[Webkit-unassigned] [Bug 88114] New: Don't hardcode target dpi of 160 (it should be 96 on desktop)

[bugzilla-daemon at webkit.org]

https://bugs.webkit.org/show_bug.cgi?id=88114

           Summary: Don't hardcode target dpi of 160 (it should be 96 on
                    desktop)
           Product: WebKit
           Version: 528+ (Nightly build)
          Platform: Unspecified
        OS/Version: Unspecified
            Status: NEW
          Severity: Enhancement
          Priority: P2
         Component: New Bugs
        AssignedTo: webkit-unassigned at lists.webkit.org
        ReportedBy: johnme at chromium.org
                CC: mjs at apple.com, bdakin at apple.com, abarth at webkit.org,
                    simon.fraser at apple.com, dino at apple.com,
                    kenneth at webkit.org, fsamuel at chromium.org,
                    aelias at chromium.org


Since bug 87407 (and bug 88047) the deviceScaleFactor is computed as deviceDPI/160, where deviceDPI is supposedly the true dpi of the device.

This works on phones, but is incorrect on laptops/desktops. The definition of a CSS pixel (http://www.w3.org/TR/css3-values/#reference-pixel) requires a pixel density of 96dpi on devices used at arms length (the reason that 160 works well on phones is because they're held at about 60% of an arm length).

It's also common to discretize deviceScaleFactor to the nearest 0.25 or so.

So laptop/desktop ports that are considering turning on ENABLE(VIEWPORT) should consider factoring out 160 into a value passed by the embedding port (e.g. in ChromeClient.h) based on it's knowledge of the device's typical distance from the viewer.

Alternatively, instead of passing a value for the target dpi (e.g. 160), have the embedder provide the deviceScaleFactor directly (for example Android would probably provide DisplayMetrics.density[1], while iOS would just return 2 if retina else 1). This might be more flexible than having WebCore decide how to discretize fractional values.

ChromeOS folks, this might be of interest since afaik you plan to ship ENABLE(VIEWPORT).
Apple MacBook folks might also be interested, given rumors of high resolution displays which will require appropriate deviceScaleFactor.

[1]: http://developer.android.com/reference/android/util/DisplayMetrics.html#density

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