The alphamask example: The web preview doesn't match the Processing example

[soegaard]

Issue description

The alphamask example https://processing.org/examples/alphamask.html has a P5.js preview over the code. When the Processing example is loaded from the Processing editor - the sketch doesn't behave the same way as the preview.

My impression is that the P5js preview on top of the code needs to invert the alphamask in order to behave the same way as the Processing example below.

See screenshots of the sitatuation here:

https://github.com/processing/p5.js/issues/5343

URL(s) of affected page(s)

https://processing.org/examples/alphamask.html

Proposed fix

Invert alphamask of the image used for the P5.js preview (and do nothing for the image used as alphamask in the Processing examples).