1229200 - Remove incorrect 'devicePixelRatio' usage in gesture handling.

Status: RESOLVED FIXED

(Firefox OS Graveyard :: Gaia::System::Window Mgmt, defect)

Description

[Marcus Cavanaugh [:mcav]]

We should not use devicePixelRatio when calculating gesture distance thresholds. (I am equally guilty!)

Touch Event coordinates are already DIP (device-independent pixels), corresponding to the same pixels we'd see in CSS. Including devicePixelRatio is incorrect and causes device-specific differences.

We've mostly gotten this right, except for a few instances, including one by me. :)

Comment 1

[GitHub Autolander Bot]

Attachment: [gaia] mcav:dpr > mozilla-b2g:master

Comment 2

[Marcus Cavanaugh [:mcav]]

Comment on attachment 8693821 [details] [review]
[gaia] mcav:dpr > mozilla-b2g:master

Please correct me if I misunderstand what's going on here:

const kSignificant = 5 * window.devicePixelRatio;

On flame, that's 7.5 DIP; on Aries, 11.25 DIP. (Where DIP = device-independent pixels). However, when we do the math for these constants, we're comparing them to touch event coordinates, which are already DIP, not device pixels. 

So if we assume that devicePixelRatio is scaled such that interface elements are the same physical size on Aries and Flame, the bug is that kSignificant is physically, arbitrarily larger on an Aries than a Flame.

In other words, we shouldn't be multiplying by devicePixelRatio in these places, because touch event coordinates are already multiplied by devicePixelRatio.

In bug 1170863, this made things better because it made the touch area bigger, both on Flame _and_ Aries, but both by different amounts.

For this patch, I just chose to assume an average devicePixelRatio of 2 as a happy medium, hence the values are doubled. I could be convinced to change it to other constant values.

Comment 3

[Etienne Segonzac (:etienne)]

Comment on attachment 8693821 [details] [review]
[gaia] mcav:dpr > mozilla-b2g:master

Makes sense.
I think it worked out before because screens with higher DPI tend to have better touchscreens too.

Comment 4

[Marcus Cavanaugh [:mcav]]

master: https://github.com/mozilla-b2g/gaia/commit/a287b7383e3666cac446c1d2c503c53453f76bed