• Announcements

    • khawk

      Download the Game Design and Indie Game Marketing Freebook   07/19/17

      GameDev.net and CRC Press have teamed up to bring a free ebook of content curated from top titles published by CRC Press. The freebook, Practices of Game Design & Indie Game Marketing, includes chapters from The Art of Game Design: A Book of Lenses, A Practical Guide to Indie Game Marketing, and An Architectural Approach to Level Design. The GameDev.net FreeBook is relevant to game designers, developers, and those interested in learning more about the challenges in game development. We know game development can be a tough discipline and business, so we picked several chapters from CRC Press titles that we thought would be of interest to you, the GameDev.net audience, in your journey to design, develop, and market your next game. The free ebook is available through CRC Press by clicking here. The Curated Books The Art of Game Design: A Book of Lenses, Second Edition, by Jesse Schell Presents 100+ sets of questions, or different lenses, for viewing a game’s design, encompassing diverse fields such as psychology, architecture, music, film, software engineering, theme park design, mathematics, anthropology, and more. Written by one of the world's top game designers, this book describes the deepest and most fundamental principles of game design, demonstrating how tactics used in board, card, and athletic games also work in video games. It provides practical instruction on creating world-class games that will be played again and again. View it here. A Practical Guide to Indie Game Marketing, by Joel Dreskin Marketing is an essential but too frequently overlooked or minimized component of the release plan for indie games. A Practical Guide to Indie Game Marketing provides you with the tools needed to build visibility and sell your indie games. With special focus on those developers with small budgets and limited staff and resources, this book is packed with tangible recommendations and techniques that you can put to use immediately. As a seasoned professional of the indie game arena, author Joel Dreskin gives you insight into practical, real-world experiences of marketing numerous successful games and also provides stories of the failures. View it here. An Architectural Approach to Level Design This is one of the first books to integrate architectural and spatial design theory with the field of level design. The book presents architectural techniques and theories for level designers to use in their own work. It connects architecture and level design in different ways that address the practical elements of how designers construct space and the experiential elements of how and why humans interact with this space. Throughout the text, readers learn skills for spatial layout, evoking emotion through gamespaces, and creating better levels through architectural theory. View it here. Learn more and download the ebook by clicking here. Did you know? GameDev.net and CRC Press also recently teamed up to bring GDNet+ Members up to a 20% discount on all CRC Press books. Learn more about this and other benefits here.
Sign in to follow this  
Followers 0
panicq

DeltaTime and acceleration

9 posts in this topic

Hello,

 

I've a little problem of logic in a game i'm currently working on.

 

I've vX += acceleration * dT which represent the velocity in X

and x += vX * dT;

 

I've done so because of the reading of book. But after thinking about it and tying with values It seams not really accurate:

 

imagine acceleration = 1 and two computers A with dT = 1s and B with dt = 2s.

 

With A:

 

at the beginning vX = x = 0;

 

vX += 1 * 1; // = 1

x += 1 * 1; // = 1

 

vX += 1 * 1; // = 2

x += 2 * 1; // = 3

 

vX += 1 * 1; // = 3

x += 6 * 1;  // = 6

 

so at 1s x = 1 px

at 2s x = 3 px

at 3s x = 6 px

at 4s x = 10 px

 

With B:

 

vX += 1 * 2; // = 2

x += 2 * 2; // = 4

 

vX += 1 * 2; // = 4

x += 4 * 2; // = 12

 

so at 2s x = 4 px

at 4s x = 12 px

 

 

So here we clearly see that at 4s both x values are not at the same position, wich is clearly not ideal for my gameplay.

 

Any Idea on what should be really done ?

 

Thx

0

Share this post


Link to post
Share on other sites

Hum ok, i'll see. And if I don't wan to fix the timestep, wich is the case in many games ?

Edited by panicq
-1

Share this post


Link to post
Share on other sites

You can keep track of the elapsed time and instead of doing:

v += a*dt;
x += v*dt;

you can do:

elapsedTime += dt;
v = v0 + a*elapsedTime;
x = x0 + v*elapsedTime;

where v0 and x0 are the initial values of velocity and speed when the object starts moving.

 

If the object can change it's direction (meaning a different acceleration), you set a to the new value, v0 to the velocity at that moment and x0 to the position at that moment.

Edited by DiegoSLTS
2

Share this post


Link to post
Share on other sites

Had you used correct formula you'd have identical result.

// dT = 1

x += (vX + vX + acceleration * dT) * dT * 1/2; // == 0.5
vX += acceleration * dT; // == 1

x += (vX + vX + acceleration * dT) * dT * 1/2; // == 2
vX += acceleration * dT; // == 2

x += (vX + vX + acceleration * dT) * dT * 1/2; // == 4.5
vX += acceleration * dT; // == 3

x += (vX + vX + acceleration * dT) * dT * 1/2; // == 8
vX += acceleration * dT; // == 4
// dT = 2

x += (vX + vX + acceleration * dT) * dT * 1/2; // == 2
vX += acceleration * dT; // == 2

x += (vX + vX + acceleration * dT) * dT * 1/2; // == 8
vX += acceleration * dT; // == 4

However as other's mentioned this isn't the only place that gets undetermined without fixed timestep.

Edited by Zaoshi Kaba
0

Share this post


Link to post
Share on other sites


Had you used correct formula you'd have identical result.

You've got a typo, 5.5 should be 4.5.

---

Apart from that, one of the reasons you (panicq) should use a fixed timestep is related to collision checking/handling. You can actually see it from Zaoshi Kaba's example; what if there's something you'd collide with at x = 4.5 (after 3 seconds), but which you would NOT collide with at x = 8 (after 4 seconds)?

How would you solve this issue? Now imagine having to deal with/special case all other issues like this. OR you could fix your timestep (a 1 time thing), and have the results deterministic regardless of what framerate the game is running at.

 

Note that even though the logical simulation (handing movement, collision, etc.) should have a fixed timestep, the render part does not need to have a fixed timestep. If you wanted (for some reason), you could force rendering to occur every 3 seconds, without impacting any of the logical/gameplay stuff.

---

Can you give an example of why a game would not want to have a fixed timestep? You claim this is the case for many games, but I can't think of a single game for which this is true.

2

Share this post


Link to post
Share on other sites

I'm not sure why you think a fixed time step is not the case in "many games"? Where are you getting that from? This isn't something you can tell from playing a game (other than deducing its absence as the cause of bugs).


Indeed, the common physics middleware most games use all but mandate a fixed timestep.
0

Share this post


Link to post
Share on other sites

Ok, thank for advises, I'll fix my frame rate for physics and let a free frame rate for my logic.

 

Have a nice day.

0

Share this post


Link to post
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!


Register a new account

Sign in

Already have an account? Sign in here.


Sign In Now
Sign in to follow this  
Followers 0