Baekalfen/PyBoy

Input always changes on same scanline

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#116 opened on 2020年3月22日

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enhancementgood first issue

説明

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

コントリビューターガイド

技術スタック
python
領域
backend
Issue 種別
bug
難度新しい貢献者にとっての実装難度です。1 は非常に小さな変更、5 は専門的な作業です。
4
推定時間経験ある貢献者が調査、実装、テスト、pull request 準備にかけるおおよその時間範囲です。
half day
活動状況Issue が今どれだけ取り組みやすいかを示します。新しい、活発、古い、ブロック中、またはメンテナー入力待ちなどです。
stale
明確さ期待される変更、受け入れ条件、次の手順がどれだけ明確かを示します。
clear
前提条件
Game Boy LY registerPythonPyBoy input handlingscanline timing
初心者向け度初回貢献者にどれだけ取り組みやすいかを 1-100 で推定したスコアです。
35
調査方針
Investigate the input polling mechanism in PyBoy's emulation loop, focusing on when input changes are applied relative to the LCD status. The issue references BGB's behavior of changing input on the first scanline of vblank to reduce latency while randomizing for programs that poll multiple times per frame. Look at the LY register handling and where input is read from the BIOS/game. The test ROM 'Telling LYs' checks for exact scanline of input change; adjust PyBoy's input update timing to match hardware behavior.