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Re: Review of caret and selection proposal in current canvas API draft

From: Benjamin Hawkes-Lewis <bhawkeslewis@googlemail.com>
Date: Sun, 30 Jan 2011 18:09:26 +0000
Message-ID: <AANLkTiktH7piLscvwrNqiT1cFZ1XXDCN9zM3EmDqKbhO@mail.gmail.com>
To: Richard Schwerdtfeger <schwer@us.ibm.com>
Cc: public-canvas-api@w3.org, public-canvas-api-request@w3.org, public-html-a11y@w3.org
On Sun, Jan 30, 2011 at 4:45 PM, Richard Schwerdtfeger
<schwer@us.ibm.com> wrote:

>> + Suggestion 2: Define effects of non-canvas transformations on
>> setCaretSelectionRect()

[snip]

>> An example of the subtleties here would be when a canvas element is
>> subjected to warping using CSS 3D transforms:
>>
> This API is targeted at Canvas and not CSS and SVG. It sounds like you want
> to extract this API and put it elsewhere.

That would be good, if possible (James's draft suggests a generic
focusPosition() method for magnifiers), but it's not what this suggestion is
about. Rather this is about how behaviors external to the canvas element can
change where canvas content appears on screen, and so change the information
that needs to be given to magnifiers if they are going to correctly focus the
current caret or selection.

>> How must user agents compute a "rectangle" in such a case? For example,
>> must
>> they compute the rectangle within the canvas context, then compute the
>> (transformed) pixel coordinates on the screen, then compute a rectangle
>> that
>> embraces all four corners?
>>
> You use the drawing path relative to the canvas upper left position. You
> then add the coordinates of the upper left corner of the upper left hand
> corner of the canvas. The coordinates of the rectangle are a best fit:
>
> Assuming coordinates are relative to the top left of a device context (Only
> OS/2 did it cartesian coordinate based)
>
> - minium top, left coordinate = top left
> - max bottom, minumum left coordinate = bottom left
> - max bottom, max right = bottom right
> - minimum top, maxium right - top right
>
> There's your rectangle.

You *cannot* assume screen coordinates of content drawn into a canvas context
from the screen coordinates of the top left corner of the canvas element.

For example, imagine you have a canvas element's top left corner at 5,5. And
you have a selection rectangle at the top left of the canvas context, so also
at 5,5, and imagine it is 10 wide and 10 long. If the canvas element is not
subject to any transforms, then we can compute the bottom right is at screen
coordinates 15,15. But now imagine the entire canvas element has been rotated
clockwise 90 degrees clockwise using its top left corner as an anchor. If a
magnifier draws focus between 5,5 and 15,15, it will miss the text it was
supposed to be focusing as on screen it will be rotated out of the magnifiers
focus. Things get even more funky if the transform applied to the canvas
element (or an ancestor element) warps it rather than rotating it.

>> + Suggestion 3: Define caretBlinkRate() on a canvas-independent interface

I did not see a reply to this point. Do you agree or disagree this would be
better?

>> + Suggestion 4: Allow privacy exemption to caretBlinkRate()
>>
>> caretBlinkRate() could be used for profiling users, so UAs must be allowed
>> not
>> to share this data if the user does not want to be profiled.
>>
>
> I don't see how a blink rate could be used to profile a user. People may just
> not like a high blink rate.

Their system defaults and their expressed preferences are
information that can be used to track them.

I refer you to the EFF's research into user fingerprinting:

http://panopticlick.eff.org/

If people with disabilities are more likely to have an unusual blink rate,
then in addition to tracking them, their blink rate can be used to infer
that they have a disability.

--
Benjamin Hawkes-Lewis
Received on Sunday, 30 January 2011 18:11:04 GMT

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