[NTG-context] Shading and transparency

Hi! It's shadings and transparency again. For most things transparency is not required since the background colour (white) can be used instead, to achieve the same effect (thanks Hans for the reimplementation/cleanup a while ago). For one use case I worked around this by including an external file and overlay the external file (see attached example). Background: In the actual documents the transparent overlays overlay images to have the edges fade into the page background (only the centre is of interest). Is there a method that does not require an external file or pre-processing? Using the external file works. Just wondering if there's a better - more integrated - way. Marco