How do I write tests for a graphics library?

Question

I'm writing an OpenGL 2D library in Python. Everything is going great, and the codebase is steadily growing.

Now I want to write unit tests so I don't accidently bring in new bugs while fixing others/making new features. But I have no idea how those would work with graphics libraries.

Some things I thought of:

• make reference screenshots and compare them with autogenerated screenshots in the tests
• replace opengl calls with logging statements and compare logs

But both seem a bad idea. What is the common way to test graphics libraries?

Answer 1

The approach I have used in the past for component level testing is:

• Use a uniform colored background, with a few different colors.
• Use uniform colored rectangles as graphical objects in tests (with a few different colors).
• Place rectangles in known places where you can calculate their projected position in the image by yourself.
• Calculate expected intensity of each channel of each pixel (background, foreground or mixture).
• If you have a test scenario that results in non-round positions, use a non-accurate compare (e.g. correlation)
• Use calculations to create expected result images.
• Compare output images to expected result images.
• If you have a blur effect, compare sum of intensity instead of discrete intensities.

As graham stated, internal units may be unit-testable free from graphics calls.