@dc42 Yes, actually you are right about that. I have done some calculations with respect to the extruder and axis accelerations, velocities, times and distances etc in order to put some numbers to the principles so it is easier to see how these parameters relate to each other at line start and end.
What I see is that at 30mm/s print speed, the extruder must extrude a 0.067mm length of filament (1mm track, 0.24mm high) during the acceleration move (with a PA factor of 0.15 for my printer configuration) and the additional length that is pushed into the extruder for the PA compensation is 0.449mm. When the deceleration move starts at this speed the extruder switches directly to retraction during which time 0.382mm is retracted leaving a balance of 0.067mm to be extruded over the length of the deceleration. This is what it looks like for the present symmetric PA setup. For higher speeds the deceleration move would not result in an immediate retraction but would be made up of an initial deceleration followed by a retraction.
Getting back to the gap I mentioned in my previous post it must then be coming from a lack of plastic at the line start (not end, as previously thought) Reviewing the parameters I was using on that test I can see why that was happening and we should probably disregard that result from this analysis.
Lastly, while some of my reasoning (post from 21 September) was incorrect with respect to why the present implementation of the PA line is causing problems, I remain convinced that I am right in saying that this implementation of PA is responsible for some of the problems we are seeing, particularly at line starts. The reason for that is obviously a different one and I would like to present that below.
Firstly, the theory on modelling plastic flow through a nozzle as established in other plastics extrusion industries and as I presented in an earlier post. This would point to the requirement that the extruder velocity at the start of acceleration should be zero and for deceleration it should be the requested print speed and both should change from there at a faster rate than the requested rate with a step change/jerk at the end of that (but not at the start).
Secondly the mere presence of blobs at line starts (See the table I supplied in my first post for reference) indicates excess material/pressure there. Since it seems this is not due to residual pressure it has to come from somewhere else and a simple consideration of the present PA implementation with initial jerk, in itself points to the likelihood that this would be a cause of blobs. The only reason why these blobs are not always there in my view can be explained by what I outlined in my first post under the heading "Line starts".
As a simplification/starting point one may try to keep the shape of the PA curve/line linear but at the very least the starting offset would have to be eliminated. Note : This is based on the assumption that positional axis line starts and ends do not implement jerk, which I remember @dc42 saying in another post but I am not sure whether this is still the case. At corners in a print line where positional axis jerk is implemented there would obviously have to be an jerk in the PA line as well. As a final note here, in theory at least it may be possible to have a much higher jerk value for the extruder when releasing pressure than when building it and some use could possibly be made of this principle to speed up printing where jerk values cause a slowing down of the print.
In summary my request would be to make PA asymmetric and speed sensitive and to eliminate the starting offset from the PA acceleration/deceleration line at line starts and ends (at least in a beta version). This would allow for further testing to validate the theories I have put forward.