[cairo] Write ups on Microsoft Avalon

Keith Packard keithp at keithp.com
Tue May 18 12:43:19 PDT 2004

Around 11 o'clock on May 18, Owen Taylor wrote:

> Most of the lines on my ~125dpi display are 1 pixel and I think should be
> 1 pixel.

That, at least, is debatable.  Certainly with the old X wide lines, using 
anything other than '0' width was a visual disaster.  But, at 125 dpi, I 
believe the default 2-pixel width is generally a good idea, especially 
when anti-aliased (1-pixel anti-aliased lines are pretty hard to see on a 
monitor these days).

> The most relevant metric is what is the readable font size for the
> user. I think that the right user interface is to make it very easy
> for the user to zoom the primary UI font size in and out and then
> base other sizes on that.

Yeah, the UI should scale based on the font size.  But, in other areas, 
where the font is specified in points, you really need to know what the 
DPI is.  And, if there are graphics associated with the image, then those
graphics better be presented at the same logical DPI on all screens.  In 
this case, the font 'point size' is fixed, so the only variable you've got 
to make things readable is 'DPI'.

Essentially, what cairo is saying is that all (current) screens are 96 dpi.
That's the same solution used by Microsoft and Apple (well, 72 dpi in their

The question is what to do with higher resolution screens. We can either
continue to assert that they are 96 dpi or we can pick some other
resolution and hope things work out.  By picking 192dpi, we have selected
the same mechanism as used in the Palm OS world -- we double the pixels up
to make sure everything remains pixel-grid aligned.

I think this is a better solution than just leaving things at 96dpi and 
having all of the graphics become microscopic.

For a UI, you'd want to set the transformation to an integer scale (perhaps
identity) and hand-scale the graphics to fit the font size; this would 
ensure that things matched the text size without just getting unreadably 
fuzzy.  We can leave cairo out of this discussion though; I don't want it 
to become too 'smart' about snapping coordinates.


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