Simple question - how to get the last value of a variable which changes continually

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flexdec

Simple question - how to get the last value of a variable which changes continually

in Programming Questions • 1 year ago

I have been coding all day and seriously, I can't think anymore.. My brain has exploded many many times. I am relatively new to processing so forgive me for my newbie-ness. And I'm french too so I'm also sorry for my english.

Ok here's my poject. I'm using OpenCV for blob detection and well, everythings working really fine. I'm using OSC to pass the Y position of the blob to another computer which passes it to an ellipse. The goal is to detect the jump motion of a human and use his Y position to create a game. The problem is, I need to do a lot of configurations to set up the ellipse movement so it's fluid and working.

I want to evaluate the last position of the ellipse so I can determine if the ellipse is ascending or descending. Should I use an array to store the Y positions and use a for ? Is it possible to check the value of the Y position at the last frame so if posY=posYLastFrame || posY<posYLastFrame, i know the object is descending? (which would be much easier and less head-bashing)

Thank you very much for your help and if you don't understand my problem I'll be glad to re-explain it! thank you!

Replies(5)

jbum

Re: Simple question - how to get the last value of a variable which changes continually

1 year ago

You don't need an array, but you do need a global variable, to store the previous value in.

float lastPosY =-1;

void draw()

{

// determine current posY

posY = ???

// do something with posY and lastPosY

boolean ellipseIsAscending = posY > lastPosY;

// do anything else that needs 'ellipseIsAscending here...

// remember posY for next time

lastPosY = posY;

}

flexdec

Re: Simple question - how to get the last value of a variable which changes continually

1 year ago

Thank you very much for your clear answer! Wow it was so simple haha it proves all that programmation i did today killed my braincells.

One last question:

What if i want to check posY lets say 5 frames before instead of only the last frame?

Do i have to use an array for this?? thanks again!

jbum

Re: Simple question - how to get the last value of a variable which changes continually

1 year ago

Yeah, then an array might be appropriate, particularly if you need the 4 intermediate values at some point (like in the classic "snakes" program).

If you simply want to sample the value every 5 frames, you can still do it with a single variable. Just only trigger your sampling code when (frameCount % 5) == 0

If using an array, you can use a counter, which goes 0,1,2,3,4,0,1,2,3,4, etc.

so you have

int lengthOfSnake = 5;

int[] saveYPos = new int[lengthOfSnake];

int snakeCtr = 0;

void draw()

{

// determine last position from 5 moves ago...

posY = saveYPos[ snakeCtr ];

// do something with posY and lastPosY here

// ...

// remember posY for next time

saveYPos[ snakeCtr ] = posY;

snakeCtr = (snakeCtr + 1) % lengthOfSnake; // make the counter wrap around the array

}

flexdec

Re: Simple question - how to get the last value of a variable which changes continually

1 year ago

Thank you!

One issue is remaining though, when i use the array to store the last 5 posY as you told me, it says my array is out of bounds.

import oscP5.*;
import netP5.*;
boolean ascending;
OscP5 oscP5;
int posY;
float posYLast = -1;
float y = height/2;
float gravity = 0.0981;
float speed = 0;
int longueur;
int [] saveY = new int[longueur];
int ctr = 0;
void setup() {
size(500, 500);
smooth();
noStroke();
oscP5 = new OscP5(this, "239.0.0.1", 6666);
}
void draw() {
background(255);
float m = millis();
saveY[ctr] = posY;
fill(0);
if (frameCount == 0) {
println(posY);
}
//ascending = true;
if (saveY[longueur]>posY) {
ascending = true;
}
else {
ascending = false;
}
if (ascending) {
fill(0);
ellipse(height/2, posY, 50, 50);
y = posY;
}
if (!ascending) {
fill(0);
ellipse(width/2, height-y, 50, 50);
y += speed;
speed += gravity;
if (y>height) {
speed = 0;
}
}
//posYLast = posY;
saveY[ctr] = posY;
ctr = (ctr+1)%longueur;
}
/* incoming osc message are forwarded to the oscEvent method. */
void oscEvent(OscMessage theOscMessage) {
/* print the address pattern and the typetag of the received OscMessage */
//print("### received an osc message.");
//print(" addrpattern: "+theOscMessage.addrPattern());
//println(" typetag: "+theOscMessage.typetag());
posY = theOscMessage.get(0).intValue();
}

jbum

Re: Simple question - how to get the last value of a variable which changes continually

1 year ago

In this code

int longueur;
int [] saveY = new int[longueur];

You have not assigned a value to longueur. It's zero, so your array is empty. Use this

int longueur = 5;

Also, you confused longueur for ctr in a statement. Change this:

if (saveY[longueur]>posY) {

to this

if (saveY[ctr]>posY) {

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