430825 - Use Cairo coordinate transformation matrix to convert from layout to device units

Status: RESOLVED WONTFIX

(Core :: Graphics, defect)

Description

[Zack Weinberg (:zwol)]

Layout does all its work in "layout units" which are defined to be 1/60th of a CSS pixel, which is itself defined to be as many device pixels as fit in 1/96th of an inch.

The Thebes and Cairo layers are somewhat unit-agnostic, in that there is a global coordinate transform matrix which could set them up to work in layout units as well.  However, we are not doing that right now; instead we are leaving that matrix as the identity, which means Thebes APIs expect coordinates in device pixels.  nsRenderingContextThebes has to convert everything itself, and so do the places in layout that have already been changed to call Thebes directly.

We should use the CTM to do the unit conversion and feed coordinates to the Thebes APIs in layout units.

Comment 1

[Zack Weinberg (:zwol)]

I no longer work for Mozilla, I am deassigning myself from bugs I have no intention of working on as a volunteer.

Comment 2

[Bas Schouten (:bas.schouten)]

Device pixel snapping makes this a little complicated. We're also moving stuff to Azure so this probably isn't valid anymore.