Show only specific part of UI.Image

Hello guys. I need to reach next effect, and have a big problems with that, even dont know how to reach that.

1) We have 100% stretched background image inside Canvas. 2) This image should be invisible by default(set alpha 0, or what). 3) I have a kinect representation of my body, it is 3d Avatar, without renderers. I do convert the coordinates of this 3d avatar to 2D space, and now, I need to get the shape of this avatar, it's silhouette, and enable the area of image, on this background one, with the shape of my silhouette, so in short words, I would only render the part of ui.Image, where my body, or any 3d object is, with the shape of the body(what is also dynamic, because I can turn hands, or any other parts)

Do you guys have any ideas how to reach that?

First what I was thinking about was UI.Mask, but it is not really, what Im looking for, or do I wrong? The second what I have in mind - is a shaders, but Im not the spec in it, so dont know, how they would work.

Thanks.