Baekalfen/PyBoy

Input always changes on same scanline

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#116 opened on Mar 22, 2020

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Description

A program can tell that it's running on PyBoy because the buttons always change right at vblank.

To reproduce:

  1. Install PyBoy
  2. Start the test ROM "Telling LYs?" (which I have precompiled for your convenience here)
  3. Press all 8 buttons in any order
  4. Watch the arrow sprite at the right side

Expect: Arrow moves after each press, followed by "Pass", as on a real GameBoy

Actual: Arrow stays at upper right of the screen, followed by an "Incorrect behavior" message that clearly isn't "Pass", as below:

tellinglys

The Game Boy emulator BGB by beware passes this test. Normally it changes the input on the first scanline of vertical blanking to reduce input latency. But if a program reads input multiple times during a frame, the emulator starts to randomize on which scanline the input changes. This way, the program can't discern that it's being emulated and freeze on a copy protection screen, but it adds one frame of lag. Once the program returns to polling once a frame, BGB returns to the low-latency behavior.

Related forum topic: https://forums.nesdev.com/viewtopic.php?f=20&t=18026

Contributor guide

Research direction
Investigate the input handling code in PyBoy to determine where the button state is latched. Compare the behavior with the BGB emulator's approach of changing input on the first scanline of vblank but randomizing the scanline if input is polled multiple times per frame. Adjust the timing logic to pass the test ROM while maintaining input responsiveness.
Tech stack
python
Domain
backend
Issue type
Bug
DifficultyEstimated implementation difficulty for a new contributor, from 1 for very small changes to 5 for expert-level work.
2
Estimated timeA rough time range for an experienced contributor to investigate, implement, test, and prepare a pull request.
1-3 hours
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Active
ClarityHow clearly the issue explains the expected change, acceptance criteria, and next step.
Clear
Prerequisites
Python
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
60