Mouse State

Calculating Mouse Pointer Angle

How can I get the mouse pointer's angle relative to a position on the screen?

Abstract art representing computer programming

Here's a simple example that shows how querying mouse state can be useful. We'll create an object that reports the direction from the center of the window to the mouse cursor when the space bar is pressed.

First, let’s create a constructor that receives an SDL_Window*, and stores where the center of the window is:

1#include <SDL.h>
2#include <iostream>
3#include "Window.h"
4
5class DirectionTracker {
6public:
7  DirectionTracker(SDL_Window* Window) {
8    int windowWidth, windowHeight;
9    SDL_GetWindowSize(Window, &windowWidth,
10                      &windowHeight);
11
12    centerX = windowWidth / 2;
13    centerY = windowHeight / 2;
14
15    std::cout <<
16      "Window center initialized at: ("
17      << centerX << ", " << centerY << ")\n";
18  }
19
20private:
21  int centerX, centerY;
22};
23
24int main(int argc, char** argv) {
25  SDL_Init(SDL_INIT_VIDEO);
26  Window GameWindow;
27  DirectionTracker Tracker{
28    GameWindow.SDLWindow};
29  SDL_Event Event;
30
31  while (true) {
32    while (SDL_PollEvent(&Event)) {
33      // ...
34    }
35    GameWindow.Update();
36  }
37
38  SDL_Quit();
39  return 0;
40}

1Window center initialized at: (350, 150)

Next, when our event loop detects the user pressing the space bar, we’ll notify the DirectionTracker:

1#include <SDL.h>
2#include <iostream>
3#include "Window.h"
4
5class DirectionTracker {
6public:
7  DirectionTracker(SDL_Window*){/*...*/}
21
22  void KeyboardEvent(SDL_KeyboardEvent& e) {
23    if (e.type == SDL_KEYDOWN
24      && e.keysym.sym == SDLK_SPACE) {
25      ReportAngle();
26    }
27  }
28
29private:
30  void ReportAngle() {
31    std::cout << "TODO: Report Angle";
32  }
33
34  int centerX, centerY;
35};
36
37int main(int argc, char** argv) {
38  SDL_Init(SDL_INIT_VIDEO);
39  Window GameWindow;
40  DirectionTracker Tracker{
41    GameWindow.SDLWindow};
42  SDL_Event Event;
43
44  while (true) {
45    while (SDL_PollEvent(&Event)) {
46      if (Event.type == SDL_KEYDOWN) {
47        Tracker.KeyboardEvent(Event.key);
48      }
49    }
50    GameWindow.Update();
51  }
52
53  SDL_Quit();
54  return 0;
55}

1Window center initialized at: (350, 150)
2TODO: Report Angle

The final step is to complete the ReportAngle() function. We query the mouse position using SDL_GetMouseState(), and implement the required calculations to determine its direction from the center of the screen:

1#include <SDL.h>
2#include <iostream>
3#include "Window.h"
4
5class DirectionTracker {
6public:
7  DirectionTracker(SDL_Window*){/*...*/}
21
22  void KeyboardEvent(SDL_KeyboardEvent&){/*...*/}
29
30private:
31  void ReportAngle() {
32    // Get current mouse position
33    int mouseX, mouseY;
34    SDL_GetMouseState(&mouseX, &mouseY);
35
36    // Calculate direction from center to mouse
37    int dirX = mouseX - centerX;
38    int dirY = mouseY - centerY;
39
40    // Calculate angle in degrees
41    double angle{
42      atan2(dirY, dirX) * 180.0 / M_PI};
43
44    std::cout << "\nDirection from center: "
45      << dirX << ", " << dirY
46      << " (angle: " << angle << " degrees)";
47  }
48
49  int centerX, centerY;
50};
51
52int main(int argc, char** argv){/*...*/}

1Window center initialized at: (350, 150)
2Direction from center: -228, -77 (angle: -161.339 degrees)
3Direction from center: 248, 113 (angle: 24.4962 degrees)