Work on code whose correctness can only be judged by looking at what it drew — image processing, tracing, SVG or canvas generation, chart rendering, layout and positioning, PDF or print output. Render an annotated diagnostic image at every step and read it back, rather than reasoning about the code. Use whenever the output is a picture, whenever a fix means typing coordinates or thresholds, or when the user says something "looks wrong", "is ugly", "is cut off", or "doesn't line up".
