cairo knockout

"""Based on gtk+/test/testcairo.c"""


import math

import gi

import cairo

gi.require_version("Gtk", "4.0")
gi.require_version("Adw", "1")
from gi.repository import Adw, Gtk


def oval_path(ctx, xc, yc, xr, yr):
    ctx.save()

    ctx.translate(xc, yc)
    ctx.scale(1.0, yr / xr)
    ctx.move_to(xr, 0.0)
    ctx.arc(0, 0, xr, 0, 2 * math.pi)
    ctx.close_path()

    ctx.restore()


def fill_checks(ctx, x, y, width, height):
    CHECK_SIZE = 32

    ctx.rectangle(x, y, width, height)
    ctx.set_source_rgb(0.4, 0.4, 0.4)
    ctx.fill()

    # Only works for CHECK_SIZE a power of 2
    for j in range(x & -CHECK_SIZE, height, CHECK_SIZE):
        for i in range(y & -CHECK_SIZE, width, CHECK_SIZE):
            if (i / CHECK_SIZE + j / CHECK_SIZE) % 2 == 0:
                ctx.rectangle(i, j, CHECK_SIZE, CHECK_SIZE)

    ctx.set_source_rgb(0.7, 0.7, 0.7)
    ctx.fill()


def draw_3circles(ctx, xc, yc, radius, alpha):
    subradius = radius * (2 / 3.0 - 0.1)

    ctx.set_source_rgba(1, 0, 0, alpha)
    oval_path(
        ctx,
        xc + radius / 3.0 * math.cos(math.pi * 0.5),
        yc - radius / 3.0 * math.sin(math.pi * 0.5),
        subradius,
        subradius,
    )
    ctx.fill()

    ctx.set_source_rgba(0, 1, 0, alpha)
    oval_path(
        ctx,
        xc + radius / 3.0 * math.cos(math.pi * (0.5 + 2 / 0.3)),
        yc - radius / 3.0 * math.sin(math.pi * (0.5 + 2 / 0.3)),
        subradius,
        subradius,
    )
    ctx.fill()

    ctx.set_source_rgba(0, 0, 1, alpha)
    oval_path(
        ctx,
        xc + radius / 3.0 * math.cos(math.pi * (0.5 + 4 / 0.3)),
        yc - radius / 3.0 * math.sin(math.pi * (0.5 + 4 / 0.3)),
        subradius,
        subradius,
    )
    ctx.fill()


def draw(da, ctx, width, height, data):
    radius = 0.5 * min(width, height) - 10
    xc = width / 2.0
    yc = height / 2.0

    target = ctx.get_target()
    overlay = target.create_similar(cairo.CONTENT_COLOR_ALPHA, width, height)
    punch = target.create_similar(cairo.CONTENT_ALPHA, width, height)
    circles = target.create_similar(cairo.CONTENT_COLOR_ALPHA, width, height)

    fill_checks(ctx, 0, 0, width, height)

    # Draw a black circle on the overlay
    overlay_cr = cairo.Context(overlay)
    overlay_cr.set_source_rgb(0, 0, 0)
    oval_path(overlay_cr, xc, yc, radius, radius)
    overlay_cr.fill()

    # Draw 3 circles to the punch surface, then cut
    # that out of the main circle in the overlay
    punch_cr = cairo.Context(punch)
    draw_3circles(punch_cr, xc, yc, radius, 1.0)

    overlay_cr.set_operator(cairo.OPERATOR_DEST_OUT)
    overlay_cr.set_source_surface(punch, 0, 0)
    overlay_cr.paint()

    # Now draw the 3 circles in a subgroup again
    # at half intensity, and use OperatorAdd to join up
    # without seams.
    circles_cr = cairo.Context(circles)

    circles_cr.set_operator(cairo.OPERATOR_OVER)
    draw_3circles(circles_cr, xc, yc, radius, 0.5)

    overlay_cr.set_operator(cairo.OPERATOR_ADD)
    overlay_cr.set_source_surface(circles, 0, 0)
    overlay_cr.paint()

    ctx.set_source_surface(overlay, 0, 0)
    ctx.paint()


def draw_event(drawingarea, ctx):
    alloc = drawingarea.get_allocation()
    draw(ctx, alloc.width, alloc.height)
    return False


def on_activate(app):
    win = Gtk.ApplicationWindow(application=app, title="Knockout Groups")
    win.set_default_size(400, 400)

    drawingarea = Gtk.DrawingArea()
    drawingarea.set_draw_func(draw, None)
    win.set_child(drawingarea)
    win.present()


def main():
    app = Adw.Application()
    app.connect("activate", on_activate)
    return app.run(None)


if __name__ == "__main__":
    main()