);
isequal(J, ind);

In general, due to MATLAB's limitations, GIFs with transparency may not be the best test case for playing with stenography.

edited May 23 '17 at 12:31

Community

1
1

answered Mar 06 '16 at 01:28

Suever

64,497
14
82
101

Thanks for the answer, but then i have this [image](http://postimg.org/image/ov45k1ypr/). It has no transparency and the color map is unique, why they still being different after convert to rgb and back to indexed? – Fitri Mar 07 '16 at 21:39
@Fitri what are you using to generate these images? – Suever Mar 07 '16 at 22:08
`[J map]=imread('RoFox.gif'); [rows cols color frame]=size(J); for i = 1:frame Jrgb(:,:,:,i)=ind2rgb(J(:,:,:,i),map); Jind(:,:,:,i)=rgb2ind(Jrgb(:,:,:,i),map); end` – Fitri Mar 07 '16 at 23:18
when i check with `for i = 1:frame isequal(Jind(:,:,:,i),J(:,:,:,i)) end` why are they not equal? – Fitri Mar 07 '16 at 23:21
@Fitri I totally see that there is an issue. I'm just trying to figure out what the input data was created with to maybe do some more debugging. – Suever Mar 07 '16 at 23:23
I take the image from google. Some images have this issue too, not only this. – Fitri Mar 07 '16 at 23:47
@Fitri I'm going to dig into this some more. This is very strange behavior and this may be a bug within MATLAB. I'll keep you posted on what I find. – Suever Mar 08 '16 at 14:55

Convert and indexed image to RGB and back without losing data

1 Answers1