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Avoid arithmetic overflow in area calculation

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Eric Fischer 2015-12-09 15:02:59 -08:00
parent 4bde17f8ff
commit f04c5e153a
1 changed files with 37 additions and 25 deletions
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62
geometry.cc
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@ -357,41 +357,53 @@ drawvec reduce_tiny_poly(drawvec &geom, int z, int detail, bool *reduced, double
double area = 0;
for (unsigned k = i; k < j; k++) {
area += geom[k].x * geom[i + ((k - i + 1) % (j - i))].y;
area -= geom[k].y * geom[i + ((k - i + 1) % (j - i))].x;
area += (long double) geom[k].x * (long double) geom[i + ((k - i + 1) % (j - i))].y;
area -= (long double) geom[k].y * (long double) geom[i + ((k - i + 1) % (j - i))].x;
}
area = area / 2;
if (fabs(area) <= pixel * pixel || (area < 0 && !included_last_outer)) {
// printf("area is only %f vs %lld so using square\n", area, pixel * pixel);
// XXX There is an ambiguity here: If the area of a ring is 0 and it is followed by holes,
// we don't know whether the area-0 ring was a hole too or whether it was the outer ring
// that these subsequent holes are somehow being subtracted from. I hope that if a polygon
// was simplified down to nothing, its holes also became nothing.
*accum_area += area;
if (*accum_area > pixel * pixel) {
// XXX use centroid;
if (area != 0) {
// These are pixel coordinates, so area > 0 for the outer ring.
// If the outer ring of a polygon was reduced to a pixel, its
// inner rings must just have their area de-accumulated rather
// than being drawn since we don't really know where they are.
out.push_back(draw(VT_MOVETO, geom[i].x - pixel / 2, geom[i].y - pixel / 2));
out.push_back(draw(VT_LINETO, geom[i].x + pixel / 2, geom[i].y - pixel / 2));
out.push_back(draw(VT_LINETO, geom[i].x + pixel / 2, geom[i].y + pixel / 2));
out.push_back(draw(VT_LINETO, geom[i].x - pixel / 2, geom[i].y + pixel / 2));
out.push_back(draw(VT_LINETO, geom[i].x - pixel / 2, geom[i].y - pixel / 2));
if (fabs(area) <= pixel * pixel || (area < 0 && !included_last_outer)) {
// printf("area is only %f vs %lld so using square\n", area, pixel * pixel);
*accum_area -= pixel * pixel;
}
*accum_area += area;
if (area > 0 && *accum_area > pixel * pixel) {
// XXX use centroid;
if (area >= 0) {
included_last_outer = false;
}
} else {
// printf("area is %f so keeping instead of %lld\n", area, pixel * pixel);
out.push_back(draw(VT_MOVETO, geom[i].x - pixel / 2, geom[i].y - pixel / 2));
out.push_back(draw(VT_LINETO, geom[i].x + pixel / 2, geom[i].y - pixel / 2));
out.push_back(draw(VT_LINETO, geom[i].x + pixel / 2, geom[i].y + pixel / 2));
out.push_back(draw(VT_LINETO, geom[i].x - pixel / 2, geom[i].y + pixel / 2));
out.push_back(draw(VT_LINETO, geom[i].x - pixel / 2, geom[i].y - pixel / 2));
for (unsigned k = i; k <= j && k < geom.size(); k++) {
out.push_back(geom[k]);
}
*accum_area -= pixel * pixel;
}
*reduced = false;
if (area > 0) {
included_last_outer = false;
}
} else {
// printf("area is %f so keeping instead of %lld\n", area, pixel * pixel);
if (area >= 0) {
included_last_outer = true;
for (unsigned k = i; k <= j && k < geom.size(); k++) {
out.push_back(geom[k]);
}
*reduced = false;
if (area > 0) {
included_last_outer = true;
}
}
}