[mapserver-dev] MapServer png vs pdf output

Tamas Szekeres szekerest at gmail.com
Thu Jun 9 13:41:25 PDT 2016

Hi Devs,

I've looked into the code and the problem described in the mail below still
persists and the image being drawn by cairo/pdf and cairo/svg is incorrect.
To summarize the things, the width and height passed to
 or cairo_svg_surface_create
should be
in points (1 point == 1/72.0 inch) which is not the case at the moment we
are passing the width and heights in pixels therefore we get a different
image when rendering the same with the cairo/png format.

Is this considered as a bug that should be corrected in MapServer?



2014-11-04 22:11 GMT+01:00 Tamas Szekeres <szekerest at gmail.com>:

> Hi Devs,
> We may experience a difference regarding to the (visual) image size when
> saving the map to pdf vs png. The problem is already described in the
> following issue:
> Cairo SVG and PDF output larger than PNG
> <https://github.com/mapserver/mapserver/issues/4487> which is related to
> the pdf/svg canvas where the size is specified in points and not in pixels.
> That means: each point has a fixed size of 1/72 inches regardless of the
> device used.
> Our problem here is that we pass the image size directly to
> cairo_pdf_surface_create
> <http://cairographics.org/manual/cairo-PDF-Surfaces.html#cairo-pdf-surface-create>
> or cairo_svg_surface_create
> <http://cairographics.org/manual/cairo-SVG-Surfaces.html#cairo-svg-surface-create> which
> is incorrect. We should rather pass
> 72.0 / map->resolution * map->width and 72.0 / map->resolution *
> map->height respectively. And we should also set map->resolution to 72
> before the image is drawn to let the symbols scale with the image size
> correctly.
> My question is what would be the correct handling for this in MapServer?
> 1. Don't do anything in MapServer let the client programs (like mapscript
> applications) to change the image size and scale.
> 2. Apply these corrections at the beginning of msDrawMap (for the pdf/svg
> renderers specifically) and restore the original values at the end of the
> drawing operation.
> 3. Apply these corrections at driver level (createImageCairo etc.)
> For me #2 seems to be the most reasonable approach (with just a few lines
> of code).
> #1 would not help for the existing apps (mapserv, shp2img) which should be
> fixed separately. And I cannot really see how #3 would work, since the
> image size and resolution is not wired directly into the drivers
> (createImage doesn't get the resolution for example).
> Any ideas or alternatives?
> Thanks,
> Tamas
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