pygame crashes using Surface.fill with a rect that goes outside its bounds and any nonzero PYGAME_BLEND flag

Question

When I try to use fill with a Surface using both the rect and special_flags arguments, pygame lags for a few seconds, then crashes without notification, if I have named rect dimensions that escape the Surface's area.

Here're the results of a test using IDLE:

>>> import pygame
>>> test_surf = pygame.Surface((50,50))
>>> test_rgba = 50,100,150,200 # arbitrary
>>> test_rect = 33,33,33,33 # these dimensions are 16px too large on each axis.
>>> test_surf.fill(test_rgba, test_rect, pygame.BLEND_RGBA_MULT)

>>> ================================ RESTART ================================
>>>

This restart occurs regardless of how I interact, or do not interact, with the software. It occurs whether I call pygame.init() after import pygame or not.

If a rect argument entirely within the surface's bounds (in this case, (0,0,50,50)) is used, it functions normally. Alternatively, omitting or sending 0 for the special_flags argument does not trigger this problem; pixels contained by the Surface are altered as normal and any that 'spill' off the edge are ignored.

What I would like to know is what causes pygame to crash in this mysterious and unexplained way?

I'm using pygame 1.9.1 and python 2.7.10.

Answer 1

For any future reader who also suffers from this problem, the problem can be sidestepped in one of at least three ways. I'll share them with you here:

• Use a clipped rect argument.

Replacing rect argument in fill with test_surf.get_rect().clip(test_rect). clip will constrain test_rect to the bounds of the destination Surface; if there are no common pixels, it will return a safe rect with shape parameters <0, 0, 0, 0>.

• Use a second surface and blit instead.

  Create a new Surface, fill it with the rgb value you want (excluding alpha) without using a BLEND mode, use set_alpha to apply the desired opacity to the Surface, and finally blit the new surface to test_surf, or whatever your target is, using the appropriate dest value (in my case, (33,33)) and BLEND mode (for me, pygame.BLEND_RBGA_MULT).

• Fight bugs with bugs.

  A third and more-puzzling solution is to call test_surf.fill(test_rgba, (0,0,0,0), pygame.BLEND_RGBA_MULT) before running the out-of-bounds rect. Whatever this does seems to fix pygame in such a way that you can safely go out of bounds with subsequent BLENDed fills without trouble. Moreover, attempting out-of-bounds, BLENDed fills on any Surface thereafter seems safe, and whatever the zero-pixel BLENDed-fill does seems to correct the dysfunctional element in pygame so that the crash cannot occur again that session.

  Be aware that this third solution has not been thoroughly tested!!