What is the correct expression syntax to autofill output extent while batch processing in QGIS?

Question

In QGIS (3.2), I'm using the Rasterize (vector to raster) in batch mode. This tool requires an output extent be set for each layer (leaving this parameter blank is not acceptable). I would like to autofill output extent using Calculate by expression. The extent should match the input layer's extent. What is the correct expression syntax to accomplish this?

Answer 1

Something like this should work.

It collects all the features of the input layer together and gets their bounds. The various components of the bounds are placed in an array which is finally converted to a (comma-separated) string.

with_variable(
 'bounds', 
 bounds(
 aggregate(
 @INPUT, -- the layer name is calculated from the tool input
 'collect', 
 $geometry
 ) 
 ),
 array_to_string( 
 array(
 x_min(@bounds),
 x_max(@bounds),
 y_min(@bounds),
 y_max(@bounds)
 )
 )
)

Example output from the Batch Processing dialog window after calculation:

8.167027224,9.826959383,38.880448354,41.246319006 [EPSG:4326]

The CRS is added automatically based on the input layer