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, 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.