The density and densityDpi is an abstract density bucket the device
manufacturer has decided makes sense for their UI to run in. This is what
is used to evaluate things like "dp" units and select and scale bitmaps from
resources.
The xdpi and ydpi are supposed to be the real DPI of the screen... though
as you've seen, many devices don't set it correctly. :( This is our fault,
it isn't actually used anywhere in the platform, so people don't realize
they have a bad value, and we haven't had a CTS test to try to make sure it
is sane (it's not clear how that test should work). Worse, we shipping the
original Droid with a graphics driver that reports the wrong value here...
in fact that reported that same damnable 96.
Unfortunately, I don't have a good solution if you want to get the real
exactly screen dots per inch. One thing you could do is compare xdpi/ydpi
with densityDpi and if they are significantly far apart, assume the values
are bad and just fall back on densityDpi as an approximation. Be careful on
this, because a correctly working device may have densityDpi fairly
different than the real dpi -- for example the Samsung TAB uses high density
even though its screen's really density is a fair amount lower than 240.
On Wed, Jan 12, 2011 at 11:31 AM, Phil Endecott <
spam_from_goo...@chezphil.org> wrote:
> Hi Richard,
>
> On Jan 12, 6:54 pm, Richard Schilling <richard.rootwirel...@gmail.com>
> wrote:
> > Did you try fetching the scale that will allow you to convert from DPI
> > to actual screen pixels? I found this very reliable:
> >
> > final scale =
> > getContext().getResources().getDisplayMetrics().density;
>
> The description of that says:
>
> "on a 160dpi screen this density value will be 1; on a 120 dpi
> screen
> it would be .75; etc."
>
> > Then multiply scale by one inch of pixels in DPI resolution (depending
> > on the screen you're on) - 120, 160, 240, 320.
> >
> > This API call ...
> >
> > int densityDpi =
> > getContext().getResources().getDisplayMetrics().densityDpi;
> >
> > will tell you the density of the screen you're on.
>
> Approximately.
>
> > So, this will tell you how many screen pixels equate to an inch.
>
> I'm not at all sure that it does.
>
> > Then
> > you can use that to lay out your ruler markings:
> > int pxPerInch = 340; // default value - note it's a new density
> > in API version 9.
> >
> > switch(densityDpi){
> > case DisplayMetrics.DENSITY_LOW:
> > pxPerInch = 120 * scale;
> > break;
> > case DisplayMetrics.DENSITY_MEDIUM:
> > pxPerInch = 160 * scale;
> > break;
> > case DisplayMetrics.DENSITY_HIGH:
> > pxPerInch = 240 * scale;
> > break;
> > }
>
> Note that the values of those constants DENSITY_* are their numeric
> values, so there is no need for the case statement; you can just
> write:
>
> pxPerInch = densityDpi * scale;
>
> However, I really don't think that does what you believe it does. I
> think that densityDpi reliably gives the approximate dpi, and density
> just gives densityDpi/160. Multiplying them together doesn't do
> anything useful. But I could be wrong!
>
> Thanks, Phil.
>
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--
Dianne Hackborn
Android framework engineer
hack...@android.com
Note: please don't send private questions to me, as I don't have time to
provide private support, and so won't reply to such e-mails. All such
questions should be posted on public forums, where I and others can see and
answer them.
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