Also, find the real source of what you're copying. The formula they cite for brightness is a rounded version of BT601 luminance. Citing the standard in the comment is way better than just linking a random SO answer.

Links are mutable, links die, SO answers can be edited. Include any information needed to understand the code into the comment and proper copyright acknowledgements if you copied it (assuming the license allows it).

What search engine prioritizes links in comments or code blocks as any sort of signal?

I have no particular attachment to SO, especially after the way they handled their public drama recently.

That said I put a link to SO any time I have to look something up there and it’s not immediately obvious from the naming/docs why it does what it does. I also try to sum it up in a sentence or two if I can and if it doesn’t distract from understanding the larger goal of that section of code.

Sure, but also it may be worth noting that the content license on Stack Overflow posts requires this. And it's the poster's content, not Stack Overflow's, so there's an element of respecting a fellow coder who helped you. In fact, most open source licenses require attribution at a minimum.

That was my first cynical thought. It is a good rule regardless.