I'm using Photoshop CS6 on a Macintosh and have created a layer mask and wish to erase at the edges of a painting in a photograph, but the edges are not quite true, so even after doing a perspective crop on the photo, the edges are not quite all straight. I would like to select a straight line that follows the edge and draw upon it in the layer mask. I've been using the selection tool and the Single Row Marquee Tool and Single Column Marquee Tool together with the shift key to add lines and option key to remove lines. In this way as I work my way along the line, drawing on the selected part in the layer mask, I add or remove columns or rows as needed. Using the Quick Selection Tool or Magic Wand do not work well as I don't get very sharp edges and in some cases the background is too close in color to the edge.
There must be a way to draw a straight line that is not quite horizontal or vertical. How can I do this more easily in Photoshop CS6?
