602647 - Fennec needs a scriptable interface to GetDPI

Status: RESOLVED FIXED

(Core :: Graphics, defect)

Description

[Matt Brubeck (:mbrubeck)]

Fennec multiplies values from the "viewport" meta tag by a factor that is intended to correct for differences in display density.  This factor is effectively a device-pixel to css-px ratio, which applies only to pages that set their scale with <meta name="viewport">.  For some more background, see: http://limpet.net/mbrubeck/2010/05/11/fennec-meta-viewport.html

Right now this factor a preference (zoom.dpiScale) with a hard-coded default of 150%.  (This default is targeted at a handheld display with about 160 dpi.)

We'd like to change this value based on the actual hardware display density.  To do this in the front-end as we do now, we would like a scriptable way to call nsWindow::GetDPI.

Comment 1

[Doug Turner (:dougt)]

this is just chrome, right?

Rationale for it blocking 2.0?

Comment 2

[Matt Brubeck (:mbrubeck)]

We need this to adapt our chrome to different-resolution screens, and also to choose the a CSS-to-device pixel ratio to apply to meta[name=viewport] values.

Both of these are necessary if we decide to implement bug 590777 (support lower-resolution displays), which is currently blocking-fenec:2.0+.

Comment 3

[Matt Brubeck (:mbrubeck)]

Sorry, actually this isn't needed for adapting our chrome theme - we can do that using mozmm and media queries.  It's only  needed for adjusting our content scaling.

Comment 4

[Stuart Parmenter]

can we just add this to nsIScreen?

Comment 5

[Doug Turner (:dougt)]

sure, or nsIDOMWindowUtils.

Comment 6

[Mike Kristoffersen (:MikeK)]

When should we use a readonly attribute and when should we use a function to get such a value from an interface?

Comment 7

[Doug Turner (:dougt)]

It doesn't matter that much.  An attribute is fine

Comment 8

[Mike Kristoffersen (:MikeK)]

Attachment: Fix V1

Comment 9

[Doug Turner (:dougt)]

Comment on attachment 483183 [details] [diff] [review]
Fix V1

roc, is it fine to expose dpi in this way?

Comment 10

[Robert O'Callahan (:roc) (email my personal email if necessary)]

You can compute this already by calling DOMWindowUtils.screenPixelsPerCSSPixel on a window with no zoom (e.g., the chrome window) and multiplying by 96.

Comment 11

[Matt Brubeck (:mbrubeck)]

(In reply to comment #10)
> You can compute this already by calling DOMWindowUtils.screenPixelsPerCSSPixel
> on a window with no zoom (e.g., the chrome window) and multiplying by 96.

On my Android device where GetDPI should return 240 (and GetDefaultScale returns 1.0), DOMWindowUtils.screenPixelsPerCSSPixel*96 == 96.  Is this expected?

Comment 12

[Robert O'Callahan (:roc) (email my personal email if necessary)]

Er, never mind, I don't know what I was thinking.

Comment 13

[Matt Brubeck (:mbrubeck)]

Pushed: http://hg.mozilla.org/mozilla-central/rev/179ea37e0a57