RPi4 can use an iPad Pro as an HDMI screen (click link below)


Scroll down - there is a follow up for video, works for ANY iPad!

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You can also use an Android device with an X Server application such as XSDL. I did this quite a bit whilst testing different screen sizes and touchscreen. It works fairly well but there is always the worry that the network will drop out so I wouldn’t want to use it for a performance unit. Handy for testing though! (I liked being able to take the display to another room to sit by the fire whilst testing bits or writing bug reports :slight_smile: .)