Hide part of an image

[Karoffe]

Well, I want to use interpolateBetween to make an image come into a box from the side, my problem is I have to make the image outside the box first then move it inside,, so I want the parts of the image to appear only once it entered the box

Here's an image explaining if you don't understand

http://imgur.com/0UZ7aog

The part inside the box should be shown as normal but the other part outside the box should not appear, how can that be done ?

[Addlibs]

You could make the box a render-target, then everything outside the render-target region won't be displayed.

If you prefer, you can do some math with dxDrawImageSection and use that.

[Karoffe]

I'd prefer to use image sections but, I am not able to discover the math I need to do this

Here is a picture I made with paint to be able of better thinking of how the math is done in my situation, but it seems that I fail at math,,

http://i.imgur.com/cRMzJke.png

What am trying is to get to do an equation with all the black colored numbers to get the value 10 (the red colored one),