Would you call this way to measure velocity "correct"?

Processing Coding Questions
1 
float x;
float playbackSpeed;
float x_po, pr_x, pa_x, x_d;
float prevTime = 0;
float dt, velocity = 1;
float currTime;

void setup() {
  size(640, 360);
  background(255);
}

void draw() {
  x_po+=10; // X1-POSITION
  if (x_po > width) // CONDITIONAL STATEMENT FOR RESETTING THE X-POSITION
    x_po = 0;
  x_d = abs((pa_x-x_po));  // CALCULATE THE PAST-X AND PRESENT-X DIFFERENCE

  println("x_po = "+x_po); // PRINT X-POSITION
  println("pa_x = "+ pa_x); // PRINT PAST-X POSITION
  println("x_d = "+x_d); // DIFFERENCE 
  pa_x = x_po; // SAVE CURRENT-X VALUE INTO PAST-X VALUE


  println("");

  currTime = millis(); // TAKE TIME
  if (x_d != 0) { // IF THERE IS MOVEMENT D CALCULATE DELTA TIME
    dt = abs(prevTime-currTime);
  } else if (x_d == 0) { IF THERE IS NO MOVEMENT DO NO CALCULATE DELTA TIME
    dt = 0.1;
  }
  println("currTime"+currTime);  //PRINT PRESENT TIME
  println("prevTime"+prevTime); // PRINT PAST TIME
  println("dt ="+dt);  //PRINT DELTA TIME
  velocity =x_d/(dt/1000); // CALCULATE DELTA X/DELTA TIME = VELOCITY
  println("Velocity= "+velocity);  // PRINT VELOCITY
  prevTime = currTime; // SAVE OLD PRESENT TIME INTO PREV TIME.
  float velocity_c = map(velocity, 0, 1000,0,100); // CONVERT VELOCITY INTO A MORE "UNDERSTANDABLE" VALUE. 
  velocity_c = constrain(velocity_c, 0, 100); // CONSTRAIN NEW VELOCITY VALUE
  println("velocity_c"+velocity_c);  // PRINT NEW CONSTRAINED VELOCITY VALUE

  fill(255, 0, 0); // RED 
  ellipse(x_po, height/2, 45, 45); // DISPLAY MY X-1 COORDINATE AS A BALL


  println("");
}
2

Taking into account

  • currTime is always greater than prevTime
  • and if x_d != 0 then the else statement is only executed if x_d ==0, no need to test again

the code becomes

if (x_d != 0) { // IF THERE IS MOVEMENT D CALCULATE DELTA TIME
  dt = currTime - prevTime;
} else  {  //IF THERE IS NO MOVEMENT DO NO CALCULATE DELTA TIME
  dt = 0.1;
}

Also need to initialise prevTime and currTime in setup

void setup() {
  size(640, 360);
  background(255);
  currTime = prevTime = millis();
}

Don’t assume prevTime is actually zero, depends on how much work is done in setup

The technique you are using will compensate for changes in framerate so is good :+1:

2 Likes
3

quark!, thank you so much for your review, I will make sure to learn and implement all the things you’ve mentioned, thanks for your time and advice :sunglasses:

4

Hello,

This helps to see the status of frameCount and millis() in setup() and draw():

void setup() 
  {
  frameRate(1); // I slowed this down for testing
  currTime = prevTime = millis();
  println(frameCount, currTime, prevTime, dt);  
  }

void draw() 
  {
  currTime = millis();
  dt = currTime - prevTime;
  println(frameCount, currTime, prevTime, dt);	  
  prevTime = currTime;
  }

I also slowed down frameRate() in this example for testing.

image
image640×813 35.9 KB

I used variable names and suggestions in topic.

@G03TH3 I also provided an example of how to properly format code. Hint.

https://discourse.processing.org/faq#format-your-code

:)

1 Like