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Adding or Concatenating Transforms (Scale, Rotation, Translation)


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#21 Kwizatz   GDNet+   -  Reputation: 1167

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Posted 28 January 2013 - 05:34 PM

I am considering dropping non uniform scale support, if I do, I downsize my SRT structure from 10 floats to 8, chances are scale won't even be used at all, but the reason why I added it on the first place was because of a model that used mirror scaling on X to duplicate the piece of armor from the right to the left. Granted, I could modify, and fix the model, but, but there were thousand of these shortcuts taken by the modelers, I'd have a problem. I am using stock models so its not like I could go to the modeler and ask him/her to fix it and submit it again.


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#22 Dmytry   Members   -  Reputation: 1147

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Posted 31 January 2013 - 12:31 AM

Hi Dmytry,

I am not sure I follow, what do you mean? in the end what my SRT operations do is a simplified matrix multiplication, so if you take S,R,and T as matrices with scale, rotation and translation respectively, and I as identity the operation you describe would be something along these lines:

( I * R * I ) * (S * I * I) * ( I * *R * I ),

which is the same as RSR, wouldn't the scale vector (diagonal) on the second matrix be doing the scale on world axes rather than object axes as well?
I think I see where you're going, but I haven't had time to think more about it.

The second part of your post, I am not sure if you're pointing out a (another?) problem or providing me with an optimization smile.png

Thank you for your comments!

Not really pointing out an error in the first part, just explaining that what you originally wanted to do - concatenate sequence of SRT into a single SRT where S scales using a vector that represents scaling on different axes, is (technically speaking) impossible - you indeed need a full blown matrix here, or if you want to optimize a tiny bit, 3x3 matrix and translation. That's because sequence of R S R can produce scaling along the diagonal, which you can not represent with one S R T . (I am assuming matrices apply in left to right, i.e. directx order)

edit:However if you are certain you only use non uniform scaling as the first transformation applied, then it can work. If U is uniform scaling and you only have S R T U R T U R T U R T .... sequence, then you're in the clear.

Second is indeed providing with an optimization. In my software I have a class that does rotation using quaternion and translation using a vector, it works pretty much same from outside as a matrix. The reason I'm using that is not so much optimization as convenience when doing physics or especially when interpolating rotations between frames (which I do because I run physics at constant framerate). In your case I'd probably just convert to matrices then multiply those together, I'd probably use 3x3 matrices with translation rather than full blown 4x4 , basically, you assume the right column (or bottom row, depending to convention) is always 0 0 0 1 .

Edited by Dmytry, 31 January 2013 - 12:56 AM.

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#23 Kwizatz   GDNet+   -  Reputation: 1167

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Posted 31 January 2013 - 08:43 AM

Alright, thanks Dmytry, I am just using these for DCC animation interpolation, skeletal animation to be exact at the moment, so I think Rob is right on the money, I still need to go back and factor in scaling properly, currently busy with my resource system.

 

Thanks to all again!


Check out my OpenGL GUI Library at github.





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