Circular mask with ratio on boundaries

I know I must show some effort but I really have no idea how to solve this problem.

I know how to create circular masks: https://stackoverflow.com/a/44874588/2681662

In the example in link above a mask is a boolean array. Means it shows if the pixel is under the circle or not which I would like to call discrete.

Discrete

However I want to know how much of each pixel is under the circle. So basically I would have a float mask array and in the boundary of the circle I would have a value shows how much of the pixel is under the circle (percentage). Which I would like to call continuous.

Continuous

Please note, the numbers given are not calculated and are eyeball estimate.

If anyone can point me to right direction I will be grateful.

Answer

I wish to thank to all of you. Specially @Yves Daoust you pointed me to right direction. I wanted to share my solution for those who face similar problem:

My solution used intersections. If I could find the intersection area between a rectangle and a circle I might be able to consider a pixel as a rectangle and the circle would be, obviously, the circle.

Intersection between two shapes:

For this You can use shapely:

To create a circle shapely uses points and adds some buffer (radius) to it:

from shapely.geometry import Point
circle = Point(centerx, centery).buffer(r)

to create a rectangle shapely provides box:

from shapely.geometry import box
rect = box(minx=min_x, miny=min_y, maxx=max_x, maxy=max_y)

One can calculate numerous properties of each shape (which technically are polygons) such as area and bounding points.

from shapely.geometry import Point
circle = Point(centerx, centery).buffer(r)
print(circle.area)

One can calculate the intersection of two polygons and it would return a polygon:

from shapely.geometry import Point, box
circle = Point(centerx, centery).buffer(r)
rect = box(minx=min_x, miny=min_y, maxx=max_x, maxy=max_y)
intersection = circle.intersection(rect)

A pixel is a square with sides of 1 unit (pixel). So the intersection area of a pixel and any other shape would result in a value [0, 1] and it’s what we were looking for.

Code

Please notice I used ellipses instead of circles since it’s inclusive.

My Package:

from __future__ import annotations

from typing import Union, Tuple
from shapely.geometry import Point, Polygon, box
from shapely.affinity import scale, rotate
from matplotlib.patches import Ellipse
import numpy as np


class Pixel:
    def __init__(self, x: int, y: int) -> None:
        """
        Creates a 1x1 box object on the given coordinates
        :param x: int
                x coordinate
        :param y: int
                y coordinate
        """
        self.x = x
        self.y = y
        self.body = self.__generate()

    def __str__(self) -> str:
        return f"Pixel(x={self.x}, y={self.y})"

    def __repr__(self) -> str:
        return self.__str__()

    def __generate(self) -> Polygon:
        """returns a 1x1 box on self.x, self.y"""
        return box(minx=self.x, miny=self.y, maxx=self.x + 1, maxy=self.y + 1)


class EllipticalMask:
    def __init__(self, center: Tuple[Union[float, int], Union[float, int]],
                 a: Union[float, int], b: Union[float, int], angle: Union[float, int] = 0) -> None:
        """
        Creates an ellipse object on the given coordinates and is able to calculate a mask with given pixels.
        :param center: tuple
                (x, y) coordinates
        :param a: float or int
                sami-major axis of ellipse
        :param b:  float or int
                sami-minor axis of ellipse
        :param angle:  float or int
                angle of ellipse (counterclockwise)
        """
        self.center = center
        self.a = a
        self.b = b
        self.angle = angle

        self.body = self.__generate()

    def __generate(self) -> Polygon:
        """Returns an ellipse with given parameters"""
        return rotate(
            scale(
                Point(self.center[1], self.center[0]).buffer(1),
                self.a,
                self.b
            ),
            self.angle
        )

    def __extreme_points(self) -> dict:
        """Finds extreme points which the polygon lying in"""
        x, y = self.body.exterior.coords.xy
        return {
            "X": {
                "MIN": np.floor(min(x)), "MAX": np.ceil(max(x))
            },
            "Y": {
                "MIN": np.floor(min(y)), "MAX": np.ceil(max(y))
            }
        }

    def __intersepter_pixels(self) -> list:
        """Creates a list of pixel objects which ellipse is covering"""
        points = self.__extreme_points()
        return [
            Pixel(x, y)
            for x in np.arange(points["X"]["MIN"], points["X"]["MAX"] + 1).astype(int)
            for y in np.arange(points["Y"]["MIN"], points["Y"]["MAX"] + 1).astype(int)
            if x >= 0 and y >= 0
        ]

    def mask(self, shape: tuple) -> np.ndarray:
        """
        Returns a float mask
        :param shape: tuple
                the shape of the mask as (width, height)
        :return: ndarray
        """
        pixels = self.__intersepter_pixels()
        mask = np.zeros(shape).astype(float)
        for pixel in pixels:
            ratio = pixel.body.intersection(self.body).area
            mask[pixel.x][pixel.y] = ratio

        return mask

    def matplotlib_artist(self) -> Ellipse:
        """
        Returns a matplotlib artist
        :return: Ellipse
        """
        e = Ellipse(xy=(self.center[0] - 0.5, self.center[1] - 0.5), width=2 * self.a, height=2 * self.b,
                    angle=90 - self.angle)
        e.set_facecolor('none')
        e.set_edgecolor("red")
        return e


class CircularMask(EllipticalMask):
    def __init__(self, center: Tuple[Union[float, int], Union[float, int]],
                 r: Union[float, int]) -> None:
        """
        Uses ellipse to create a circle
        :param center:  tuple
                (x, y) coordinates
        :param r: float or int
                radius of circle
        """
        super(CircularMask, self).__init__(center, r, r, 0)

Usage:

from myLib import EllipticalMask
from matplotlib import pyplot as plt

m = EllipticalMask((50, 50), 25, 15, 20)

mask = m.mask((100, 100))
e = m.matplotlib_artist()

fig, ax = plt.subplots(1, 1, figsize=(4, 4))
ax.imshow(mask)
ax.add_artist(e)
plt.show()

Results:

enter image description here

enter image description here

Any feedback is appreciated.