How do I know if a point is in triangle?

Question

I've got a simple 2D triangle. I've been searching how do I know if a given point belongs to that triangle. Here's the algorithm (I've found it here: How to determine if a point is in a 2D triangle?), which's very good and fast (according to other people):

float sign(int x0, int y0, int x1, int y1, int x2, int y2){
    return (x0 - x2) * (y1 - y2) - (x1 - x2) * (y0 - y2);
}

int ptintr(int sx, int sy, int x0, int y0, int x1, int y1, int x2, int y2){
    int b1, b2, b3;

    b1 = sign(sx, sy, x0, y0, x1, y1) < 0.0f ? 1 : 0;
    b2 = sign(sx, sy, x1, y1, x2, y2) < 0.0f ? 1 : 0;
    b3 = sign(sx, sy, x2, y2, x0, y0) < 0.0f ? 1 : 0;

    if((b1) == b2 && (b2 == b3))
        return 1;
    return 0;
}

I call this function inside draw_ftriangle():

void draw_ftriangle(SDL_Surface * s, int x0, int y0, int x1, int y1, int x2, int y2, unsigned color){
    int ymin = min(y0, min(y1, y2));
    int xmin = min(x0, min(x1, x2));
    int ymax = max(y0, max(y1, y2));
    int xmax = max(x0, max(x1, x2));

    draw_line(s, x0, y0, x1, y1, color);
    draw_line(s, x1, y1, x2, y2, color);
    draw_line(s, x0, y0, x2, y2, color);

    for(; ymin < ymax; ymin++)
        for(; xmin < xmax; xmin++)
            if(ptintr(xmin, ymin, x0, y0, x1, y1, x2, y2))
                put_pixel(s, xmin, ymin, color);
}

Here sx and sy is the given point's coordinates, and x0, x1, x2, y0, y1, y2 are the points of triangle. But this algorithm doesn't work. Whenever I give to this function a coordinates of a point and a coordinates of triangle's point, it always return false. Could anyone tell me if this algorithm is correct, or maybe I left some mistake here?

Answer 1

Your loop only does the top line because xmin is not reset.

In other words, the first time through the inner x loop you will increase xmin until it becomes xmax. The second time you do the inner x loop, xmin is already xmax so nothing happens.

Try

int x;
for(; ymin < ymax; ymin++)
    for(x=xmin; x < xmax; x++)
        if(ptintr(x, ymin, x0, y0, x1, y1, x2, y2))
            put_pixel(s, x, ymin, color);