From f04c5e153a0e34ea3a4f72a48df4dfddfe4703f3 Mon Sep 17 00:00:00 2001 From: Eric Fischer Date: Wed, 9 Dec 2015 15:02:59 -0800 Subject: [PATCH] Avoid arithmetic overflow in area calculation --- geometry.cc | 62 ++++++++++++++++++++++++++++++++--------------------- 1 file changed, 37 insertions(+), 25 deletions(-) diff --git a/geometry.cc b/geometry.cc index 586bfa4..36ba691 100644 --- a/geometry.cc +++ b/geometry.cc @@ -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; + } } }