I opened the lesson by just being explicit about the goals and purpose of the lesson. The learning goals are explicitly listed at the beginning of the activity as well because I feel it is important to always put the goal of the lesson in front of the students. If students are unsure of what they need to learn they may miss the point of the entire lesson (I know from personal experience).
The purpose is to help students make clear connections between the art project and performing dilations on the coordinate plane.
The goal is students will see how the mechanics of drawing a perspective art piece are actually the same as the mechanics of drawing a dilation on the coordinate plane using a center of dilation point.
After making the purpose and goal explicit to my students I open the floor to conjectures about what the students think we will learn. What do you think are the clear connections between perspective art and dilations? We make a list of their ideas on the board or even better on poster paper so we can refer back to these ideas as we work through the activity. I also put their name beside their idea to show ownership, encourage participation, and promote pride when they are correct.