How can I set a variable alpha channel to an image?

I display the mean of a 2-D distribution with imagesc(mean(whatever)), and I wish to make it transparent where the distribution std reach a maximum and opaque where std is at its minimum.

Alpha channel allows me to make the whole image more or less transparent, how can I apply a variable alpha channel?

Answer 1

[Adam]

You can use a mask image of the same size as your main image, filled with 0-1 values to give any level of transparency on a per-pixel basis.

So you can apply a mask to your data e.g.

myMask = data > 0.5;

and then use this as an alpha value which would make values greater than 0.5 opaque and those less transparent, but you are not limited to 0 and 1 in the alpha channel.

Answer 2

[Image Analyst]

See demos:

http://blogs.mathworks.com/steve/2009/02/18/image-overlay-using-transparency/

http://www.mathworks.com/matlabcentral/answers/104889-how-to-overlay-a-foreground-image-over-a-background-image