The image below is a photo of my pencil drawing on illustration board for “Scholar’s Stairway,” page 138 of Dinotopia: Journey to Chandara. Over that I have superimposed a slice of the final painting in oil, so you can see how the picture ended up.
The perspective is fairly complicated, with three vanishing points, each one a few meters away from the edge of the painting. Rather than worrying about those remote VPs, I established a series of graduating slopes for each VP by means of a set of evenly spaced guide marks alongside the margins of the picture. They’re marked off on a piece of white tape superimposed over the low-tack blue tape at the edge of the painting.
Any intermediate slope for any of the three vanishing points can easily be determined by lining up the mahl stick with the guide marks along the edge. Since the painting is not tied in space to the VPs, it can be moved around or turned upside down while you're working.
Having these guides was important in a painting like this, where the line drawing itself (including the center lines for the domes) was eventually covered up by the opaque paint. When the painting was finished, I stripped off all the tape, leaving a clean white edge around the image. Thanks to Ted Youngkin and Jack Wemp for teaching me this.
Tomorrow: Eye Magnets.