How can I implement zoom functionality using the mouse scroll wheel in SDL?

Question

Ryan McCombe · Accepted Answer

Implementing zoom functionality using the mouse scroll wheel in SDL involves detecting the scroll events and adjusting the zoom level of your application accordingly.

Here's an example application that keeps track of a zoom level:

1#include <SDL.h>
2#include <iostream>
3
4int main(int argc, char* argv[]) {
5  SDL_Init(SDL_INIT_VIDEO);
6  SDL_Window* window = SDL_CreateWindow(
7    "Zoom Functionality Example",
8    SDL_WINDOWPOS_UNDEFINED,
9    SDL_WINDOWPOS_UNDEFINED,
10    800, 600,
11    SDL_WINDOW_SHOWN
12  );
13
14  SDL_Event event;
15  bool running = true;
16  float zoomLevel = 1.0f; 
17
18  while (running) {
19    while (SDL_PollEvent(&event)) {
20      if (event.type == SDL_MOUSEWHEEL) {
21        if (event.wheel.y > 0) {
22          zoomLevel += 0.1f; 
23        } else if (event.wheel.y < 0) {
24          zoomLevel -= 0.1f; 
25        }
26        std::cout << "Zoom level: "
27          << zoomLevel << '\n'; 
28      }
29      if (event.type == SDL_QUIT) {
30        running = false;
31      }
32    }
33  }
34
35  SDL_DestroyWindow(window);
36  SDL_Quit();
37  return 0;
38}

1Zoom level: 1.1
2Zoom level: 1.0

In the event loop, check for SDL_MOUSEWHEEL events and adjust the zoomLevel variable based on the y value of event.wheel. Positive values increase the zoom level, and negative values decrease it.

Once you have adjusted the zoomLevel, you need to apply it to your rendering logic. This can be done by scaling your rendering based on the zoomLevel.

Here's an example of how to integrate zooming into your rendering:

1#include <SDL.h>
2#include <iostream>
3
4void Render(
5  SDL_Renderer* renderer, float zoomLevel) {
6  SDL_SetRenderDrawColor(renderer, 0, 0, 0, 255);
7  SDL_RenderClear(renderer);
8
9  SDL_SetRenderDrawColor(
10    renderer, 255, 255, 255, 255);
11  int width = static_cast<int>(50 * zoomLevel); 
12  int height = static_cast<int>(50 * zoomLevel); 
13  SDL_Rect rect = {375, 275, width, height};
14  SDL_RenderFillRect(renderer, &rect);
15
16  SDL_RenderPresent(renderer);
17}
18
19int main(int argc, char* argv[]) {
20  SDL_Init(SDL_INIT_VIDEO);
21  SDL_Window* window = SDL_CreateWindow(
22    "Zoom Functionality Example",
23    SDL_WINDOWPOS_UNDEFINED,
24    SDL_WINDOWPOS_UNDEFINED,
25    800, 600,
26    SDL_WINDOW_SHOWN
27  );
28  SDL_Renderer* renderer = SDL_CreateRenderer(
29    window, -1, 0);
30
31  SDL_Event event;
32  bool running = true;
33  float zoomLevel = 1.0f;
34
35  while (running) {
36    while (SDL_PollEvent(&event)) {
37      if (event.type == SDL_MOUSEWHEEL) {
38        if (event.wheel.y > 0) {
39          zoomLevel += 0.1f;
40        } else if (event.wheel.y < 0) {
41          zoomLevel -= 0.1f;
42        }
43        std::cout << "Zoom level: "
44          << zoomLevel << '\n';
45      }
46      if (event.type == SDL_QUIT) {
47        running = false;
48      }
49    }
50    Render(renderer, zoomLevel);
51  }
52
53  SDL_DestroyRenderer(renderer);
54  SDL_DestroyWindow(window);
55  SDL_Quit();
56  return 0;
57}

1Zoom level: 1.1
2Zoom level: 1.0

By following these steps, you can implement zoom functionality in your SDL application using the mouse scroll wheel.

Adjust the zoomLevel based on scroll input and apply it to your rendering logic to achieve the desired zoom effect.

Handling Mouse Scrolling

Implement Zoom with Mouse Scroll

Handling Mouse Scrolling

Game Development with SDL2

Questions & Answers