I've been playing around with an approach in QGIS that uses Addition between the layers I'm mapping to build a tri-variate composite colour map - an example is here in the QGIS Flickr pool:
To achieve an output I thought I had to set the background of the image to black - in order to draw out the colours. However I've been asked (demanded!!!) to set the background to white.
Thinking about it a little, one approach that would seem to work would be to set the background of the Composer to Alpha 0.
This is a screen grab of an map produced in a similar manner. In the composer I set the background with an Alpha of 0 - meaning that it doesn't influence the layers. Great!
However when I export the final image in any format (including Pdf and SVG) I only get a white page!
There has been a similar question asked about this in ArcGIS (Exporting Graphic with transparency in layout to PDF using ArcGIS for Desktop?) - and the suggestion there was to change the graphic to a feature and insert that .... something I don't think is possible with QGIS ...
So - has anyone either any suggestions how this might be overcome? How can I export an image which is using Addition and Alpha without needing a black background?
More fundamentally ... can anyone explain (in relative layman's terms!) why the export isn't working in the first place?