I was commissioned to create a car ad and, at the time, the only source file I could find for the car was a very low-res image off the Internet. So I used the low-res file for rotoscoping and reference.
When depicting subjects such as cars or people, it is vital to get your proportions right—photo reference is invaluable for this also.
1. Start With the Original ImageA shot of the MY06 Subaru Impreza STI. The shot is not great quality, but it was sufficient enough for what I needed.