Powerpoint flickers when plugged into projector via HDMI

To troubleshoot the issue, try changing the display settings on your Mac to adjust the refresh rate or resolution when connected to the projector. Additionally, you can also try updating your PowerPoint software to see if that resolves the flickering problem.

To test if it's a hardware graphics acceleration issue in PowerPoint for Mac, try disabling it through PowerPoint preferences. To check the refresh rate, go to System Preferences > Displays and adjust the refresh rate for the projector.

<< the other possibility may be the refresh rate of the mac for the projector.. >>

As long as your projector is not connecting using AirPlay, or a DisplayLink stunt-box, the refresh rate should always be at least 30 times a second or higher, no matter what the software is doing.

Because the refresh rate for a regular wired display is so very high, refreshing an ordinary display using software is completely impossible. The Mac includes several display-generator automata -- Hardware that can fetch the entire display buffer, serialize it, and get it up on the screen as if it were one operation (from the processor point of view).

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When you Mirror displays, the Mac calculates a compromise resolution to be used on BOTH displays, and runs them in lock step at exactly the same resolution and refresh rate.

Punchline: it is not likely to be the refresh rate of the Mac that is the issue.

What model-year MacBook Pro?

Please enumerate exactly what displays (including the projector as a display) are connected, and if any are mirrored to each other.