NO PATH ! error and questions to gradient_path.cpp in global_planner

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Problem Description:

I am trying to get a global path based on a 500 * 500 static map with global_planner (indigo). I set up the parameter use_grid_path to be false, so that the path is calculated with a gradient descent method. Goal Pose is fed with the 2D Nav Goal in Rviz. Sometime it works fine by showing the global path. But sometimes it publishes rosconsole [Error] messages repetitively in the terminal, even if a potential map is successfully showed by Rviz.

[ERROR] [1444812100.339654309, 6406.380000000]: NO PATH!
[ERROR] [1444812100.340210027, 6406.380000000]: Failed to get a plan from potential when a legal potential was found. This shouldn't happen.
[ERROR] [1444812100.364187125, 6406.410000000]: NO PATH!
[ERROR] [1444812100.364835403, 6406.410000000]: Failed to get a plan from potential when a legal potential was found. This shouldn't happen.

I dug a little bit into the code in gradient_path.cpp http://docs.ros.org/jade/api/global_planner/html/gradient__path_8cpp_source.html#l00068. I think it must failed at row 80 to get this kind of error message.

00080     while (c++ < ns*4) { 

But my static map is not big, and I have already got the potential map. So I do not really understand what is going on here.

Question to the code in gradient_path.cpp

http://docs.ros.org/jade/api/global_planner/html/gradient__path_8cpp_source.html#l00068

1.

This part comes from function float GradientPath::gradCell(float* potential, int n). In my opinion, the potential from cells on the left, right, up and down side should be taken into account. But at line 285, it is a fixed cell. Why should man always take care at this fixed cell?

00276     // check for in an obstacle
00277     if (cv >= POT_HIGH) {
00278         if (potential[n - 1] < POT_HIGH)
00279             dx = -lethal_cost_;
00280         else if (potential[n + 1] < POT_HIGH)
00281             dx = lethal_cost_;00282 
00283         if (potential[n - xs_] < POT_HIGH)
00284             dy = -lethal_cost_;
00285         else if (potential[xs_ + 1] < POT_HIGH)
00286             dy = lethal_cost_;
00287     } 

2.

Why do we take stc, stc + 1, stcnx and stcnx + 1 but not other neighbors for interpolation?

00181             // get grad at four positions near cell
00182             gradCell(potential, stc);
00183             gradCell(potential, stc + 1);
00184             gradCell(potential, stcnx);
00185             gradCell(potential, stcnx + 1);
00186 
00187             // get interpolated gradient
00188             float x1 = (1.0 - dx) * gradx_[stc] + dx * gradx_[stc + 1];
00189             float x2 = (1.0 - dx) * gradx_[stcnx] + dx * gradx_[stcnx + 1];
00190             float x = (1.0 - dy) * x1 + dy * x2; // interpolated x
00191             float y1 = (1.0 - dx) * grady_[stc] + dx * grady_[stc + 1];
00192             float y2 = (1.0 - dx) * grady_[stcnx] + dx * grady_[stcnx + 1];
00193             float y = (1.0 - dy) * y1 + dy * y2; // interpolated y

3.

Why do we only consider these two horizontal bounds, why not the vertical bounds?

00091         if (stc < xs_ || stc > xs_ * ys_ - xs_) // would be out of bounds
00092         {
00093             printf("[PathCalc] Out of bounds\n");
00094             return false;
00095         }

These questions troubled me a lot. Please help ~~

Thanks in advance :-)