W3C home > Mailing lists > Public > www-style@w3.org > September 2010

Re: Linear gradients, Transforms and angles...

From: Chris Marrin <cmarrin@apple.com>
Date: Tue, 21 Sep 2010 10:31:43 -0700
Cc: Simon Fraser <smfr@me.com>, Daniel Glazman <daniel.glazman@disruptive-innovations.com>, "L. DavidBaron" <dbaron@dbaron.org>, "Tab Atkins Jr." <jackalmage@gmail.com>, www-style list <www-style@w3.org>
Message-id: <CE2DD48D-E83C-4D6A-BB6C-671C955C343C@apple.com>
To: Brad Kemper <brad.kemper@gmail.com>

On Sep 20, 2010, at 7:29 PM, Brad Kemper wrote:

> 
> 
> On Sep 20, 2010, at 2:01 PM, Chris Marrin <cmarrin@apple.com> wrote:
> 
>>> This author's expectations are that 90deg should be upward for a linear direction specified in degrees. All my prior experience in life before learning how programmers think reinforce that expectation. 
>> 
>> Angle is a concept that I believe is fairly new to CSS, so I don't think there is much of a precedent here.
> 
> Yes, but there is an long-time, pre-CSS precedent for specifying linear directions in the manner printed on traditional protractors.

And there is a long tradition in mathematics of the positive Y axis going up. But in HTML the Y axis goes down, because of the (western) tradition of reading from top to bottom. Now, assuming your protractor is the clear plastic kind, flip it upside down. Which direction do positive angles go now?

The "right-hand rule" is the tradition in mathematics. But HTML flips Y (for good reason), so we use the left hand rule, which has clockwise rotations. It's all consistent. 

> 
>> Using the math we are using in CSS transforms,
> 
> [...]
> 
>> means positive rotation angles go clockwise.
>> 
>>   -webkit-transform: rotate(10deg)
>> 
>> results in a clockwise rotation of the element to which it is applied. This is true in both WebKit and Firefox
> 
> That's fine. My point is that designers do not think of a linear direction as being the same as a rotation. At least I don't. So something that is entirely sensible for specifying an amount of rotation (an amount of turning in a circular direction) is not necessarily appropriate or relevant to specifying a straight, linear direction. To me it is counter-intuitive to do so, as it confuses two entirely different types of direction. 
> 
> When I walk north, I do not think of myself as pointing a default of east and then turning -90 degrees before starting forward. I just walk straight in the direction I want to go.

Are you saying that you don't think we should use the left hand rule for rotation just because we use it for orientation? If so, I suppose that's a valid point of view, but it is neither logically nor mathematically consistent.

-----
~Chris
cmarrin@apple.com
Received on Tuesday, 21 September 2010 17:32:17 GMT

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