Data not being filtered out by threshold

I am trying to create touch down and take off events on my centre of pressure data, however the threshold I have created seems to consistently come in one frame too early.

The threshold is allowing data below (-0.51) the set value (<2) through. Even when the threshold is increased this is happening. So i am constantly left with a negative value as first touch down before the actual touch down occurs one frame later.

This is the code i am using to create the events:

Air = zeros(length(mm3),1);

for n = 1:length(mm3)
    if mm3(n,2) < 2
        Air(n,1) = 1;
    end
end

Edges = diff(Air);             % Steigung kennzeichnet aufsteigende und abfallende Kante
TOPressure = find(Edges==1);     % Absprungzeitpunkte
TDPressure = find(Edges==-1);

plot(mm3(:,2))

TOPressure = TOPressure(5:55,1);
TDPressure = TDPressure(5:55,1);

TOPressure = TOPressure(2:47,1);
TDPressure = TDPressure(1:46,1);

[Jan]

I do not see a relation between the description and the code. The code does not contain any threshold. What does "seems to come one frame too early" exactly mean? Which variable is concerned?

By the way: You can create Air directly without pre-allocation and a loop:

Air = double(mm3(:, 2) < 2);

[Joshua Isherwood]

The threshold would be this line : if mm3(n,2) < 2 so the code should create the events TDPressure and TDPressure at the first values exceeding 2.

Sorry a little vague on that part. The variable is the centre of pressure measured during running so the events are supposed to be created during initial ground contact (TDPressure) and final ground contact (TOPressure) however the events are being created one frame too early with the foot still in the air.

So it allows one frame of data < 2 through and creates the events then as apposed to the following frame which is actually > 2.

Have included two files, mm3 which shows that the initial contact occurs at frame 206 with a value > 2, then the TDPressure which has the initial contact occurring at 205 with a value at < 2.

Answer 1

[Santhana Raj]

If you see the help of diff, it says: diff(X), for a vector X, is [X(2)-X(1) X(3)-X(2) ... X(n)-X(n-1)].

So, in your case, 80th element of Air is actually 1. And when you use diff, 79th element of TOPressure is Air(80)-Air(79), (which is actually the edge).

[Joshua Isherwood]

So my issue is with the diff not with the air? How am i best resolving the issue?

[Santhana Raj]

Yes. It looks so.

Easiest way out would be to add 1 to the result of find. another way is to adjust the result of diff by one position.