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Demo Recording & Playback

Overview

The engine supports recording and replaying gameplay. Demo files are stored as data/demo000.blp … data/demo003.blp (4 slots).

Implementation: src/event.cpp

Data Structures

struct DemoHeader {
    short version;   // demo format version
    short world;     // world number (mission)
    short skill;     // difficulty (0=easy, 1=normal, 2=hard)
    short flags;     // mode flags
};

struct DemoEvent {
    int    time;     // event time in ms from demo start
    UINT   message;  // Windows message (WM_LBUTTONDOWN, WM_MOUSEMOVE, WM_ACTION_*, etc.)
    WPARAM wParam;   // message parameter
    LPARAM lParam;   // message parameter (usually encodes mouse position)
};

Recording

void CEvent::DemoRecStart();
// Starts recording:
// - Sets m_bDemoRec = TRUE
// - Writes DemoHeader to buffer
// - Saves the initial world state

void CEvent::DemoRecStop();
// Ends recording:
// - Writes recorded DemoEvent[] to demo000.blp etc.
// - Sets m_bDemoRec = FALSE

During recording, all relevant input messages (clicks, mouse moves, toolbar button presses) are captured into DemoEvent[] with timestamps.

Playback

void CEvent::DemoPlayStart();
// Starts demo playback:
// - Loads DemoHeader and verifies world + difficulty
// - Loads world file (initial state)
// - Sets m_bDemoPlay = TRUE

void CEvent::DemoStep();
// Called every tick:
// - Compares current time with timestamp of next DemoEvent
// - When it is time for an event, injects it into the message loop (PostMessage)
// - Simulates user input exactly as during the original recording

Demo Phase

Demo playback runs in the WM_PHASE_DEMO phase. After the demo ends, the game returns to the main menu or shows the intro screen.

Limitations