Drawing a transparent line in pygame

Question

I need a way to draw a partially transparent line in pygame. I found this answer(python - Draw a transparent Line in pygame) but it only works with straight lines and doesn't work with different line thicknesses.

you can create new Surface with transparent background and draw any lines (without transparency) and later draw it in main Surface with transparency. — furas, Jul 03 '20 at 13:53
example from you link works in similar way as I described - but you don't have to use `fill()` but `draw.line()` — furas, Jul 03 '20 at 13:55
I'm new to pygame, how would you do this (with multiple surfaces)? — Robert Lucas, Jul 03 '20 at 13:55
create surface as in exeampl and use `fill()` with color `(0,0,0,0)` to create transparent surface (important is last zero which means `alpha` channel), later draw on this surface any line(s) and finally blit it on main surface. I have some [examples on GitHub](https://github.com/furas/python-examples/tree/master/pygame/transparency). Minimal example uses `draw.circle` to draw transparent circles. — furas, Jul 03 '20 at 14:00

score 3 · Accepted Answer · answered Jul 03 '20 at 14:20

You have to create new surface with any size and with alpha channel

surface = pygame.Surface((width, height)).convert_alpha()

or use main surface to create new surface with the same size

surface = screen.convert_alpha()

Fill it with transparent color (0,0,0,0). Important is last zero which means alpha channel - (R,G,B,A)

surfaces.fill([0,0,0,0])

Draw on this surface with alpha channel smaller then 255

pygame.draw.line(surface, (0, 0, 0, 32), (0, 0), (800, 600), 5)

And finally you can blit it on main page in any place

screen.blit(surface, (x, y))

For surface which has the same size as main surface it can be (0,0)

screen.blit(surface, (0,0))

Minimal example

import pygame

pygame.init()

screen = pygame.display.set_mode((800,600))#, depth=32)

surface1 = screen.convert_alpha()
surface1.fill([0,0,0,0])
pygame.draw.circle(surface1, (255, 0, 0, 128), (325, 250), 100)

surface2 = screen.convert_alpha()
surface2.fill([0,0,0,0])
pygame.draw.circle(surface2, (0, 255, 0, 128), (475, 250), 100)

surface3 = screen.convert_alpha()
surface3.fill([0,0,0,0])
pygame.draw.circle(surface3, (0, 0, 255, 128), (400, 350), 100)

surface4 = screen.convert_alpha()
surface4.fill([0,0,0,0])
pygame.draw.line(surface4, (0, 0, 0, 32), (0, 0), (800, 600), 5)
pygame.draw.line(surface4, (0, 0, 0, 32), (0, 600), (800, 0), 5)

surface5 = screen.convert_alpha()
surface5.fill([0,0,0,0])
pygame.draw.polygon(surface5, (255, 0, 0, 128), [(400, 250), (450, 300), (400, 350), (350, 300)])

screen.fill([255,255,255]) # white background
screen.blit(surface1, (0,0))
screen.blit(surface2, (0,0))
screen.blit(surface3, (0,0))
screen.blit(surface4, (0,0))
screen.blit(surface5, (0,0))

pygame.display.flip()

running = True
while running:
    for event in pygame.event.get():
        if event.type == pygame.QUIT:
            running = False
        if event.type == pygame.KEYDOWN:
            if event.key == pygame.K_ESCAPE:
                running = False

pygame.quit()

Drawing a transparent line in pygame

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