Create fixed width and length polygon buffer from endpoints of polygon in QGIS

Question

Is there a way to generate a polygonal buffer on the end of an existing polygon in QGIS? The red polygon was added using MS Paint as an example of what I would like to achieve. Ideally, the red polygon would extend from the orange polygon from the center point (red vertex) - equally or similar to a buffer. It would be even better if the red polygon could maintain the width of the orange polygon. Not sure if this is possible or something Geometry by Expression could handle.

polygon_buffer_on_end_of_polygon

• Babel: What is the "end of a polygon"? You have only rectangles and you want to create the buffer at the smaller side of it? At both ends or just one end?

Answer 1

You can use QGIS expressions with Geometry Generator and this expression for rectangles - see screenshot at the bottom.

Variants:

• If you want the "buffers" only on one side, in line 3 replace (-1,1) by (1) to get only one end or by (-1) to get only the other end.
• In case you want the "buffers" on the long side of the rectangle, in line 18 change radians (main_angle ($geometry)) into radians (main_angle( $geometry)-90)

collect_geometries (
 array_foreach(
 array(-1,1), --change here to get buffers only at one end
 with_variable (
 'int',
 intersection (
 boundary($geometry),
 make_line(
 centroid ($geometry),
 project (
 centroid($geometry),
 @element* length (
 make_line(
 centroid($geometry),
 point_n($geometry,1)
 )
 ),
 radians (main_angle ($geometry)) -- change here to get buffers at the long side of rectangles
 )
 )
 ),
 make_regular_polygon(
 @int, 
 project (
 @int, 
 length(
 make_line (
 @int,
 closest_point (
 nodes_to_points($geometry),
 @int
 )
 )
 ),
 radians (main_angle ($geometry))
 ),
 4,
 1
 )
 )
 )
)

Yellow: original rectangles, blue with red outline: created "buffers": enter image description here

• Anthony: Thank you for providing an answer to my question. What would I change in your expression if I wanted to save the results to a shapefile using Geometry by Expression?
• Babel: Should work the same for Geometry by expression
• Anthony: I'm getting ---> Evaluation error: Cannot convert to geometry
• Babel: For me, it works - testet right now. When you use geometry generator, does it work?
• Anthony: Yes - your solution works using the Geometry Generator, but returns the evaluation error listed above.