[Papervision3D] matrix multiplication order

[grizu]

I did a little experiment.
I have a vector in global space, which I want to rotate my object about. 
I multiplied this vector with my objects inverse matrix to get it into 
object space. Then I rotated my object about it with:
    do3d.transform.calculateMultiply (do3d.transform, rotationMatrix)
And it works as it should! :)

So, why if I rotate my object with flipped params:
    do3d.transform.calculateMultiply (rotationMatrix, do3d.transform)
I get the same (right) effect, without doing the inverse thing. What 
happens here?

Would be very nice if someone could shed some light on this.
Thanks a lot,
Chris
> Thank you Andy, what you said is clear to me. But I only have one 
> RotationMatrix...
> Just thinking, why does
> do3d.transform.calculateMultiply (m1, m2)
> take two parameters? I only have one rotationmatrix, and simply want 
> to transform my object with it.
>
> If I have
> do3d.transform.calculateMultiply (do3d.transform, rotationMatrix),
> I multiply my object's matrix with the rotationmatrix (at least I 
> thought so). Or is it object*matrix*object?
>
> What happens if I flip the parameters? I multiply matrix*object, then 
> with my object?.... *confused*
>
> Thanks!
> Chris
>> The order is important since matrix multiplication isn't commutative. 
>>  (Consider the differences of multiply row->colum and how its 
>> different depending on which matrix is first).  Essentially, the 
>> matrix in the first position is applied "first".  Think if you want 
>> to apply a rotation and translation on an object.  If you do 
>> translate*rotation, your object will be "moved", then rotated, 
>> meaning your object will rotate around the translated center rather 
>> than its original origin.  If you do rotate*translate, you would 
>> rotate around its center, then move out from there.
>>
>> consider code (not sure if this is correct, but you can fix the 
>> syntax if not:)
>>
>> sphere.transform.calculateMultiply4x4(Matrix3D.rotationMatrix(0, 1, 
>> 0, Math.PI/2), Matrix3D.translationMatrix(300, 0, 0));
>>
>> This code will rotate the sphere (around origin of scene) 90 degrees 
>> and move it down its new X axis (which is now the Z axis in world space)
>>
>> If you flip those two params, it will move out to the right, and then 
>> rotate around its new center.
>>
>> hopefully that will help somewhat - a more thorough explanation might 
>> be needed.
>>
>> -andy
>>
>>
>>
>>
>> On May 8, 2009, at 9:24 AM, grizu wrote:
>>
>>>
>>> Hello!
>>>
>>> Could someone tell me, what is the difference between:
>>>
>>>    do3d.transform.calculateMultiply3x3 (do3d.transform, 
>>> rotationMatrix);
>>>
>>> and
>>>
>>>    do3d.transform.calculateMultiply3x3 (rotationMatrix, 
>>> do3d.transform);
>>>
>>> I know that order is important, when multiplying matrices (rotX, 
>>> rotY, rotZ,
>>> then translation, is this right?). But I can't understand, what is 
>>> going on
>>> here. I tried the first thing, which seems logic to me: I simply 
>>> multiply an
>>> object's transform with a rotation matrix. But my simple scene only 
>>> works
>>> with the second way.
>>> It would be very nice, if someone could explain this to me. I try to 
>>> get
>>> into this matrix-thing, but it is not easy sometimes...
>>>
>>> Thanks in advance!
>>> Chris
>>> -- 
>>> View this message in context: 
>>> http://www.nabble.com/matrix-multiplication-order-tp23446997p23446997.html 
>>>
>>> Sent from the Papervision3D mailing list archive at Nabble.com.
>>>
>>>
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