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I'm considering coding a software that would read data sent from a device (game console, computer, ...) via HDMI. The program would then be able to display the shared content inside a window, which would make it possible to use a laptop screen as a monitor. More info on what I'm trying to do here.

I'm aware that an HDMI-out port of a laptop can't be used to retrieve data. I would rather use an USB port, with potentially a HDMI to USB adapter.

My question is : Is there any physical barrier blocking me from retrieving an HDMI output from USB port, manipulating it in my program to somehow display it on screen ?

In my mind, this should just be some transfer of bits. With the right protocol understanding, my program should be able to retrieve the data one way or another. But I must admit my knowledge of HDMI and hardware is really limited. Any redirection to web content, doc, tutorial or blog would be greatly appreciated by the way.

Thank you.

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    \$\begingroup\$ Sounds like what you want is an HDMI capture card that outputs to USB. They're reasonably common for game streamers, and it shouldn't be too hard to find a place that sells them. There's no need to reinvent the wheel here. On the other hand, if your main goal is to learn more about HDMI and USB, have a look at the HDMI2USB project. \$\endgroup\$
    – tangrs
    Commented Oct 7, 2020 at 10:27
  • \$\begingroup\$ @tangrs Yep, I learnt about it but I wonder why you have to get through such cards instead of just having an appropriate software on your computer... \$\endgroup\$
    – SinLey
    Commented Oct 7, 2020 at 10:42
  • \$\begingroup\$ I'm not sure why you would think that'd be possible. USB isn't GPIO. USB and HDMI aren't remotely compatible (neither electrically or protocol-wise). Say you were to somehow connect an HDMI output to a USB port, almost no USB controller hardware will expose the kind of interface that would even allow you to control it with software at the level you need. \$\endgroup\$
    – tangrs
    Commented Oct 7, 2020 at 12:22
  • \$\begingroup\$ Oh okay, I just came to understand that there are no such things as "HDMI to USB adapters". Those are juste miscalled capture cards... So yeah, it makes sense I have to get through a capture card to retrieve the output. Sorry if my question seemed silly and thanks for your help :) \$\endgroup\$
    – SinLey
    Commented Oct 7, 2020 at 12:54

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Yep, I learnt about it but I wonder why you have to get through such cards instead of just having an appropriate software on your computer...

USB is a complex bidirectional protocol implementing a bursting bus with host arbitration. HDMI is a complex, multi-lane unidirectional protocol.

They electrically aren't related to each other.

You need special hardware, including buffers, and packetization, to make that work.

Software is exactly that: instructions you run on a processor of sorts one data that has already been acquired by specific hardware. USB is not specific hardware for HDMI signals. They simply aren't compatible in any way.

with potentially a HDMI to USB adapter.

so, yeah, you need one of these commercially available devices.

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  • \$\begingroup\$ This does answer my question. Thanks for your help :) \$\endgroup\$
    – SinLey
    Commented Oct 7, 2020 at 12:59
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There are few potential problems:

  1. High speed interface - HDMI is a very high speed interface, it requires advanced board design to implement properly.
  2. Encryption - Some HDMI content is protected by HDCP. Few years ago, it was broken, still it is an additional roadblock for your implementation.
  3. Data rate - HDMI 2.1 maximum data rate is 48Gbps, Good luck transferring this over USB without compression.
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