2d index to 3d array

Hi, I'd like to set several pixels in a base image to a color. These pixels are given as a 2d array (determined from a threshold in greyscale). Its fairly easy to do in grayscale:

im= ('something.pgm') peaks = im>200 baseimage(im)= 150;

but if I have a color image I need to set the three colours (third dimension)

something like: baseimage(im,:) = [0,0,1] which obviously doesn't work, but there must be a neat way to do this.

Answer 1

[Walter Roberson]

[r, c, b] = size(baseimage);
rgb = [0 0 1];   %fill color
idx = find(im);
baseimage(idx) = rgb(1);
baseimage(idx+r*c) = rgb(2);
baseimage(idx+2*r*c) = rgb(3);

[Ben]

Works perfect, thanks.

Answer 2

[Ahmed A. Selman]

First note that adding a third dimension that is interpreted as (color) in a grayscale (2-D matrix) image is a logically false task, yet it is mathematically possible. I think there have been some discussions in this forum about this issue.

As for your question, try using imwrite function with a color map argument, that's if you want to deal with coloring grayscale images, im, as:

imwrite(im,colormap(hot(100)),'myImage','jpg'));% hot is one type of many colormaps. Try others for different results.

If you want to manually add a 3rd dimension to a 2-D matrix (im), use concatenation commands like

[a,b]=size(im);
baseimage=cat(3,im,eye(a,b));% adds a unit matrix as a third dim. Use ones(a,b) to add ones matrix.

[Ben]

Sorry, maybe my question was not clear. I have converted a grayscale to color like this:

backim=cat(3,baseim,baseim,baseim);

Which gives a grey image that can have colour.

Now I need to colour some areas of the image which are defined in another image 'im' (a 2d binary array)

I'd like to be able to choose the fill color.