How to set alpha transparency property using pygame.draw.line?

Question

I am trying to set transparency of a drawn line using pygame.draw.line(tela, (255, 255, 255), self.a, self.b, width=1), where:

• tela is the screen
• (255, 255, 255) is the color
• self.a and self.b are the position and direction respectively
• and width is its weight

Is there a direct simple way to do that? Is there a conceptual argument that prevent us to do it readily? Does everything have to have a complex answer?

Answer 1

Below is an adaptation of @Yannis Assael's answer to his own question Pygame draw anti-aliased thick line that I referred you to. I've transformed into a function named aaline() and streamlined the code to try to make it more efficient and readable.

The function has been defined to have a calling sequence similar to the existing one named pygame.draw.aaline() for drawing single-pixel-wide lines make switching from on to the other a little easier.

from math import atan2, cos, hypot, sin
import pygame
import pygame.gfxdraw
from pygame.locals import *
import sys


def aaline(surface, color, start_pos, end_pos, width=1):
    """ Draws wide transparent anti-aliased lines. """
    # ref https://stackoverflow.com/a/30599392/355230

    x0, y0 = start_pos
    x1, y1 = end_pos
    midpnt_x, midpnt_y = (x0+x1)/2, (y0+y1)/2  # Center of line segment.
    length = hypot(x1-x0, y1-y0)
    angle = atan2(y0-y1, x0-x1)  # Slope of line.
    width2, length2 = width/2, length/2
    sin_ang, cos_ang = sin(angle), cos(angle)

    width2_sin_ang  = width2*sin_ang
    width2_cos_ang  = width2*cos_ang
    length2_sin_ang = length2*sin_ang
    length2_cos_ang = length2*cos_ang

    # Calculate box ends.
    ul = (midpnt_x + length2_cos_ang - width2_sin_ang,
          midpnt_y + width2_cos_ang  + length2_sin_ang)
    ur = (midpnt_x - length2_cos_ang - width2_sin_ang,
          midpnt_y + width2_cos_ang  - length2_sin_ang)
    bl = (midpnt_x + length2_cos_ang + width2_sin_ang,
          midpnt_y - width2_cos_ang  + length2_sin_ang)
    br = (midpnt_x - length2_cos_ang + width2_sin_ang,
          midpnt_y - width2_cos_ang  - length2_sin_ang)

    pygame.gfxdraw.aapolygon(surface, (ul, ur, br, bl), color)
    pygame.gfxdraw.filled_polygon(surface, (ul, ur, br, bl), color)


if __name__ == '__main__':

    # Define some colors.
    BLACK = (0, 0, 0)
    WHITE = (255, 255, 255)
    RED = (255, 0, 0)
    GREEN = (0, 255, 0)
    BLUE = (0, 0, 255)
    AQUA = (0, 255, 255)
    ORANGE = (255, 165, 0)
    YELLOW = (255, 255, 0)

    # Window size.
    WIDTH, HEIGHT = 800, 600

    # Set up pygame.
    pygame.init()

    # Set up window for display.
    window = pygame.display.set_mode((WIDTH, HEIGHT), 0, 32)
    pygame.display.set_caption('Wide Transparent Lines')

    # Set background color of window.
    window.fill(BLACK)

    a = (0, 0)
    b = (WIDTH, HEIGHT)

    LINE_COLOR = ORANGE
    fw = 255 / (WIDTH-1)
    fh = 255 / (HEIGHT-1)
    width = 3

    # Draw an opaque diagonal line then those on either side with
    # ever-increasing transparency.
    color = LINE_COLOR + (255,)  # Add transparency to color.
    aaline(window, color, (0, 0), (WIDTH, HEIGHT), width)

    for x in range(0, WIDTH, 25):
        color = LINE_COLOR + (int(fw*x),)  # Add transparency to color.
        aaline(window, color, (0, 0), (x, HEIGHT), width)

    for y in range(0, HEIGHT, 25):
        color = LINE_COLOR + (int(fh*y),)  # Add transparency to color.
        aaline(window, color, (0, 0), (WIDTH, y), width)

    # Copy window to screen.
    pygame.display.update()

    # Run the game loop.
    while True:
        for event in pygame.event.get():
            if event.type == QUIT:
                pygame.quit()
                sys.exit()

For testing and demonstration purposes, there's a driver at the end that calls it in some loops. Below is a screenshot of what's displayed. The lines become progressively more transparent as one of their end points gets further and further away from the the center diagonal of the rectangle.