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Description
I found this while reviewing the code of #2615
If the surface pitch is not aligned to 4 bytes, it doesn't properly skip through rows.
For example:
import pygame
surf_height = 5
def create_surface_from_byte_width(byte_width):
byte_data = bytes(byte_width * surf_height)
surf_width = byte_width // 4
dest = pygame.image.frombuffer(
byte_data, (surf_width, surf_height), "RGBA", pitch=byte_width
)
dest.fill((120, 50, 70, 200))
return dest
dest = create_surface_from_byte_width(8) # 2 pixels wide
print("Surface 1: pitch =", dest.get_pitch())
print("Before premul:", dest.get_at((dest.get_width() - 1, dest.get_height() - 1)))
dest = dest.premul_alpha()
print("After premul:", dest.get_at((dest.get_width() - 1, dest.get_height() - 1)))
dest = create_surface_from_byte_width(10) # 2.5 pixels wide
print("Surface 2: pitch =", dest.get_pitch())
print("Before premul:", dest.get_at((dest.get_width() - 1, dest.get_height() - 1)))
dest = dest.premul_alpha()
print("After premul:", dest.get_at((dest.get_width() - 1, dest.get_height() - 1)))Surface 1: pitch = 8
Before premul: Color(120, 50, 70, 200)
After premul: Color(94, 39, 55, 200)
Surface 2: pitch = 10
Before premul: Color(120, 50, 70, 200)
After premul: Color(0, 0, 0, 0)
The final pixels of the 10-byte wide surface are never reached by the premultiply algorithm.