Rasterise triangle

rast.c

#define EDGE_FUNC(a, b, c) (((c).x - (a).x) * ((b).y - (a).y) - ((c).y - (a).y) * ((b).x - (a).x))

static void rasterize_triangle(old_renderer_t *r, const scene_t *scene, const triangle_t *t) {
    vec3_t a = t->a, b = t->b, c = t->c;

    float inv_az = 1.0f/a.z;
    float inv_bz = 1.0f/b.z;
    float inv_cz = 1.0f/c.z;

    // projected
    vec2_t a_2p = vec2_mults(vec2_make(a.x, a.y), inv_az);
    vec2_t b_2p = vec2_mults(vec2_make(b.x, b.y), inv_bz);
    vec2_t c_2p = vec2_mults(vec2_make(c.x, c.y), inv_cz);

    // backface culling
    vec2_t a2b2 = vec2_sub(b_2p, a_2p);
    vec2_t a2c2 = vec2_sub(c_2p, a_2p);
    float x = vec2_cross(a2b2, a2c2);
    if (x > 0.0) {
        return;
    }

    // ndc
    vec2_t a_2ndc = vec2_div(vec2_add(a_2p, vec2_make(CANVAS_WIDTH/2.0, CANVAS_HEIGHT/2.0)), vec2_make(CANVAS_WIDTH, CANVAS_HEIGHT));
    vec2_t b_2ndc = vec2_div(vec2_add(b_2p, vec2_make(CANVAS_WIDTH/2.0, CANVAS_HEIGHT/2.0)), vec2_make(CANVAS_WIDTH, CANVAS_HEIGHT));
    vec2_t c_2ndc = vec2_div(vec2_add(c_2p, vec2_make(CANVAS_WIDTH/2.0, CANVAS_HEIGHT/2.0)), vec2_make(CANVAS_WIDTH, CANVAS_HEIGHT));

    // raster
    vec2_t a_2r = vec2_mult(a_2ndc, vec2_make(r->width, r->height));
    vec2_t b_2r = vec2_mult(b_2ndc, vec2_make(r->width, r->height));
    vec2_t c_2r = vec2_mult(c_2ndc, vec2_make(r->width, r->height));

    int xmin = min3i(a_2r.x, b_2r.x, c_2r.x);
    int xmax = max3i(a_2r.x, b_2r.x, c_2r.x);
    int ymin = min3i(a_2r.y, b_2r.y, c_2r.y);
    int ymax = max3i(a_2r.y, b_2r.y, c_2r.y);

    if ((xmin < 0 && xmax < 0) || (ymin < 0 && ymax < 0)
        || (xmin > r->width && xmax > r->width)
        || (ymin > r->height && ymax > r->height)) {
        return;
    }

    xmin = max2i(xmin, 0);
    xmax = min2i(xmax, r->width-1);
    ymin = max2i(ymin, 0);
    ymax = min2i(ymax, r->height-1);

    float area = EDGE_FUNC(a_2r, b_2r, c_2r);
    if (area == 0) {
        return;
    }
    float inv_area = 1.0f / area;

    int fdid = -1;

    vec2_t v0 = a_2r, v1 = b_2r, v2 = c_2r;
    float a12 = (v2.y - v1.y);
    float b12 = (v1.x - v2.x);
    float c12 = (v2.x*v1.y - v2.y*v1.x);

    float a20 = (v0.y - v2.y);
    float b20 = (v2.x - v0.x);
    float c20 = (v0.x*v2.y - v0.y*v2.x);

    float a01 = (v1.y - v0.y);
    float b01 = (v0.x - v1.x);
    float c01 = (v1.x*v0.y - v1.y*v0.x);

    vec2_t p = vec2_make(xmin+0.5f, ymin+0.5f);

    float row_f12 = a12*p.x + b12*p.y + c12;
    float row_f20 = a20*p.x + b20*p.y + c20;
    float row_f01 = a01*p.x + b01*p.y + c01;

    for (int y = ymin; y <= ymax; y+=1) {
        bool was_inside = false;

        float f12 = row_f12;
        float f20 = row_f20;
        float f01 = row_f01;

        for (int x = xmin; x <= xmax; x+=1) {
            if (f12 < 0 || f20 < 0 || f01 < 0) {
                if (was_inside) {
                    break;
                }
                f12 += a12;
                f20 += a20;
                f01 += a01;
                continue;
            }
            was_inside = true;
            float w0 = f12 * inv_area;
            float w1 = f20 * inv_area;
            float w2 = f01 * inv_area;
            float inv_z = (w0*inv_az) + (w1*inv_bz) + (w2*inv_cz);
            int ix = (r->width*(r->height-y-1)+(r->width-x-1));
            if (inv_z < r->zbuffer[ix]) {
                f12 += a12;
                f20 += a20;
                f01 += a01;
                continue;
            }
            r->zbuffer[ix] = inv_z;

            if (fdid == -1) {
                vec2_t st0 = vec2_mults(t->ta, inv_az);
                vec2_t st1 = vec2_mults(t->tb, inv_bz);
                vec2_t st2 = vec2_mults(t->tc, inv_cz);
                vec3_t n = vec3_cross(vec3_sub(t->b, t->a), vec3_sub(t->c, t->a));
                n = vec3_normalize(n);
                float pxa = (t->a.x*-inv_az), pxb = (t->b.x*-inv_bz), pxc = (t->c.x*-inv_cz);
                float pya = (t->a.y*-inv_az), pyb = (t->b.y*-inv_bz), pyc = (t->c.y*-inv_cz);
                fragment_data_t fd = (fragment_data_t) {
                    .inv_az = inv_az,
                    .inv_bz = inv_bz,
                    .inv_cz = inv_cz,
                    .st0 = st0,
                    .st1 = st1,
                    .st2 = st2,
                    .n = n,
                    .pxa = pxa, .pxb = pxb, .pxc = pxc,
                    .pya = pya, .pyb = pyb, .pyc = pyc,
                    .tex = t->texture,
                };
                dynarray_append(&r->fragment_data, &fd);
                fdid = (int)r->fragment_data.count-1;
            }

            r->fragment_buffer[ix] = (fragment_t){
                .data_id = fdid,
                .w0 = w0,
                .w1 = w1,
                .inv_z = inv_z,
            };
            f12 += a12;
            f20 += a20;
            f01 += a01;
        }
        row_f12 += b12;
        row_f20 += b20;
        row_f01 += b01;
    }
}