Low resolution problem in MacBook Pro when you log in.

You could encounter low resolution problem in MacBook Pro

when you log in like after connecting to a projector.

Then, try to reset NVRAM (non-volatile random-access memory) as follows:

1. Shut down your Mac.
2. Locate the following keys on the keyboard: Command (⌘), Option, P, and R.
3. Turn on your Mac.
4. Press and hold the Command-Option-P-R keys immediately after you hear the startup sound.
5. Hold these keys until the computer restarts and you hear the startup sound for a second time.
6. Release the keys.

References:
https://discussions.apple.com/thread/2353284
https://support.apple.com/en-us/HT204063