[Qgis-developer] Blend modes for printer/pdf outputs - advice?

[Nyall Dawson]

Hi all,

I'm after some advice on the best way to handle pdf or printer output
of compositions when the composition or map contains blending.

Since QPrinter doesn't support the QPainter composition modes, any
blending options are currently ignored when exporting a composition to
pdf/printer. There's two options I can see to handle this, and I'm
wondering what everyone thinks is the best approach.

Option 1 - Automatically export compositions with blending using rasters.
This approach would check whether the composition contains any
blending, and if so would use the "print as raster" option to
effectively flatten the output before exporting. This is probably the
most user-friendly approach, since the user doesn't need to do
anything extra to get the printout looking correct.

Option 2 - Warn the user that blending requires 'print as raster' checked
This approach would display a warning dialog when attempting to
print/pdf a composition containing blending (similar to how svg
outputs currently show a warning). The warning would tell the user to
check "print as raster" if they want the blending modes used in the
export.

The advantage with option 2 is that it leaves the possibility for
vector-based exports even when blending has been used. This way, a
user who wants to further edit a composition in a vector illustration
program (inkscape/illustrator/etc) can still export to an editable pdf
to tweak the map and re-apply the blending in the external program.
Option 1 would make this impossible, since any blending would force
the output into raster format and prevent any manual editing of the
composition.

For this reason my gut feeling is that option 2 is a better way to go,
but I thought I'd raise it with the list for other opinions.

Nyall