Question

I was trying to draw a shaded triangle, but I get wrong colors. Here’s the example of what I get, if I call a function with red, green and blue colors:

It should be red on the top left, green on the top right and blue on the top bottom. But as you can see the colors are not like that. I should get something like this:

Here’s the code:

unsigned RGB(unsigned char R, unsigned char G, unsigned B){
    return  (R << 16) | (G << 8) | B;
}

    float get_dist(int x1, int y1, int x2, int y2){
            int xproj = x2-x1;
            int yproj = y2-y1;
            float dist = sqrt(pow(xproj, 2) + pow(yproj, 2));
            return dist;
    }
    unsigned char getR(unsigned c){
            return (c >> 16) & 0xFF;
    }
    unsigned char getG(unsigned c){
            return (c >> 8) & 0xFF;
    }
    unsigned char getB(unsigned c){
            return c & 0xFF;
    }
    void draw_ftriangle(SDL_Surface * s, int x0, int y0, unsigned c0, int x1, int y1, unsigned c1, int x2, int y2, unsigned c2){
            int x;
            signed d0, d1, d2;
            unsigned R, G, B, 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(x=xmin; x < xmax; x++)
                            if(ptintr(x, ymin, x0, y0, x1, y1, x2, y2)){
                                    d0 = get_dist(x, ymin, x0, y0);
                                    d1 = get_dist(x, ymin, x1, y1);
                                    d2 = get_dist(x, ymin, x2, y2);
                                    R = d0+d1+d2 == 1 ? getR(c0)*d0 + getR(c1)*d1 + getR(c2)*d2 :
                                                                             (getR(c0)*d0 + getR(c1)*d1 + getR(c2)*d2)/(d0+d1+d2);
                                    G = d0+d1+d2 == 1 ? getG(c0)*d0 + getG(c1)*d1 + getG(c2)*d2 :
                                                                            (getG(c0)*d0 + getG(c1)*d1 + getG(c2)*d2)/(d0+d1+d2);
                                    B = d0+d1+d2 == 1 ? getB(c0)*d0 + getB(c1)*d1 + getB(c2)*d2 :
                                                                            (getB(c0)*d0 + getB(c1)*d1 + getB(c2)*d2)/(d0+d1+d2);
                                    color = RGB(R, G, B);
                                    put_pixel(s, x, ymin, color);
                            }
    }

I call a function drraw_ftriangle like that:

draw_ftriangle(surface, 100, 100, RGB(255, 0, 0),
235, 150, RGB(0, 255, 0),
4, 254, RGB(0, 0, 255));

I’m not sure, but I think the problem should be here:

R = (d0+d1+d2) == 1 ? getR(c0)*d0 + getR(c1)*d1 + getR(c2)*d2 :
                      (getR(c0)*d0 + getR(c1)*d1 + getR(c2)*d2)/(d0+d1+d2);

Same with G and B variables…

I’d like other people to explain me, where’s the problem, because I’m searching for it all day long…

Answer 1

The R and G parameters to your RGB function are unsigned char, and you are not casting them to unsigned int before shifting. That means, you’re shifting an 8-bit number. You know what happens…

So, either convert the parameters to unsigned int or cast the values before shifting.

PS: Whatever you do, it would be nice if you made all three parameters the same type!

As in comments, turns out it was this:

So I don’t think your (x,y) coordinates are normalised in that
get_dist function. You compute the length of the hypoteneuse
connecting those two vectors, but you don’t give it any base lengths
to normalise on.