From 7f9117b2bf0636a594eb1f1ba76b9dbe0e8a4e83 Mon Sep 17 00:00:00 2001
From: EvilSpirit <anandamide@mail.ru>
Date: Wed, 11 Sep 2019 10:30:48 +0000
Subject: [PATCH] Calculate area of selected faces, if any.

---
 CHANGELOG.md       |  2 ++
 src/mesh.cpp       | 11 +++++++++++
 src/polygon.cpp    |  6 ++++++
 src/polygon.h      |  2 ++
 src/solvespace.cpp | 17 +++++++++++++++++
 5 files changed, 38 insertions(+)

diff --git a/CHANGELOG.md b/CHANGELOG.md
index 5dbdf355..b7f6d7a2 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -72,6 +72,8 @@ New measurement/analysis features:
   * New option for displaying areas of closed contours.
   * When calculating volume of the mesh, volume of the solid from the current
     group is now shown alongside total volume of all solids.
+  * When calculating area, and faces are selected, calculate area of those faces
+    instead of the closed contour in the sketch.
   * When selecting a point and a line, projected distance to current
     workplane is displayed.
 
diff --git a/src/mesh.cpp b/src/mesh.cpp
index 28484204..57aa87b4 100644
--- a/src/mesh.cpp
+++ b/src/mesh.cpp
@@ -1181,3 +1181,14 @@ double SMesh::CalculateVolume() const {
     }
     return vol;
 }
+
+double SMesh::CalculateSurfaceArea(const std::vector<uint32_t> &faces) const {
+    double area = 0.0;
+    for(uint32_t f : faces) {
+        for(const STriangle &t : l) {
+            if(f != t.meta.face) continue;
+            area += t.Area();
+        }
+    }
+    return area;
+}
diff --git a/src/polygon.cpp b/src/polygon.cpp
index e28bc424..736a1626 100644
--- a/src/polygon.cpp
+++ b/src/polygon.cpp
@@ -87,6 +87,12 @@ double STriangle::SignedVolume() const {
     return a.Dot(b.Cross(c)) / 6.0;
 }
 
+double STriangle::Area() const {
+    Vector ab = a.Minus(b);
+    Vector cb = c.Minus(b);
+    return ab.Cross(cb).Magnitude() / 2.0;
+}
+
 bool STriangle::IsDegenerate() const {
     return a.OnLineSegment(b, c) ||
            b.OnLineSegment(a, c) ||
diff --git a/src/polygon.h b/src/polygon.h
index eed7b155..b7c5a71e 100644
--- a/src/polygon.h
+++ b/src/polygon.h
@@ -191,6 +191,7 @@ public:
     bool Raytrace(const Vector &rayPoint, const Vector &rayDir,
                   double *t, Vector *inters) const;
     double SignedVolume() const;
+    double Area() const;
     bool IsDegenerate() const;
 };
 
@@ -281,6 +282,7 @@ public:
     void PrecomputeTransparency();
     void RemoveDegenerateTriangles();
     double CalculateVolume() const;
+    double CalculateSurfaceArea(const std::vector<uint32_t> &faces) const;
 
     bool IsEmpty() const;
     void RemapFaces(Group *g, int remap);
diff --git a/src/solvespace.cpp b/src/solvespace.cpp
index 0407a941..2c0157d0 100644
--- a/src/solvespace.cpp
+++ b/src/solvespace.cpp
@@ -797,6 +797,23 @@ void SolveSpaceUI::MenuAnalyze(Command id) {
 
         case Command::AREA: {
             Group *g = SK.GetGroup(SS.GW.activeGroup);
+            SS.GW.GroupSelection();
+            auto const &gs = SS.GW.gs;
+            double scale = SS.MmPerUnit();
+
+            if(gs.faces > 0) {
+                std::vector<uint32_t> faces;
+                faces.push_back(gs.face[0].v);
+                if(gs.faces > 1) faces.push_back(gs.face[1].v);
+                double area = g->displayMesh.CalculateSurfaceArea(faces);
+                Message(_("The surface area of the selected faces is:\n\n"
+                          "    %s\n\n"
+                          "Curves have been approximated as piecewise linear.\n"
+                          "This introduces error, typically of around 1%%."),
+                    SS.MmToStringSI(area, /*dim=*/2).c_str());
+                break;
+            }
+
             if(g->polyError.how != PolyError::GOOD) {
                 Error(_("This group does not contain a correctly-formed "
                         "2d closed area. It is open, not coplanar, or self-"