[GRASS-dev] Cairo monitor driver

Glynn Clements glynn at gclements.plus.com
Sun Oct 14 21:54:15 EDT 2007

Lars Ahlzen wrote:

> >> Additionally, the fact that GRASS monitors (unfortunately) receive
> >> integer data (screen coordinates, line widths, etc) somewhat limits the
> >> effectiveness of the high-precision output from the CAIRO driver. For
> >> example, in the "Massachusetts Counties and roads" example, the
> >> antialiasing breaks down for certain lines because they are divided into
> >> smaller segments whose coordinates are rounded before they are drawn.
> >> Similarly, zooming in on vector output will also reveal this drawback.
> > 
> > Perhaps that rounding can be toggled based on the output device...
> I wish, and please prove me wrong here, but I believe that this would
> require a (minor) architectural change to GRASS. All monitors drivers
> get their drawing coordinates as integer data, e.g. "Draw a line from
> position 4, 23 to 5, 45", rather than floating point values, like "Draw
> a line from 3.84, 22.96 to 5.41, 45.11".

One of the main changes in 7.x is that graphics coordinates will use
floating-point. For geographic data, the expected approach will be to
set an appropriate transform then use cartographic coordinates.

> That makes some sense as long as you only display data on a pixel based
> screen/file with no antialiasing or oversampling (like the X or PNG
> monitors), but not for higher quality or vector format output.
> The PS driver that is currently in the CVS tree seems to suffer from the
> same problem, btw, which limits its usefulness as well.
> Changing this would probably mean adjusting all monitor drivers (which I
> don't think would be overly complicated, but it would take a bit of work).

Changing the drivers is the easy part; it's changing all of the d.*
commands which takes the effort.

Glynn Clements <glynn at gclements.plus.com>

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