How can I create an action that increases the canvas size by a specified number of pixels?
I want to batch process a number of files of unknown size to add a consistent white border around each one, without affecting the scaling/cropping of the original picture. In other words, if the original picture is 200px x 350px I would like to automatically increase the canvas to 300px x 450px, leaving the image centered inside a 50px border.
I know how to adjust the canvas size by the total number of pixels, or based on a percentage, but neither of those seem to give me the results I need.
If it matters, a previous step in the process will be to scale the image by percentage (i.e. scale to 75%, then add a 20px border).