Merge and export two layers that utilize blending modes

Question

I have spent more time than I can to admit trying to come up with a solution that doesn't require bringing the layers into Photoshop or the GIMP (which would take an insane amount of RAM/swap space to work on this).

Here's the setup: Using QGIS 2.8.1 on Ubuntu. 2 layers imported, both have srs and align properly.

Layer 1: Low resolution color imagery for the entire planet Layer 2: Higher resolution B&W imagery for an entire planet

I'm using the color information from Layer 1 and overlaying it on the higher resolution B&W imagery of layer 2. To do this, I use the following:

Layer one: Properties: Color rendering: Blending mode: Hard Light Layer two: Properties: Band rendering: Contrast Enhancement: No enhancement Layer two: Properties: Color rendering: Contrast: -15

After setting these values, the resultant two datasets look very good over one another.

Now, my issues as I see them:

1. I need to merge the two layers into one
2. I need to export the resultant merged layer into a geotiff

I can figure out #2 okay using layer: Save As: Output mode: Rendered Image, etc

But, what I cannot figure out is #1.

Answer 1

You could try exporting the whole project with extent and pixel size=resolution set to match the input files:

qgis --project myproject.qgs --snapshot image.png --width 1500 --height 1000 --extent xmin,ymin,xmax,ymax

cf. http://anitagraser.com/2010/11/30/creating-high-resolution-images-using-qgis/

edit: that's comparable to Project->Save As Image (creates a world file) but allows to specify extent and size manually