Using PyQGIS to buffer the width different than the height of a rectangle

Question

So I have a polygon in QGIS (3.18) and it will always be a rectangle (I create it using minimum bounding geometry, oriented rectangle) I want to buffer the width out using some math to get the total width to a number divisible by a number input by the user, and I want to do the same to the height but the buffer amount will need to be different to get it divisible by the same number. What is the best way to go about this?

This is for a plugin so I need to do this in PyQGIS.

EDIT: the end goal here is to divide this rectangle up into a grid of squares (side length of whatever the user inputs), and the squares should fill the rectangle up without any partial squares. If there is an even better way to get to this end goal then great. But I figure I will need to get the dimensions of the rectangle correct first.

here is the code to create the rectangle:

        newconvexhulllayer = processing.run("qgis:minimumboundinggeometry", {
          'INPUT': self.poly_layer,
          'TYPE': 1,
          'OUTPUT': 'memory:'})

        QgsProject.instance().addMapLayer(newconvexhulllayer['OUTPUT'])

I don't know where to begin for code for the buffer since I need to buffer the width and height by a different amount. I'm not worried about the math I just need to know how to buffer the width and height differently in qgis.

Answer 1

Here a PyQGIS solution for 1 feature :

from functools import lru_cache

@lru_cache(maxsize=6)
def orientation_choices(num_coords: int, axis: str = "x") -> list:
    if axis == "x":
        if num_coords == 4:  # diagonal rectangle
            return ["L", "L", "R", "R"]
        elif num_coords == 2:  # aligned with North rectangle
            return ["L", "R"]
        elif num_coords == 3:  # diamond rectangle
            return ["L", "0", "R"]

    if axis == "y":
        if num_coords == 4:  # diagonal rectangle
            return ["B", "B", "T", "T"]
        elif num_coords == 2:  # aligned with North rectangle
            return ["B", "T"]
        elif num_coords == 3:  # diamond rectangle
            return ["B", "0", "T"]


def which_rect_corner(vertices: list, vertex: QgsPoint) -> str:
    x_coords, y_coords = zip(*set((v.x(), v.y()) for v in vertices))
    x_coords = sorted(x_coords)
    y_coords = sorted(y_coords)
    pos_x_vertex = x_coords.index(vertex.x())
    pos_y_vertex = y_coords.index(vertex.y())
    choices_x = orientation_choices(len(x_coords), "x")
    choices_y = orientation_choices(len(y_coords), "y")
    return f'{choices_y[pos_y_vertex]}{choices_x[pos_x_vertex]}'


# replace here the variables with true values
rect_feature_index = 1
rect_width = 99
rect_height = 23
user_input = 20

# replace layer by the vector layer output of the QGIS alg
layer = iface.activeLayer()
current_rect_geom = layer.getFeature(rect_feature_index).geometry()


new_rect_width = (rect_width // user_input + [1, 0][rect_width % user_input == 0]) * user_input
new_rect_height = (rect_height // user_input + [1, 0][rect_width % user_input == 0]) * user_input

x_coord_delta = (new_rect_width - rect_width) / 2
y_coord_delta = (new_rect_height - rect_height) / 2

corners = [
    (
        vertex,
        which_rect_corner(current_rect_geom.vertices(), vertex)
    )
    for vertex in current_rect_geom.vertices()
]

for i, (vertex, corner) in enumerate(corners):
    vertex_x, vertex_y = vertex.x(), vertex.y()
    if corner == "BL":  # Bottom Left
        vertex_x -= x_coord_delta
        vertex_y -= y_coord_delta
    elif corner == "TL":  # Top Left
        vertex_x -= x_coord_delta
        vertex_y += y_coord_delta
    elif corner == "TR":  # Top Right
        vertex_x += x_coord_delta
        vertex_y += y_coord_delta
    elif corner == "BR":  # Bottom Right
        vertex_x += x_coord_delta
        vertex_y -= y_coord_delta
    elif corner == "B0":
        vertex_y -= y_coord_delta
    elif corner == "T0":
        vertex_y += y_coord_delta
    elif corner == "0L":
        vertex_x -= x_coord_delta
    elif corner == "0R":
        vertex_x += x_coord_delta

    # modify the vertex
    current_rect_geom.moveVertex(vertex_x, vertex_y, i)

with edit(layer):
    layer.changeGeometry(rect_feature_index, current_rect_geom)