static inline cairo_int128_t
det64x32_128 (cairo_int64_t a, int32_t b,
cairo_int64_t c, int32_t d)
{
/* det = a * d - b * c */
return _cairo_int128_sub (_cairo_int64x32_128_mul (a, d),
_cairo_int64x32_128_mul (c, b));
}
static const cairo_fixed_64_64_t
_det32_64 (cairo_fixed_32_32_t a,
cairo_fixed_32_32_t b,
cairo_fixed_32_32_t c,
cairo_fixed_32_32_t d)
{
return _cairo_int128_sub (_cairo_int64x64_128_mul (a, d),
_cairo_int64x64_128_mul (b, c));
}
/*
* We need to compare the x-coordinates of a pair of lines for a particular y,
* without loss of precision.
*
* The x-coordinate along an edge for a given y is:
* X = A_x + (Y - A_y) * A_dx / A_dy
*
* So the inequality we wish to test is:
* A_x + (Y - A_y) * A_dx / A_dy ∘ B_x + (Y - B_y) * B_dx / B_dy,
* where ∘ is our inequality operator.
*
* By construction, we know that A_dy and B_dy (and (Y - A_y), (Y - B_y)) are
* all positive, so we can rearrange it thus without causing a sign change:
* A_dy * B_dy * (A_x - B_x) ∘ (Y - B_y) * B_dx * A_dy
* - (Y - A_y) * A_dx * B_dy
*
* Given the assumption that all the deltas fit within 32 bits, we can compute
* this comparison directly using 128 bit arithmetic. For certain, but common,
* input we can reduce this down to a single 32 bit compare by inspecting the
* deltas.
*
* (And put the burden of the work on developing fast 128 bit ops, which are
* required throughout the tessellator.)
*
* See the similar discussion for _slope_compare().
*/
static int
edges_compare_x_for_y_general (const cairo_bo_edge_t *a,
const cairo_bo_edge_t *b,
int32_t y)
{
/* XXX: We're assuming here that dx and dy will still fit in 32
* bits. That's not true in general as there could be overflow. We
* should prevent that before the tessellation algorithm
* begins.
*/
int32_t dx;
int32_t adx, ady;
int32_t bdx, bdy;
enum {
HAVE_NONE = 0x0,
HAVE_DX = 0x1,
HAVE_ADX = 0x2,
HAVE_DX_ADX = HAVE_DX | HAVE_ADX,
HAVE_BDX = 0x4,
HAVE_DX_BDX = HAVE_DX | HAVE_BDX,
HAVE_ADX_BDX = HAVE_ADX | HAVE_BDX,
HAVE_ALL = HAVE_DX | HAVE_ADX | HAVE_BDX
} have_dx_adx_bdx = HAVE_ALL;
/* don't bother solving for abscissa if the edges' bounding boxes
* can be used to order them. */
{
int32_t amin, amax;
int32_t bmin, bmax;
if (a->edge.line.p1.x < a->edge.line.p2.x) {
amin = a->edge.line.p1.x;
amax = a->edge.line.p2.x;
} else {
amin = a->edge.line.p2.x;
amax = a->edge.line.p1.x;
}
if (b->edge.line.p1.x < b->edge.line.p2.x) {
bmin = b->edge.line.p1.x;
bmax = b->edge.line.p2.x;
} else {
bmin = b->edge.line.p2.x;
bmax = b->edge.line.p1.x;
}
if (amax < bmin) return -1;
if (amin > bmax) return +1;
}
ady = a->edge.line.p2.y - a->edge.line.p1.y;
adx = a->edge.line.p2.x - a->edge.line.p1.x;
if (adx == 0)
have_dx_adx_bdx &= ~HAVE_ADX;
bdy = b->edge.line.p2.y - b->edge.line.p1.y;
bdx = b->edge.line.p2.x - b->edge.line.p1.x;
if (bdx == 0)
have_dx_adx_bdx &= ~HAVE_BDX;
dx = a->edge.line.p1.x - b->edge.line.p1.x;
if (dx == 0)
have_dx_adx_bdx &= ~HAVE_DX;
#define L _cairo_int64x32_128_mul (_cairo_int32x32_64_mul (ady, bdy), dx)
#define A _cairo_int64x32_128_mul (_cairo_int32x32_64_mul (adx, bdy), y - a->edge.line.p1.y)
#define B _cairo_int64x32_128_mul (_cairo_int32x32_64_mul (bdx, ady), y - b->edge.line.p1.y)
switch (have_dx_adx_bdx) {
default:
case HAVE_NONE:
return 0;
case HAVE_DX:
/* A_dy * B_dy * (A_x - B_x) ∘ 0 */
return dx; /* ady * bdy is positive definite */
case HAVE_ADX:
/* 0 ∘ - (Y - A_y) * A_dx * B_dy */
return adx; /* bdy * (y - a->top.y) is positive definite */
case HAVE_BDX:
/* 0 ∘ (Y - B_y) * B_dx * A_dy */
return -bdx; /* ady * (y - b->top.y) is positive definite */
case HAVE_ADX_BDX:
/* 0 ∘ (Y - B_y) * B_dx * A_dy - (Y - A_y) * A_dx * B_dy */
if ((adx ^ bdx) < 0) {
return adx;
} else if (a->edge.line.p1.y == b->edge.line.p1.y) { /* common origin */
cairo_int64_t adx_bdy, bdx_ady;
/* ∴ A_dx * B_dy ∘ B_dx * A_dy */
adx_bdy = _cairo_int32x32_64_mul (adx, bdy);
bdx_ady = _cairo_int32x32_64_mul (bdx, ady);
return _cairo_int64_cmp (adx_bdy, bdx_ady);
} else
return _cairo_int128_cmp (A, B);
case HAVE_DX_ADX:
/* A_dy * (A_x - B_x) ∘ - (Y - A_y) * A_dx */
if ((-adx ^ dx) < 0) {
return dx;
} else {
cairo_int64_t ady_dx, dy_adx;
ady_dx = _cairo_int32x32_64_mul (ady, dx);
dy_adx = _cairo_int32x32_64_mul (a->edge.line.p1.y - y, adx);
return _cairo_int64_cmp (ady_dx, dy_adx);
}
case HAVE_DX_BDX:
/* B_dy * (A_x - B_x) ∘ (Y - B_y) * B_dx */
if ((bdx ^ dx) < 0) {
return dx;
} else {
cairo_int64_t bdy_dx, dy_bdx;
bdy_dx = _cairo_int32x32_64_mul (bdy, dx);
dy_bdx = _cairo_int32x32_64_mul (y - b->edge.line.p1.y, bdx);
return _cairo_int64_cmp (bdy_dx, dy_bdx);
}
case HAVE_ALL:
/* XXX try comparing (a->edge.line.p2.x - b->edge.line.p2.x) et al */
return _cairo_int128_cmp (L, _cairo_int128_sub (B, A));
}
#undef B
#undef A
#undef L
}