Example curtain tool macro; uses partial image overlay.

curtain_tool.ijm

macro "Curtain Tool - C00cT0f20A" {
   getCursorLoc(x, y, z, flags);
   setBatchMode(true);
   id=getImageID;
   run("Duplicate...","title=temp");
   // here goes the image transform to preview
   run("Gaussian Blur...","sigma=5");
   // 
   id2=getImageID;
   px=x;
   while (flags&16>0) {
       selectImage(id);
       getCursorLoc(x, y, z, flags);
       if (x!=px) {
         selectImage(id2);
         makeRectangle(x,0,getWidth-x,getHeight);
         run("Duplicate...","title=part");
         id3=getImageID;
         selectImage(id);
         run("Add Image...", "image=[part] x="+x+" y=0 opacity=100 zero");
         while (Overlay.size>1) Overlay.removeSelection(0);
         selectImage(id3);
         close();
         px=x;
      }
   }
}