Here’s how I would do it. The challenge is that the result can’t be precisely defined because it’s essentially art. But with TDD the assertions don’t actually have to actually live in code. All we have to do is make incremental verifiable progress that lets us fearlessly make changes.

So I would set up my viewport as a grid where in each square there will eventually live a rendered image or animation. The first one blank, the second one a dot, the third a square, the fourth with color, the fifth a rhombus, the sixth with two disjoint rhombuses …

When you’re satisfied with each box you copy/paste the code into the next one and work on the next test always rendering the previous frames. So you can always reference all the previous working states and just start over if needed.

So the TDD flow becomes

1. Write down what you want the result of the next box to look like.

2. Start with the previous iteration and make changes until it looks like what you wanted.

3. Write down what you want…