Why does MATLAB read a picture and add noise on it?

Hi,

I typed in matlab the following command:

x=imread('pic.gif')
imshow(x);

I would expect that the output image look exactly like the one MATLAB read. However, it didn't happen. MATLAB gave an output that looked like the original one, but with a lot of noise... My question is why don't I get exactly the same image?

[Adam]

I don't have 'pic.gif' so it's rather difficult to say without any information or screenshots of the original and the Matlab imshow version.

[M B]

Hi Adam, The picture is attached.

[Titus Edelhofer]

M B, I can't see the attachment either ...

[M B]

I attatched it now to the original message.

Answer 1

[Titus Edelhofer]

Hi,

your image is an indexed image, not RGB. Therefore you need to read the colormap as well:

[a,b] = imread('pic.gif');
imshow(a);
colormap(b);

Titus

[M B]

thanks :-)

Answer 2

[Jan]

Or:

[c, map] = imread('pic.gif');
img = ind2rgb(c, map);
imshow(img)

[M B]

thanks a lot

[M B]

I wish to ask how MATLAB decided how to present the picture.

MATLAB got only one arguement, a data matrix (of an indexed image), without a colormap. How did MATLAB present the picture? How did MATLAB know what color to place in each pixel.

[Image Analyst]

If you didn't supply a color map, and the image was uint8, then it just chose the gray scale colormap. But since the pixel value is really a row number into a colormap instead of an intensity value, the image most likely looked like a bunch of black/white/gray garbage/noise.

[Titus Edelhofer]

It was not 100% noise, since the colormap of the image had some similarity to simple gray scale. That's why it did not look completely weird but just "with some noise".