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I have been tasked with setting up what will essentially be a large screen displaying information, with the pi running it fixed beneath behind a small touch screen where users can input info and control the display on the large screen.

Googling has turned up nothing useful and I don't know where to start.

I have near total freedom over the pi and already have all the hardware I'm going to get, IE pi, touchscreen and display screen, I just have no idea where to start with this

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You could use a hdmi splitter for this. So from you hdmi out on the PI you connect it to a powered hdmi splitter and then connect your large screen and the touch screen that way your video source is split between the 2 screens. You will need to see what resolution the touch screen supports and have your big screen match it.

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Another thought would be to have two Pi and attach one screen to each. The one with the touch screen could tell the other Pi what to display. They could be connected back to back with ethernet using ssh to pass information.

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Can I use HDMI output alongside my LCD?

Yes and no. As explained in the official Pi blog on the subject, only applications which know how to output over HDMI can be used. An example is given for OMXPlayer: https://www.raspberrypi.org/blog/the-eagerly-awaited-raspberry-pi-display/2.4k

It is possible to use both display outputs at the same time, but it does require software to choose the right display. Omxplayer is one application that has been modified to enable secondary display output.

To start displaying a video onto the LCD display (assuming it is the default display) just type:

# omxplayer video.mkv

To start a second video onto the HDMI then:

# omxplayer --display=5 video.mkv

Please note, you may need to increase the amount of memory allocated to the GPU to 128MB if the videos are 1080P, adjust the gpu_mem value in config.txt for this. The Raspberry Pi headline figures are 1080P30 decode, so if you are using two 1080P clips it may not play correctly depending on the complexity of the videos.

Display numbers are:

  • LCD: 4
  • HDMI/TV: 5
  • Auto-select non-default display: 6

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