The mirror filter can be used in a variety of ways. One example is if you are sketching a person or object, and you want both sides to be an exact match of one another.
For this tutorial, a completed drawing of a butterfly will be used, but this filter can also easily be used while you are in the process creating your sketch or image. Open your image in Acorn, File ▸ Open.
Choose Filter ▸ Tile Effect ▸ Mirror. The mirror point will be placed in the center of your canvas.
Using the on-canvas control, select the move tool then drag the mirror point directly vertical until it snaps with the canvas boundary. Alternatively, you can enter the x,y coordinates in the filters window.