DPI Scale Calculation

The decision to use only width for DPI scale calculation is based on how modern displays and operating systems handle DPI scaling. Let's explore why this works and when you might need to consider both dimensions.

Why Width is Usually Sufficient

Most modern displays maintain consistent DPI scaling across both dimensions. Here's how we can verify this:

1#include <SDL.h>
2#include <iostream>
3
4void CheckScaling(SDL_Window* Window) {
5  int w1, h1, w2, h2;
6  SDL_GetWindowSize(Window, &w1, &h1);
7  SDL_GetWindowSizeInPixels(Window, &w2, &h2);
8
9  float ScaleX{float(w2) / w1};
10  float ScaleY{float(h2) / h1};
11
12  std::cout << "Scale X: " << ScaleX << "\n"
13            << "Scale Y: " << ScaleY << "\n";
14}
15
16int main() {
17  SDL_Init(SDL_INIT_VIDEO);
18  SDL_Window* Window{SDL_CreateWindow(
19    "Scale Test",
20    SDL_WINDOWPOS_UNDEFINED,
21    SDL_WINDOWPOS_UNDEFINED,
22    800, 600, 0)};
23
24  CheckScaling(Window);
25
26  SDL_DestroyWindow(Window);
27  SDL_Quit();
28  return 0;
29}

On a typical 125% DPI scale, you'll see:

1Scale X: 1.25
2Scale Y: 1.25

When to Consider Both Dimensions

However, there are cases where you might want to check both dimensions:

• Non-square pixel displays (rare in modern systems)
• Custom scaling settings
• Platform-specific quirks

Here's an improved version of GetDPIScale():

1struct Scale2D {
2  float X, Y;
3};
4
5Scale2D GetDPIScale2D(SDL_Window* Window) {
6  int w1, h1, w2, h2;
7  SDL_GetWindowSize(Window, &w1, &h1);
8  SDL_GetWindowSizeInPixels(Window, &w2, &h2);
9
10  return Scale2D{
11    float(w2) / w1,
12    float(h2) / h1
13  };
14}
15
16void HandleDifferentScales(SDL_Window* Window) {
17  Scale2D Scale{GetDPIScale2D(Window)};
18  if (std::abs(Scale.X - Scale.Y) > 0.01f) {
19    std::cout << "Warning: Non-uniform "
20      "scaling detected\n";
21  }
22}

In practice, using width alone is usually sufficient because:

• Operating systems maintain uniform scaling
• Different X/Y scales can cause visual problems
• Most UI frameworks assume uniform scaling