Debugging Motion Issues

How do I debug incorrect relative motion readings?

Debugging relative motion issues requires a systematic approach to identify whether the problem lies with SDL, your code, or the system configuration. Here's a comprehensive debugging toolkit:

#include <SDL.h>
#include <chrono>
#include <fstream>
#include <iostream>
#include "Window.h"

class MotionDebugger {
public:
  MotionDebugger() {
    // Open log file
    Log.open("mouse_motion.log");

    // Log system information
    Log << "SDL Version: " << SDL_MAJOR_VERSION
      << "." << SDL_MINOR_VERSION
      << "." << SDL_PATCHLEVEL << '\n';

    // Log relative mode state
    Log << "Relative Mode: "
      << (SDL_GetRelativeMouseMode()
            ? "Enabled"
            : "Disabled") << '\n';
  }

  void LogMotionEvent(SDL_MouseMotionEvent& E) {
    auto Now = std::chrono::system_clock::now();
    auto Timestamp = std::chrono::duration_cast<
        std::chrono::milliseconds>(
        Now.time_since_epoch()).count();

    Log << Timestamp << "," << E.xrel << ","
      << E.yrel << "," << E.x << "," << E.y <<
      '\n';

    // Also check for potential issues
    CheckForAnomalies(E);
  }

private:
  void CheckForAnomalies(
    SDL_MouseMotionEvent& E) {
    // Check for unusually large movements
    if (std::abs(E.xrel) > 100
      || std::abs(E.yrel) > 100) {
      std::cout << "Warning: Large movement "
        "detected\n";
    }

    // Check for stuck positions
    if (E.x == LastX && E.y == LastY
      && E.xrel != 0 && E.yrel != 0) {
      std::cout <<
        "Warning: Position stuck but "
        << "relative motion reported\n";
    }

    LastX = E.x;
    LastY = E.y;
  }

  std::ofstream Log;
  int LastX{0};
  int LastY{0};
};

int main() {
  SDL_Init(SDL_INIT_VIDEO);
  Window GameWindow;
  MotionDebugger Debugger;

  // Enable relative mode
  if (SDL_SetRelativeMouseMode(SDL_TRUE) < 0) {
    std::cout <<
      "Failed to enable relative mode: "
      << SDL_GetError() << '\n';
  }

  SDL_Event E;
  while (true) {
    while (SDL_PollEvent(&E)) {
      if (E.type == SDL_MOUSEMOTION) {
        Debugger.LogMotionEvent(E.motion);
      } else if (E.type == SDL_QUIT) {
        SDL_Quit();
        return 0;
      }
    }
    GameWindow.Update();
    GameWindow.Render();
  }
}

Common issues to check for:

  • Inconsistent event timing
  • Stuck cursor positions
  • Unusually large movements
  • Missing or duplicate events
  • System-specific behavior differences

Additional debugging steps:

  • Compare raw input vs. processed values
  • Test with different mouse hardware
  • Check system mouse settings
  • Verify window focus handling
  • Monitor CPU usage during mouse movement

Relative Mouse Mode

Learn how to restrict cursor movement to a window whilst capturing mouse motion continuously.

Questions & Answers

Answers are generated by AI models and may not have been reviewed. Be mindful when running any code on your device.

Handling SDL Relative Mode Failure
What happens if SDL_SetRelativeMouseMode() fails?
Smoothing Mouse Movement
Can relative motion data be smoothed for a better user experience?
Common Relative Mode Failures
What are common reasons for relative mode to fail on some systems?
Center Mode Impact
How does SDL_HINT_MOUSE_RELATIVE_MODE_CENTER affect gameplay?
Handling Small Movements
Why might an application ignore small movements in relative mode?
SDL2 vs SDL3 Relative Mode
How does relative mode work differently in SDL2 vs. SDL3?
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