Get texture coordinates of mouse position in SDL2? - c

Question

I have the strict requirement to have a texture with resolution (let's say) of 512x512, always (even if the window is bigger, and SDL basically scales the texture for me, on rendering). This is because it's an emulator of a classic old computer assuming a fixed texture, I can't rewrite the code to adopt multiple texture sizes and/or texture ratios dynamically.
I use SDL_RenderSetLogicalSize() for the purpose I've described above.
Surely, when this is rendered into a window, I can get the mouse coordinates (window relative), and I can "scale" back to the texture position with getting the real window size (since the window can be resized).
However, there is a big problem here. As soon as window width:height ratio is not the same as the texture's ratio (for example in full screen mode, since the ratio of modern displays would not match of the ratio I want to use), there are "black bars" at the sides or top/bottom. Which is nice, since I want always the same texture ratio, fixed, and SDL does it for me, etc. However I cannot find a way to ask SDL where is my texture rendered exactly inside the window based on the fixed ratio I forced. Since I need the position within the texture only, and the exact texture origin is placed by SDL itself, not by me.
Surely, I can write some code to figure out how those "black bars" would change the origin of the texture, but I can hope there is a more simple and elegant way to "ask" SDL about this, since surely it has the code to position my texture somewhere, so I can re-use that information.
My very ugly (can be optimized, and floating point math can be avoided I think, but as the first try ...) solution:
static void get_mouse_texture_coords ( int x, int y )
{
int win_x_size, win_y_size;
SDL_GetWindowSize(sdl_win, &win_x_size, &win_y_size);
// I don't know if there is more sane way for this ...
// But we must figure out where is the texture within the window,
// which can be changed since the fixed ratio versus the window ratio (especially in full screen mode)
double aspect_tex = (double)SCREEN_W / (double)SCREEN_H;
double aspect_win = (double)win_x_size / (double)win_y_size;
if (aspect_win >= aspect_tex) {
// side ratio correction bars must be taken account
double zoom_factor = (double)win_y_size / (double)SCREEN_H;
int bar_size = win_x_size - (int)((double)SCREEN_W * zoom_factor);
mouse_x = (x - bar_size / 2) / zoom_factor;
mouse_y = y / zoom_factor;
} else {
// top-bottom ratio correction bars must be taken account
double zoom_factor = (double)win_x_size / (double)SCREEN_W;
int bar_size = win_y_size - (int)((double)SCREEN_H * zoom_factor);
mouse_x = x / zoom_factor;
mouse_y = (y - bar_size / 2) / zoom_factor;
}
}
Where SCREEN_W and SCREEN_H are the dimensions of the my texture, quite misleading by names, but anyway. Input parameters x and y are the window-relative mouse position (reported by SDL). mouse_x and mouse_y are the result, the texture based coordinates. This seems to work nicely. However, is there any sane solution or a better one?
The code which calls the function above is in my event handler loop (which I call regularly, of course), something like this:
void handle_sdl_events ( void ) {
SDL_Event event;
while (SDL_PollEvent(&event)) {
switch (event.type) {
case SDL_MOUSEMOTION:
get_mouse_texture_coords(event.motion.x, event.motion.y);
break;
[...]