Why EDID is not trustworthy for DPI

Adam Jackson ajax at redhat.com
Wed Oct 5 21:54:12 UTC 2011

On Wed, 2011-10-05 at 23:11 +0200, Benny Amorsen wrote:
> Matthew Garrett <mjg59 at srcf.ucam.org> writes:
> > We have no technological solution for dealing with the fact that 
> > applications may move from one DPI to another at runtime, and may even 
> > be displaying on both displays at once.
> >From a technology viewpoint, that is actually theoretically easy to
> handle on modern hardware: Render everything as 3D objects and let the
> graphics hardware scale as appropriate.

Your use of the word "theoretically" reveals much.  You would almost
certainly be appalled by just how much geometry information is necessary
to render a single glyph.  Which is why we - and Windows, and OSX -
don't do that.  When you ask for a glyph at a given transformation
matrix, it gets rasterized down to an A8 mask, and we reuse those from
then on.  (Okay, it's A8R8G8B8 if you're doing subpixel antialiasing).
That's the only way you get anything like acceptable performance.

If it were easy, we'd already be doing it.

- ajax
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