[mapserver-dev] RFC 89 Layer Geomtransform: Call for vote

Daniel Morissette dmorissette at mapgears.com
Tue Feb 19 07:12:39 PST 2013

Alan, All,

As we looked into the implementation details with Alan, we found out 
that the STYLE-level GEOMTRANSFORM receives a shape in pixel 
coordinates, whereas the LAYER-level GEOMTRANSFORM will receive the raw 
shape in ground coordinates (meters, degrees, etc.)

This poses a little potential for confusion in the user's mind as the 
argument to methods such as simplify() must be in the same units as the 
coordinates of the shapes at that point of the rendering workflow, i.e. 
pixels at the STYLE-level and in ground units at the LAYER-level.


LAYER NAME "my_layer"
   DATA "lines.shp"
   GEOMTRANSFORM (simplify([shape], 10))  ## 10 ground units
       GEOMTRANSFORM (buffer([shape], 5)  ## 5 pixels
       WIDTH 2
       COLOR 255 0 0

It would be possible (but messy) to include some maths at the 
layer-level to allow passing a value in pixels at the layer level as 
well. This way things would be consistent between STYLE level and LAYER 
level as we would always pass args in pixels, but this may not be what 
we want in lots of cases, for instance if a LAYER-level GEOMTRANSFORM is 
used to simplify shapes in a WFS context where pixel size makes no or 
little sense.

It is also not possible at all to deal with values in ground units at 
the style level because we do not have information about map cellsize at 
that level in the code (so passing values in ground units at the style 
level is not an option without messy code changes)

Note that in cases where we want to pass a pixel value at the layer 
level, a [map_cellsize]  variable will be available, e.g.

LAYER NAME "my_layer"
   DATA "lines.shp"
   # 10 * [map_cellsize] == 10 pixels converted to ground units
   GEOMTRANSFORM (simplify([shape], [map_cellsize]*10))

It seems that our best option would be to keep the values as pixels at 
the style level and ground units at the layer level, and properly 
document the difference, and hope that users won't be too confused.

What do you all think?


On 13-02-14 6:58 AM, Alan Boudreault wrote:
> Hi devs,
> Please vote on RFC 89, Layer Geomtransform (
> http://mapserver.org/trunk/development/rfc/ms-rfc-89.html )
> I'll start with my +1.
> Thanks,
> Alan

Daniel Morissette
Provider of Professional MapServer Support since 2000

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