Setting a pygame surface to have rounded corners

Question

This Rectangle class extends from pygame.sprite. I'd like to use set_rounded to modify how round the corners of the rect are. For example https://i.sstatic.net/uyKWC.jpg

class Rectangle(pg.sprite.Sprite):
    def __init__(self):
        pg.sprite.Sprite.__init__(self)
        self.original_image = pg.Surface((10, 10))
        self.image = self.original_image
        self.rect = self.image.get_rect()


    def set_rounded(self, roundness):
        pass

The roundness argument would determine the radius of the rounded rect.

Answer 1

You can achieve what you want by setting the key word argument border_radius of the function pygame.draw.rect.

Create a rectangle the same size as the image and and per pixel alpha (SRCALPHA) and draw a completely white, opaque image with round corners on it:

size = self.original_image.get_size()
self.rect_image = pg.Surface(size, pg.SRCALPHA)
pg.draw.rect(self.rect_image, (255, 255, 255), (0, 0, *size), border_radius=roundness)

Copy the original image and use the BLEND_RGBA_MIN blending mode to blend the rectangle with the image (see pygame.Surface.blit):

self.image = self.original_image.copy().convert_alpha()
self.image.blit(self.rect_image, (0, 0), None, pg.BLEND_RGBA_MIN) 

Note, the keyword attribute border_radius is a new feature in Pygame 2.0.

---

If you can't use version 2.0, you'll need to stick the rounded rectangle together yourself:

class Rectangle(pg.sprite.Sprite):
    # [...]

    def set_rounded(self, roundness):
        size = self.original_image.get_size()
        self.rect_image = pg.Surface(size, pg.SRCALPHA)
        
        #pg.draw.rect(self.rect_image, (255, 255, 255), (0, 0, *size), border_radius=roundness)

        r, c = roundness, (255, 255, 255)
        pg.draw.rect(self.rect_image, c, (r, 0, size[0]-2*r, size[1]))
        pg.draw.rect(self.rect_image, c, (0, r, size[0], size[1]-2*r))
        for cpt in [(r, r), (size[0]-r, r), (r, size[1]-r), (size[0]-r, size[1]-r)]:  
            pg.draw.circle(self.rect_image, c, cpt, r)

        self.image = self.original_image.copy().convert_alpha()
        self.image.blit(self.rect_image, (0, 0), None, pg.BLEND_RGBA_MIN) 

---

See the example:

import pygame as pg

class Rectangle(pg.sprite.Sprite):
    def __init__(self):
        pg.sprite.Sprite.__init__(self)
        self.original_image = pg.Surface((100, 100))
        self.original_image.fill((255, 0, 0))
        self.image = self.original_image
        self.rect = self.image.get_rect()

    def set_rounded(self, roundness):
        size = self.original_image.get_size()
        self.rect_image = pg.Surface(size, pg.SRCALPHA)
        pg.draw.rect(self.rect_image, (255, 255, 255), (0, 0, *size), border_radius=roundness)

        self.image = self.original_image.copy().convert_alpha()
        self.image.blit(self.rect_image, (0, 0), None, pg.BLEND_RGBA_MIN) 

pg.init()
window = pg.display.set_mode((200, 200))

rect_object = Rectangle()
rect_object.set_rounded(30)
rect_object.rect.center = window.get_rect().center
group = pg.sprite.Group(rect_object)

run = True
while run:
    for event in pg.event.get():
        if event.type == pg.QUIT:
            run = False

    window.fill((128, 128, 128))
    group.draw(window)
    pg.display.flip()