• 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
Guoshima

rotation using quaternions in a shader

6 posts in this topic

Hello, I want to use quaternions to represent my rotation of an object which is rendered using instancing, because a quaternion only takes 4 floats, while a rotation matrix uses 9. But what's the fastest way of multiplying a float3 (position) with a quaternion which represent the model rotation. Regards, Kenzo
0

Share this post


Link to post
Share on other sites
Whatever it is it'll probably be slower than doing the vector-matrix multiply, GPUs are *really* good at that [smile]
0

Share this post


Link to post
Share on other sites
yip .. but for the moment I am setting my per instance data as const values for the vertex shader and copying upto 21 floats per instance seems rather slow to me. (and I think I only have a 1024 constants so when I use 21 floats I can only render 48 instances with 1 drawcall, and if I use quaternions I "only have to use" 16 floats which gives me 64+ instances)

Regards,
Kenzo
0

Share this post


Link to post
Share on other sites
Rotating using Matrices is 6 additions and 9 multiplications. Rotating using quaternions has the following costs:

Generic quaternion multiplies: 24 add, 32 mul
specialized quaternion multiplies (best case): 17 add, 24 mul
convert to matrix, then transform: 18 add, 21 mul (including conversion cost of 12 add and 12 mul)

As you can see, the best solution is probably to convert the quaternion to a matrix and use that to rotate the vector. It's obviously slower than using a matrix directly, but as you say 4 floats is less than 9 (or 16 as the case may be)


Edit: Note: be sure to check out the cost of converting from matrix to quaternion in the pdf below.. It could get pretty expensive esp. since you have to do it on the CPU.

source: site: Geometric tools. link: Rotation Representations and Performance Issues

[Edited by - frostburn on July 11, 2006 7:12:20 AM]
0

Share this post


Link to post
Share on other sites
He's wanting to send the quaternions to reduce the number of constants sent to the GPU, so converting to a matrix on the CPU won't save him anything. Converting to a matrix on the GPU would be more expensive than just doing the quaternion*vector*inv(quaternion) required for the rotation. I think the cheapest way would be to do it like this:

let q = rotation quaternion = (w,x) where x is a 3d-vector of the axis components
let v = 3d-vector to rotate
let a = cross(x,v) + w*v
v' = (cross(a,-x) + dot(x,v)*x + w*a) / dot(q,q)

So you've got 2 cross-products, 2 dot-products, 3 vector-scaler multiplies, 3 vector additions and 1 vector-scaler division.

EDIT: I *think* I did the working correctly for the q*v*inv(q), but no guarenttees [smile]
EDIT2: dot(q,q) should be 1

[Edited by - joanusdmentia on July 11, 2006 8:55:00 AM]
0

Share this post


Link to post
Share on other sites
AFAIK a single quaternion-vector product could be done on a CPU sequentially w/ 16 ADDs and 15 MULs, due to an algorithm using 2 cross products, a scalar addition, a vector scale, and 3 vector additions. On a GPU the vector scale and vector additions are single OPs, and I assume the cross product is provided by the GPU as well. So it may happen that a total of 8 OPs are sufficient on a GPU (but I'm not sure).

However, a quaternion-matrix conversion is needed to be done once for all vertices but only if the same matrix could be applied to all vertices. AFAIK this prohibits the conversion to be done on the GPU, since it isn't possible to pass parameters from one shader run to the next.

So, if the memory consumption and bandwidth wouldn't be a limitation, in fact this would become a question of where the break-through is w.r.t. the count of vertices to be transformed.


EDIT: joanusdmentia came to the same conclusion but a bit faster :)
0

Share this post


Link to post
Share on other sites
thanx for the help. So in the end it isn't so expensive to do the rotation then using a quaternion (a little bit more than 2 times more). It's for object of maximum 24 vertices so this shouldn't be the bottle neck then.

Gonna try it out later!

Regards,
Kenzo
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