I was talking to a friend who is trying to make his own LED matrix (they're making their own LEDs from scratch in university so they can't just buy a module) and he asked me if he could short all of the cathodes of their LEDs together and use a single resistor, instead of using 1 resistor per LED. I said that he could, but then that would mean the brightness of the LEDs would change as the total current through an LED would depend on how many LEDs were powered.
After doing some online reading though I have found that this is actually what they do in a number of commercial LED matrices:
Image from circuitstoday
My question is, does this actually happen to matrices in real life? is it the change in current so negligible that the change in brightness isn't observable?
I have also found this image from circuitstoday and it does appear that the brightness of a row/column is dependant on how many LEDs or powered: