Expected behavior of updating fish positions

[hujiajie]

Proposal A: Update fish positions according to wall clock time (current behavior, as well as that of the original WebGL implementation)

• Pros: Fish appear to have approximately the same speed on different devices
• Cons: Introduces nondeterminism. E.g., fish positions of the frame at the beginning of the tenth second may be calculated using an elapse of 9.000s in the first run, but 9.001s in the second run, resulting in slightly different rendering results, which is bad for auto testing. How to ensure the frame is captured at the exact time (9.000s) can be another problem.

Proposal B: Update fish positions according to frame number

• Pros: Every frame is deterministic
• Cons: Fish may appear fast on some devices but slow on others

Proposal C: Add an option for switching between A and B

• Pros: Switchable between A and B
• Cons: Code path in auto testing (assuming proposal B) may diverge more from the default (assuming proposal A), which is undesired

[gyagp]

For a real application, the position has to be calculated by the wall clock time. So Proposal A should be the default behavior. For test, actually the expectation can be impacted by many factors, including fish count, MSAA, screen resolution, elapsed time and so on. Regarding the impact of elapsed time, I'd prefer we can make the whole screen stationary. That is we have exactly the same result for different frames.

[hujiajie]

Proposal D: Update fish positions according to wall clock time, and hook the time function in testing so it returns mocked wall clock time

Hooking can be implemented with LD_PRELOAD on Linux, but I'm not sure about the feasibility on Windows and macOS.

The method is analogous to using a patched libc during testing, where the time function returns deterministic values and ignores the hardware clock. Aquarium will not need a separate path for testing then.

[gyagp]

We shouldn't introduce too much complexity for the tests. I still prefer some changes within Aquarium to facilitate the tests.