how to draw transparent rect in pygame

Question

Can I draw a rect which is transparent inside, but not the outline? It would be better if I could set the outline's width.

Answer 1

If you just want to draw the outline of a rect, you can pass an integer as the fourth (the width) argument to pygame.draw.rect:

pygame.draw.rect(screen, (0, 100, 255), (50, 50, 162, 100), 3)  # width = 3

The problem with this approach is that the corners won't look sharp and clean and the outline can't be transparent.

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You could also use the gfxdraw module to draw several outlines with a for loop:

def draw_rect_outline(surface, rect, color, width=1):
    x, y, w, h = rect
    width = max(width, 1)  # Draw at least one rect.
    width = min(min(width, w//2), h//2)  # Don't overdraw.

    # This draws several smaller outlines inside the first outline. Invert
    # the direction if it should grow outwards.
    for i in range(width):
        pygame.gfxdraw.rectangle(screen, (x+i, y+i, w-i*2, h-i*2), color)

draw_rect_outline(screen, (250, 50, 162, 100), (0, 100, 255, 155), 9)

That also allows you to pass a color with an alpha channel to make the outline transparent.

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It would also be possible to create a transparent surface and draw a rect onto it (you can pass a transparent color here as well):

surf = pygame.Surface((162, 100), pygame.SRCALPHA)
pygame.draw.rect(surf, (0, 100, 255, 155), (0, 0, 162, 100), 21)

If you want a filled, transparent rectangle, just fill the complete surface:

surf.fill((0, 100, 255, 155))