Document extensively the generated SupportPoint structure,

fix wrong estimation of weight torque for connections

src/libslic3r/SupportSpotsGenerator.cpp

@@ -463,7 +463,7 @@ public:
 #ifdef DETAILED_DEBUG_LOGS
             BOOST_LOG_TRIVIAL(debug) << "bed_centroid: " << bed_centroid.x() << "  " << bed_centroid.y() << "  " << bed_centroid.z();
             BOOST_LOG_TRIVIAL(debug) << "SSG: bed_yield_torque: " << bed_yield_torque;
-            BOOST_LOG_TRIVIAL(debug) << "SSG: bed_weight_arm: " << bed_weight_arm;
+            BOOST_LOG_TRIVIAL(debug) << "SSG: bed_weight_arm: " << bed_weight_arm_len;
             BOOST_LOG_TRIVIAL(debug) << "SSG: bed_weight_torque: " << bed_weight_torque;
             BOOST_LOG_TRIVIAL(debug) << "SSG: bed_movement_arm: " << bed_movement_arm;
             BOOST_LOG_TRIVIAL(debug) << "SSG: bed_movement_torque: " << bed_movement_torque;
@@ -492,7 +492,7 @@ public:
                                       params.material_yield_strength;
 
             float conn_weight_arm    = (conn_centroid.head<2>() - mass_centroid.head<2>()).norm();
-            float conn_weight_torque = conn_weight_arm * weight * (conn_centroid.z() / layer_z);
+            float conn_weight_torque = conn_weight_arm * weight * (1.0f - conn_centroid.z() / layer_z);
 
             float conn_movement_arm    = std::max(0.0f, mass_centroid.z() - conn_centroid.z());
             float conn_movement_torque = movement_force * conn_movement_arm;
@@ -732,17 +732,24 @@ SupportPoints check_stability(const PrintObject *po, const PrintTryCancel& cance
             // and the support presence grid and add the point to the issues.
             auto reckon_new_support_point = [&part, &weakest_conn, &supp_points, &supports_presence_grid, &params,
                                              &layer_idx](const Vec3f &support_point, float force, const Vec2f &dir) {
+                // if position is taken and point is for global stability (force > 0) or we are too close to the bed, do not add
+                // This allows local support points (e.g. bridging) to be generated densely 
                 if ((supports_presence_grid.position_taken(support_point) && force > 0) || layer_idx <= 1) {
                     return;
                 }
+
                 float area = params.support_points_interface_radius * params.support_points_interface_radius * float(PI);
+                // add the stability effect of the point only if the spot is not taken, so that the densely created local support points do not add 
+                // unrealistic amount of stability to the object (due to overlaping of local support points)
+                if (!(supports_presence_grid.position_taken(support_point))) {
                     part.add_support_point(support_point, area);
+                }
 
                 float radius = params.support_points_interface_radius;
                 supp_points.emplace_back(support_point, force, radius, dir);
-                if (force > 0) {
                 supports_presence_grid.take_position(support_point);
-                }
+                
+                // The support point also increases the stability of the weakest connection of the object, which should be reflected
                 if (weakest_conn.area > EPSILON) { // Do not add it to the weakest connection if it is not valid - does not exist
                     weakest_conn.area += area;
                     weakest_conn.centroid_accumulator += support_point * area;

src/libslic3r/SupportSpotsGenerator.hpp

@@ -63,11 +63,33 @@ struct Params {
     }
 };
 
+// The support points are generated for two reasons:
+// 1. Local extrusion support for extrusions that are printed in the air and would not 
+//    withstand on their own (too long bridges, sharp turns in large overhang, concave bridge holes, etc.)
+//    These points have negative force (-EPSILON) and Vec2f::Zero() direction
+//    The algorithm still expects that these points will be supported and accounts for them in the global stability check
+// 2. Global stability support points are generated at each spot, where the algorithm detects that extruding the current line 
+//    may cause separation of the object part from the bed and/or its support spots or crack in the weak connection of the object parts
+//    The generated point's direction is the estimated falling direction of the object part, and the force is equal to te difference 
+//    between forces that destabilize the object (extruder conflicts with curled filament, weight if instable center of mass, bed movements etc)
+//    and forces that stabilize the object (bed adhesion, other support spots adhesion, weight if stable center of mass)
+//    Note that the force is only the difference - the amount needed to stabilize the object again.
 struct SupportPoint {
     SupportPoint(const Vec3f &position, float force, float spot_radius, const Vec2f &direction);
+    bool is_local_extrusion_support() const { return force < 0; }
+    bool is_global_object_support() const { return !is_local_extrusion_support(); }
+
+    //position is in unscaled coords. The z coordinate is aligned with the layers bottom_z coordiantes
     Vec3f position;
+    // force that destabilizes the object to the point of falling/breaking. It is in g*mm/s^2 units
+    // values gathered from large XL print: Min : 0 | Max : 18713800 | Average : 1361186 | Median : 329103
+    // For reference 18713800 is weight of 1.8 Kg object, 329103 is weight of 0.03 Kg
+    // The final printed object weight was approx 0.5 Kg
     float force;
+    // Expected spot size. The support point strength is calculated from the area defined by this value. 
+    // Currently equal to the support_points_interface_radius parameter above
     float spot_radius;
+    // direction of the fall of the object (z part is neglected)
     Vec2f direction;
 };