[cairo] Write ups on Microsoft Avalon

Owen Taylor otaylor at redhat.com
Tue May 18 14:11:41 PDT 2004

On Tue, 2004-05-18 at 15:43, Keith Packard wrote:
> 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).

How many lines do I have on my display that are not horizontal or 
vertical? I see two - the two 45-degree lines of the WM close button.
If I have a diagram or vector art occupying most of my screen then
double pixel lines probably aren't issue, but for focus lines, 
accelerator underlines, etc, a single pixel thickness is still

> > 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.

More precisely, you need a way to translate points to pixels: the
"logical DPI". But my feeling is that tying this logical DPI to the
default transformation matrix is a mistake.

>  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
> case). 

Actually, Microsoft allows you to choose between 96 and 120. (Though
some apps don't do very well at the 120 setting.)

> 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.

Do you really think the average screen is going to shoot up beyond the
120-150 dpi range? I just don't see the demand for that.

And on a 150dpi monitor, keeping the graphics the same size they are on
a 100dpi monitor is an activity for people with better eyesight than
mine. (This mostly speculation, though if someone wants to lend me a
150dpi monitor, I'd be happy to test it out in practice.)

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

I argue that in the world of current display technology, apps need
to be aware of two scales:

 A) The pixel scale in order to get sharp graphics
 B) The appropriate logical DPI for the user

and I secondly argue that B) cannot be quantized to an integral multiple
of A) without causing some real sizing problems. So to me, introducing
the quantized scale as the default  for Cairo just adds
a *third* scale that an app have to worry about.


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