# Re: Transformations with units

From: Rik Cabanier <cabanier@gmail.com>
Date: Fri, 2 Mar 2012 16:22:26 -0800
Message-ID: <CAGN7qDBchCZM7ViYw_orhdvGhWq3MnHM+yxQxnyiFm4UEbjb2w@mail.gmail.com>
To: Tanguy Ortolo <tanguy+svg@ortolo.eu>

```Hi Tanguy,

there were many discussion on this on the www-style mailing list.
At the last CSS face-to-face, everyone agree to drop the units on the tx/ty
part in a matrix.
You can find the discussion here:
http://lists.w3.org/Archives/Public/www-style/2012Feb/0550.html

Rik

On Fri, Mar 2, 2012 at 11:21 AM, Tanguy Ortolo <tanguy+svg@ortolo.eu> wrote:

> Hello,
>
> According to the current recommentation, transformations can only be
> defined with numbers. While this can seem a logical constraint since
> transformations are matrices, which are multiplicated with lenghts, the
> reality is a bit more subtle:
>
>    [ x']   [ a c e ]   [ x ]
>    [ y'] = [ b d f ] ⋅ [ y ]
>    [ 1 ]   [ 0 0 1 ]   [ 1 ]
>
> Here, x, y, x' and y' are homogeous to lengths, but because 1 is
> adimensional, not all coefficients of the transformation matrix are.
> Indeed, e and f are multiplicated with the adimensional number 1 to make
> lengths. Thus, e and f are homogeneous to lengths.
>
> These coefficients appear only in one simple transformation, the
> translation. This is quite intuitive since a translation is basically
> defined by a length vector.
>
> Now, when drawing lengths in a natural unit distinct from the pixel, it
> is very inconvenient to translate these units to pixels just for
> translations. So, do you think not that it would be appropriate to
> modify the recommendation to indicate that coefficients e and f for
> matrices, and tx and ty for translations, are lengths and not numbers?
>
> Regards,
>
> --
> Tanguy Ortolo
>
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```
Received on Saturday, 3 March 2012 00:22:55 UTC

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