[GRASS-dev] [GRASS GIS] #733: display lib: support for curveto

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1

#733: display lib: support for curveto
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Reporter: hamish | Owner: grass-dev@lists.osgeo.org
     Type: enhancement | Status: new
Priority: minor | Milestone: 7.0.0
Component: Display | Version: svn-trunk
Keywords: | Platform: All
      Cpu: All |
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Hi,

it would be nice if the display library had a D_curve_abs() and
D_curve_rel() functions to handle Bezier Curves. Among other things this
would make displaying EPS/SVG/DXF graphics directly on the display
monitor/canvas/whatever you want to call it now a whole lot easier. I am
not sure if it is better to expose a variable to control the number of
vertices along the line to be produced, or for the display lib to decide
that dynamically. I assume the coord input needs would be 3 or more pairs
of coords. Again, not sure if either be like D_polyline_*() and require a
"int n" argument or just have it loop while coord != NULL.

In the short term it could immediately be added to d.graph as a new
`curve` instruction and as a demonstration/testbed. (See the LIMITATIONS
section of the d.graph man page)
I expect the cairodriver and psdriver could use it directly, but not sure
of GIS uses for it beyond decorations. Hmmm... new line drawing tool in
wxVdigit?

thanks,
Hamish
(who has just spent more time than I care to calculating vertices by hand)

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Ticket URL: <https://trac.osgeo.org/grass/ticket/733&gt;
GRASS GIS <http://grass.osgeo.org>

2

#733: display lib: support for curveto
--------------------------+-------------------------------------------------
  Reporter: hamish | Owner: grass-dev@lists.osgeo.org
      Type: enhancement | Status: new
  Priority: minor | Milestone: 7.0.0
Component: Display | Version: svn-trunk
Resolution: | Keywords:
  Platform: All | Cpu: All
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Comment (by glynn):

Replying to [ticket:733 hamish]:

> it would be nice if the display library had a D_curve_abs() and
D_curve_rel() functions to handle Bezier Curves.

Are you volunteering to update the clipping/culling code to handle Bezier
curves?

> Among other things this would make displaying EPS/SVG/DXF graphics
directly on the display monitor/canvas/whatever you want to call it now a
whole lot easier. I am not sure if it is better to expose a variable to
control the number of vertices along the line to be produced, or for the
display lib to decide that dynamically. I assume the coord input needs
would be 3 or more pairs of coords. Again, not sure if either be like
D_polyline_*() and require a "int n" argument or just have it loop while
coord != NULL.

Are you planning on having the application supply explicit control points,
or have the display library determine the tangents itself?

For the former, an open path with N segments requires 3N+1 vertices
(versus N+1 for a polyline), while a closed path requires 3N (versus N).
For the latter, the number of vertices remains unchanged.

> I expect the cairodriver and psdriver could use it directly, but not
sure of GIS uses for it beyond decorations. Hmmm... new line drawing tool
in wxVdigit?

None of the drivers can use anything directly, because:

1. Some people want to be able to pass non-Euclidean geometry directly to
the display library, but none of the underlying graphics libraries or
formats support that, so the display library has to "euclidify" the data.

2. Some people want the display library to clip/cull the data to the
current region, because letting the driver render the entire map is too
much work for the driver, and having the module clip/cull the data to the
current region is too much work for the module's author, so the display
library has to clip/cull the data.

--
Ticket URL: <https://trac.osgeo.org/grass/ticket/733#comment:1&gt;
GRASS GIS <http://grass.osgeo.org>