What's the difference between relative and global mouse coordinates? When should I use each?

Question

Ryan McCombe · Accepted Answer

SDL provides two main ways to get mouse coordinates through SDL_GetMouseState() and SDL_GetGlobalMouseState(). The key difference lies in their frame of reference and typical use cases.

Relative Coordinates

Relative coordinates are reported by SDL_GetMouseState() and are measured relative to the top-left corner of your window. These coordinates start at (0,0) at your window's top-left corner and increase as you move right and down. They're most useful when you need to:

Handle mouse interactions within your window's UI elements
Implement window-specific features like buttons or drag handles
Calculate if the mouse is within specific regions of your window

Here's an example showing how to use relative coordinates:

1#include <SDL.h>
2#include <iostream>
3
4int main(int argc, char** argv) {
5  SDL_Init(SDL_INIT_VIDEO);
6  SDL_Window* window =
7    SDL_CreateWindow("Relative Coordinates",
8                     SDL_WINDOWPOS_CENTERED,
9                     SDL_WINDOWPOS_CENTERED,
10                     800, 600,
11                     SDL_WINDOW_SHOWN);
12
13  bool running = true;
14  while (running) {
15    SDL_Event event;
16    while (SDL_PollEvent(&event)) {
17      if (event.type == SDL_QUIT) {
18        running = false;
19      }
20    }
21
22    int x, y;
23    SDL_GetMouseState(&x, &y); 
24
25    // Check if mouse is in top-left quadrant
26    if (x < 400 && y < 300) {
27      std::cout <<
28        "Mouse in top-left quadrant at "
29        << x << "," << y << "\n";
30    }
31  }
32
33  SDL_DestroyWindow(window);
34  SDL_Quit();
35  return 0;
36}

Global Coordinates

Global coordinates are reported by SDL_GetGlobalMouseState() and are measured relative to the top-left corner of the primary display. These coordinates can be negative in multi-monitor setups. They're most useful when you need to:

Track mouse movement across multiple windows
Implement drag-and-drop between windows
Handle multi-monitor scenarios
Position windows relative to the mouse cursor

Here's an example demonstrating global coordinates:

1#include <SDL.h>
2#include <iostream>
3
4int main(int argc, char** argv) {
5  SDL_Init(SDL_INIT_VIDEO);
6
7  // Create two windows side by side
8  SDL_Window* window1 =
9    SDL_CreateWindow("Window 1", 100, 100, 400,
10                     300, SDL_WINDOW_SHOWN);
11  SDL_Window* window2 =
12    SDL_CreateWindow("Window 2", 550, 100, 400,
13                     300, SDL_WINDOW_SHOWN);
14
15  bool running = true;
16  while (running) {
17    SDL_Event event;
18    while (SDL_PollEvent(&event)) {
19      if (event.type == SDL_QUIT) {
20        running = false;
21      }
22    }
23
24    int globalX, globalY;
25    SDL_GetGlobalMouseState(&globalX, &globalY);
26
27    std::cout << "Mouse at global position: "
28      << globalX << "," << globalY << "\n";
29  }
30
31  SDL_DestroyWindow(window1);
32  SDL_DestroyWindow(window2);
33  SDL_Quit();
34  return 0;
35}

When choosing between relative and global coordinates, consider your specific needs:

Use relative coordinates when working within a single window
Use global coordinates when dealing with multi-window or multi-monitor scenarios
Consider using both if you need to implement features like drag-and-drop between windows

Mouse Capture and Global Mouse State

Relative vs Global Mouse Coordinates

Relative Coordinates

Global Coordinates

Mouse Capture and Global Mouse State

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