How to get pixel data in canvas given rotation?

Question

JavaScript

Dealt with canvas. I have coded myself a small image editor in which we can scale and rotate the image. I decided to add a change in the transparency of the image and ran into a problem. The area where the pixel data is taken does not take into account that the context is rotated. What should I do?
Maybe I'm wording it wrong, sorry. I hope the image helps to understand the essence of the problem.
Here is the app itself

Answer 1

https://ru.wikipedia.org/wiki/%D0%9C%D0%B0%D1%82%D...