/wiki/formulas

Object Rotation Around Midpoint

/* Convert to degrees */
double angle = radians * 180 / PI;

/* rotate around origin point */
double dx = ox + cos(angle)*radius;
double dy = oy + sin(angle)*radius;

Midpoint Circle Algorithm

int diam = (radius * 2);

int x = radius - 1; int y = 0; int tx = 1; int ty = 1; int error = tx - diam;

while (x >= y) {

SDL_RenderDrawPoint(gRen, cx + x, cy + y);
SDL_RenderDrawPoint(gRen, cx + x, cy - y);
SDL_RenderDrawPoint(gRen, cx - x, cy + y);
SDL_RenderDrawPoint(gRen, cx - x, cy - y);
SDL_RenderDrawPoint(gRen, cx + y, cy - x);
SDL_RenderDrawPoint(gRen, cx + y, cy + x);
SDL_RenderDrawPoint(gRen, cx - y, cy - x);
SDL_RenderDrawPoint(gRen, cx - y, cy + x);

if (error <= 0)

{
  ++y;
  error += ty;
  ty += 2;
}

if( error > 0)
{
  --x;
  tx += 2;
  error += (tx - diam);