during save to image file to the disk, there are some noise...how to fix these issue?

[lovedancer075]

My code is like below:

During mpf.show() is called, the image is llike below:

and When I save the image to the png format, some noise appear like below, I wanna to know that how can I save a good png file without this noise?

[DanielGoldfarb]

I have seen this issue before. Here is one example. Please note the various comments there. Thank you.

[alexpvpmindustry]

wow, u guys have sharp eyes. not sure how to solve this tho...