How can I “plot” an image on top of another image with a different colormap?

Question

I've got two images, one 100x100 that I want to plot in grayscale and one 20x20 that I want to plot using another colormap. The latter should be superimposed on the former.

This is my current attempt:

A = randn(100);
B = ones(20);
imagesc(A);
colormap(gray);
hold on;
imagesc(B);
colormap(jet);

There are a couple of problems with this:

1. I can't change the offset of the smaller image. (They always share the upper-left pixel.)
2. They have the same colormap. (The second colormap changes the color of all pixels.)
3. The pixel values are normalised over the composite image, so that the first image changes if the second image introduces new extreme values. The scalings for the two images should be separate.

How can I fix this?

I want an effect similar to this, except that my coloured overlay is rectangular and not wibbly:

Answer 1

Just change it so that you pass in a full and proper color matrix for A (ie 100x100x3 matrix), rather than letting it decide:

A = rand(100); % Using rand not randn because image doesn't like numbers > 1
A = repmat(A, [1, 1, 3]);
B = rand(20); % Changed to rand to illustrate effect of colormap
imagesc(A);
hold on;
Bimg = imagesc(B);
colormap jet;

To set the position of B's image within its parent axes, you can use its XData and YData properties, which are both set to [1 20] when this code has completed. The first number specifies the coordinate of the leftmost/uppermost point in the image, and the second number the coordinate of the rightmost/lowest point in the image. It will stretch the image if it doesn't match the original size.

Example:

xpos = get(Bimg, 'XData');
xpos = xpos + 20; % shift right a bit
set(Bimg, 'XData', xpos);