Date: Thu, 5 Nov 2009 00:14:22 -0800
Message-Id: <129AAAB5-C1C3-4BD8-83F9-D41663DB7AC2@gmail.com>
To: Brendan Kenny <bckenny@gmail.com>
Cc: Simon Fraser <smfr@me.com>, www-style list <www-style@w3.org>, "Tab Atkins Jr." <jackalmage@gmail.com>
```
On Nov 4, 2009, at 10:48 PM, Brendan Kenny <bckenny@gmail.com> wrote:

> On Thu, Nov 5, 2009 at 12:17 AM, Brad Kemper <brad.kemper@gmail.com>
> wrote:
>> [cc'ing my reply to whole list, assuming you meant to do the same...]
>>
>> I think that clockwise rotation makes total sense when actually
>> rotating
>> something, instead of indicating a linear direction. But for linear
>> directions, I think that there is not really anything that is
>> turning.
>>
>> Thus using .25turn instead of 90deg for a linear-gradient direction
>> feels a
>> little unnatural to me (even though it means the same to CSS),
>> because
>> nothing is turning aside from a point of reference. For just
>> indication a
>> straight direction, the protractor directions seem most intuitive.
>> Like this
>> (showing degrees and radians, and with 0 going to the right, and 90
>> going
>> straight up):
>>
>>
>> Here is a Java applet from a math site, which shows directional
>> arrows, and
>> the resulting angles:
>>
>> http://www.mathopenref.com/degrees.html
>>
> [...]
> My intuition says the same, but I also think that the only angle unit
> should be radians, so I can't trust myself here =]
>
> I think there are two arguments to be made here. The weaker one is
> from the mathematical side: there is an implicit transform in the
> entire view, so any directional vector is also transformed.

OK. I tend to think of it more conceptually than mathmatically, but I
can see this.

> Simon's
> point about consistency is much better. I've run across several blog
> entries that already assume that the existing transform
> implementations (giving clockwise rotations for positive angles) are
> buggy or are simplifying things for those who might not remember their
> last geometry course.

Interesting. I wouldn't say it is buggy, just adhering to a different
convention. An angle is an angle, no matter where the reference line
starts. At 2:25 on a round clock, the big hand is 90 degrees (and 270
degrees) from the little hand, and vice versa. It is just a common
convention in geometry to show angles as measured from a horizontal
baseline with zero to the right, when no other reference line exists.
Clockwise is similarly a common defaul for rotational motion, due to
long-term familarity with clocks, I suppose (and screws, volume knobs,
door knobs, etc.). For knobs, counter-clockwise often means turning
them towards their original un-turned position. It's possible I have
some Western bias here.

> As an example, if an element with a directional
> gradient is near element rotated at the same angle, the intuitive
> result to the author would be for them to be oriented similarly. The
> opposite would probably seem broken.

Ah. That's a pretty good argument. I hadn't considered that. Hmm. I
don't know if I'd say it's "broken" per se, but counter intuitive in
that situation. So you are saying that transforms should change?

> (to say nothing of gradients on rotated elements. i assume the points
> and the angle specified are also transformed; if so, the assumed
> behavior would probably be that angles of the same sign would result
> in a cumulative rotation)

Sure. That argument flows naturally from the previous one.
```
Received on Thursday, 5 November 2009 08:15:13 UTC

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