CODEWITHSUNDEEP
c++mathsdl-2
Aquac
Aquac

Reputation: 31

SDL2 fill circle - Same operations, different results

I've tried to draw a filled circle with SDL2 and I used Pythagoras to calculate the points.

The problem is that I'm getting different results depending if I'm calculating the X coordinate or the Y coordinate. Someone could explain me why when I'm calculating the X coordinate the circle is not drawn correctly?

200 and 500 are hardcoded positions.

void DrawBall()
{

    for(int y = 1; y <= this->iRadius; y++)
    {

        int x = sqrt((this->iRadius * this->iRadius) - (y * y));
        SDL_RenderDrawLine(this->renderer, 200 - x, 200 + y , 200 + x, 200 + y);
        SDL_RenderDrawLine(this->renderer, 200 - x, 200 - y  + 1, 200 + x, 200 - y + 1);

    }


    for (int x = 1; x <= this->iRadius; x++) {

        int y = sqrt((this->iRadius * this->iRadius) - (x * x));

        SDL_RenderDrawLine(this->renderer, 500 - x, 500 + y, 500 + x, 500 + y);
        SDL_RenderDrawLine(this->renderer, 500 - x, 500 - y + 1, 500 + x, 500 - y + 1);

    }
}

Top-left calculating x, bottom-right calculating y

Upvotes: 1

Views: 316

Answers (1)

MBo
MBo

Reputation: 80187

If you output x/y values for the second case and radius 10, you'll see dense y-values for small x and sparse y-values for larger x. Some lines (y=9) are drawn many times, some horizontals aren't drawn at all (y=1,2,5 here)

9.9 9.8 9.5 9.1 8.6 8.0 7.1 6.0 4.3 0

So horizontal line drawing shows such picture. But you can draw vertical lines - in this case you fill all the circle. (I omitted +1 in code).

Also note that for circle with radius R you should draw 2*r+1 lines, not 2*r. 

 SDL_RenderDrawLine(this->renderer, 500 - x, 500 - y, 500 - x, 500 + y);
 SDL_RenderDrawLine(this->renderer, 500 + x, 500 - y, 500 + x, 500 + y);

Upvotes: 2

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