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# Why my Straight line planner doesn't work on all maps?

I'm working on an elementary but dramatic global planner. It should plan a simple straight-line path from a starting point, which is the baseline center, and a goal, which is a 2D nav goal that I set from the GUI of RVIZ, or by hands putting the coordinates of the goal. The job is simple, but I can't work it out. What I have done till now is, pick up the coordinates of the starting point and the goal (expressed on the map frame), calculate the angular coefficient, and using this coefficient calculate the x coordinate of each waypoint on the straight by applying a translation of delta (which is the increment I want to apply) and a rotation of angular coefficient. Then the y coordinate is calculated by simple linear interpolation. It turns out that the path is correct for some maps (generated using the famous Gmapping), but for others, it is not. It really depends on how the baseline frame is positioned relative to the map frame (sometimes the path is turned 180 degrees from where it should be), and I'm wondering if I'm encountering a problem with the representation of the Euler angles. I know it is a stupid question and I'm wrong on something, but currently, I'm really struggling.

float yaw = 0.f;

if(flag_){
ROS_INFO("STRAIGHT");
yaw = tf::getYaw(new_start.pose.orientation);
}else{
ROS_INFO("non STRAIGHT");
yaw = (goal_.pose.position.y - start.pose.position.y) / (goal_.pose.position.x - start.pose.position.x); //angular coefficient
}

int id = 0;
int n = 0;
float goal_distance = sqrt(pow((start.pose.position.x - goal_.pose.position.x), 2) + pow((start.pose.position.y - goal_.pose.position.y), 2));

while(n * delta < goal_distance){

geometry_msgs::PoseStamped new_goal = goal_;

if (flag_)
{
//finding the points of the path that connect the goal (which is the projection of the start, n meters away) and the start, currently working on all maps
new_goal.pose.position.x = (n * delta) * cos(yaw) + start.pose.position.x;
new_goal.pose.position.y = (n * delta) * sin(yaw) + start.pose.position.y;
new_goal.pose.orientation = new_start.pose.orientation;
}

else
{
//finding the points of the path that connect the goal and the start, currently not working on some maps
sign = (goal_.pose.position.x - start.pose.position.x >= 0) ? 1 : -1; //doesn't make sense, but it works
new_goal.pose.position.x = sign * (n * delta) * cos(yaw) + start.pose.position.x;
new_goal.pose.position.y = (((new_goal.pose.position.x - start.pose.position.x) * (goal_.pose.position.y - start.pose.position.y)) / (goal_.pose.position.x - start.pose.position.x)) + start.pose.position.y;
new_goal.pose.orientation = goal_.pose.orientation;

}
++n;
plan.push_back(new_goal);
}


This planner draws a straight line using as a goal the projection of the current pose meters away, or a straight line using as a goal the point selected from RVIZ. As you can see, in the second part, there is a lot going on, that's 100% wrong. Firstly I ...

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Are you asking about generating nav_msgs/Path, or about geometry_msgs/Twist?

Hi, it's a global planner so the result should be a nav_msgs/Path

1
1. All poses in your plan (except maybe the last) should have the same yaw value (expressed as a quaternion, of course.) It is the angle of the vector drawn from start_xy to goal_xy.

2. Make sure you passing the correct units to the sine/cosine functions (radians vs. degrees.)

If this isn't enough of a hint, you probably need to add some code to your description.

thanks for the answer. 1. working with quaternions is a pain in the ass, yeah they resolve lots of problems, but yet a pain in the ass For the angle, I calculate it as a delta on y / delta on x, I think that's correct. 2. always use radians.

I made a Pastebin with the interesting part. This planner draws a straight line using as a goal the projection of the current pose meters away, or a straight line using as a goal the point selected from RVIZ. As you can see, in the second part, there is a lot going on, that's 100% wrong. Firstly I'm using a sign to calculate the x coordinate of the n-th point of the path, which is incorrect cause it is a transformation, but this was the only way I can make it work if the map frame is rotated ...(more)

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To compute your yawin radians I think it should be yaw = atan2(goal.y - start.y, goal.x - start.x) instead of yaw = (goal.y - start.y) / (goal.x - start.x)

Otherwise, here is a tutorial for a plugin that plans a straight line. While the wrapping code is for ROS2 the core logic to compute the straight line is fully described: https://navigation.ros.org/plugin_tut...

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Also, your new_goal.pose.orientation is not correct. While traveling your robot needs to point toward the goal (yaw in your code), but the goal_.orientation is usually not this same value. When your robot reaches the goal_ x,y, it should then rotate in place to the requested goal_.orientation.

mmmm, yeah, it should be equal to the yaw I calculate. In the end, the rotational components of the pose of each waypoint on the path are not so important, the local planner is kinda agnostic to that components. In any case, I will fix it too. Thanks also to @PatoInso, on Monday I will apply the modifications and test them. I will let you know how it goes.