Re: [css3-transforms] Multiplication order

On Sat, Nov 10, 2012 at 2:10 PM, Simon Sapin <simon.sapin@kozea.fr> wrote:

> Hi,
>
> css3-transforms recently got a change to specify more precisely the
> multiplication order of transformation matrices:
>
> http://dvcs.w3.org/hg/csswg/**rev/a88c3b8d6b27<http://dvcs.w3.org/hg/csswg/rev/a88c3b8d6b27>
>
> This is good, but still somewhat ambiguous: matrix multiplication is not
> commutative, A * B is not always the same as B * A.
>
> §6 says:
>
>  Multiply by each of the transform functions in ‘transform’ property from
>> left to right
>>
>
> Is this a left-multiply or a right multiply? In other words, which of
> these is correct?
>
>     accumulator = accumulator * to_matrix(function)
> or
>     accumulator = to_matrix(function) * accumulator
>

It's always left to right so:

accumulator = accumulator * to_matrix(function)




>
>
> I remember trying stuff blindly and comparing with other engines in order
> to get this right in WeasyPrint…
>
> Similar questions arise with nested "transformed" stacking contexts or
> anywhere an implementation would multiply matrices. It might just be me who
> is confused, but the spec could still be more explicit than it is currently.
>

Is your confusion with preserver-3d or with regular 2d transforms?
In the case of 2d, the outer transform happens first (= to the left)


>
> Thanks,
> --
> Simon Sapin
>
>