3d sphere with Z coordinate transorms in sdl3

tilt_sphere.cpp

#include <SDL3/SDL.h>
#include <SDL3/SDL_main.h>
#include <cmath>

// Calculates the X and Y screen coordinates for a circle centered anywhere
void get_arbitrary_circle_point_trig(
    float centerX, float centerY, 
    float radius, float angle_radians, 
    float* out_x, float* out_y
) {
    // 1. Calculate the local x distance, then shift it by the center X
    *out_x = centerX + (radius * cosf(angle_radians));
    
    // 2. Calculate the local y distance, then shift it by the center Y
    *out_y = centerY + (radius * sinf(angle_radians));
}




void gen_circle_points(float centerX, 
                       float centerY, 
                       float radius, 
                       SDL_FPoint* points) {
    int total_steps = (int)(2.0f * SDL_PI_F * radius); 
    float angle_step = (2.0f * SDL_PI_F) / total_steps;

    for (int i = 0; i < total_steps; i++)
    {
        float angle = i * angle_step;
        float x, y;
        get_arbitrary_circle_point_trig(centerX, centerY, radius, angle, &x, &y);
        points->x = x;
        points->y = y;
        ++points;
    }
}


// treat x y as if it was x z for a transform, semi 3d looking circle overlay for sphere
void transform_z_points(const SDL_FPoint* src, SDL_FPoint* dst, int amount, float centerY) {
    for (int i = 0; i < amount; ++i) {
        dst[i].x = src[i].x;
        
        // 1. Get distance from center (Local Y)
        float localY = src[i].y - centerY; 
        
        // 2. Squash/tilt it around its center, then move back to screen position
        dst[i].y = centerY + (localY * 0.5f);  // low tilt is more constrained
    }
}



int main(int argc, char *argv[])
{
  bool quit = false;
  SDL_Window *window = SDL_CreateWindow("Triangle Example", 800, 600, 0);

  SDL_Renderer *renderer = SDL_CreateRenderer(window, NULL);

  static constexpr float centerX = 400.0f;
  static constexpr float centerY = 300.0f;

  static constexpr float circleRads = 40.0f;

  static constexpr int circlePointCount = (int)(2.0f * SDL_PI_F * circleRads); 

  SDL_FPoint topPoints[circlePointCount];

  SDL_FPoint topPoints2[circlePointCount];

  gen_circle_points(centerX, centerY, circleRads, topPoints);

  transform_z_points(topPoints, topPoints2, circlePointCount, centerY);



  while (!quit) {
   SDL_Event ev;
   while (SDL_PollEvent(&ev) != 0) {
      switch(ev.type) {
        case SDL_EVENT_QUIT:
        quit = true;
        break;
      }
    }
    SDL_SetRenderDrawColor(renderer, 0, 0, 0, 255);
    SDL_RenderClear(renderer);


    SDL_SetRenderDrawColor(renderer, 233, 0, 0, 255);

    SDL_RenderPoints(renderer, topPoints, circlePointCount);

    SDL_SetRenderDrawColor(renderer, 0, 233, 0, 255);

    SDL_RenderPoints(renderer, topPoints2, circlePointCount);

    SDL_Delay(16);

    SDL_RenderPresent(renderer);
  }

  SDL_DestroyRenderer(renderer);
  SDL_DestroyWindow(window);
  SDL_Quit();
  return 0;
}