Fixed image plugin

Hello, i would like to know if there is some kind of plugin that outputs an image unscaled, i mean, if it has the same image size as the canvas, to full occupy the area, whereas if i resize the source, the image scale should stay the same, just cropping the image from the side i resize the source, or at least, to be configurable which corner of the image to be always visible.
I haven't found any way to do this.

Tried to do this with the included filter files, but it seems to me that the information about the source size (relative to the scene it is contained) or the texture size to adjust within inside the .effect code aren't available.

said in other words, how can i know the source inner size from within the .effects code when a source is resized inside its scene.I need to react to these changes in my code but i don't know from where to get such values