Fix Revolve clipping (#1200)

fixed sign error

Author: elalish

include/manifold/common.h

@@ -20,7 +20,7 @@
 #include <chrono>
 #endif
 
-#include "manifold/linalg.h"
+#include "linalg.h"
 
 namespace manifold {
 /** @addtogroup Math

src/constructors.cpp

@@ -15,6 +15,7 @@
 #include "./csg_tree.h"
 #include "./impl.h"
 #include "./parallel.h"
+#include "manifold/manifold.h"
 #include "manifold/polygon.h"
 
 namespace manifold {
@@ -315,7 +316,7 @@ Manifold Manifold::Revolve(const Polygons& crossSection, int circularSegments,
       }
       const size_t next = i + 1 == poly.size() ? 0 : i + 1;
       if ((poly[next].x < 0) != (poly[i].x < 0)) {
-        const double y = poly[next].y + poly[next].x *
+        const double y = poly[next].y - poly[next].x *
                                             (poly[i].y - poly[next].y) /
                                             (poly[i].x - poly[next].x);
         polygons.back().push_back({0, y});
@@ -351,8 +352,8 @@ Manifold Manifold::Revolve(const Polygons& crossSection, int circularSegments,
   const int nSlices = isFullRevolution ? nDivisions : nDivisions + 1;
 
   for (const auto& poly : polygons) {
-    std::size_t nPosVerts = 0;
-    std::size_t nRevolveAxisVerts = 0;
+    size_t nPosVerts = 0;
+    size_t nRevolveAxisVerts = 0;
     for (auto& pt : poly) {
       if (pt.x > 0) {
         nPosVerts++;

test/manifold_test.cpp

@@ -19,8 +19,6 @@
 #ifdef MANIFOLD_CROSS_SECTION
 #include "manifold/cross_section.h"
 #endif
-#include "../src/tri_dist.h"
-#include "samples.h"
 #include "test.h"
 
 namespace {
@@ -282,6 +280,16 @@ TEST(Manifold, Revolve3) {
 }
 #endif
 
+TEST(Manifold, RevolveClip) {
+  Polygons polys = {{{-5, -10}, {5, 0}, {-5, 10}}};
+  Polygons clipped = {{{0, -5}, {5, 0}, {0, 5}}};
+  Manifold first = Manifold::Revolve(polys, 48);
+  Manifold second = Manifold::Revolve(clipped, 48);
+  EXPECT_EQ(first.Genus(), second.Genus());
+  EXPECT_EQ(first.Volume(), second.Volume());
+  EXPECT_EQ(first.SurfaceArea(), second.SurfaceArea());
+}
+
 TEST(Manifold, PartialRevolveOnYAxis) {
   Polygons polys = SquareHole(2.0);
   Polygons offsetPolys = SquareHole(10.0);