Creating Single sided buffer with Virtual layer in QGIS

Question

I'm creating a layer with single sided buffers based on a line layer as virtual layer in QGIS with the following query:

SELECT
    id,
    ST_SingleSidedBuffer(layer.geometry, layer.buffer_dist, layer.side) 
FROM
    layer;

The buffer distance and the buffer side for each feature ist stored in the attribute table.

I would like to the set the join style to 'MITRE'. I found that there should be a way to set the option like this (https://www.gaia-gis.it/fossil/libspatialite/wiki?name=BufferOptions):

SELECT BufferOptions_SetJoinStyle('MITRE');

Unfortunately I get the following error message:

Query preparation error on PRAGMA table_info(_tview): no such function: BufferOptions_SetJoinStyle

Hence my question: Is there a way to set the buffer options globally in the QGIS project? Or is there a way to set the buffer options within the query?

Answer 1

As it does not seem to be possible right now using a virtual layer (as stated in the comment by @geozelot), you might want to use another method to achieve the same result. You can use QGIS expressions with Geometry generator (for visualization only) or Menu processing / Toolbox / [Geometry by expression]2 to create actual geometries (see here for details).

Use the function single_sided_buffer, it has the following syntax:

single_sided_buffer(geometry,distance[,segments=8][,join=1][,miter_limit=2.0])

Help says:

join - join style for corners, where 1 = round, 2 = miter and 3 = bevel

The whole expression thus looks something like (change the fieldnames "buffer_dist" and buffer_side):

single_sided_buffer(
    $geometry, 
    "buffer_dist" * "buffer_side",
    join:=2
)

The expression, here with Geometry genrator - initial line in red, buffers in blue: