Cheap HDMI cables typically leak image information quite nicely. With the right equipment, it's really easy to get a decent resolution image of the radiation picked up from an HDMI cable even over tens of meters.
You can also search for "side-channel" and "TEMPEST".
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The signals analyzed below are LVDS and VGA, not exactly HDMI, but also an interesting read - because the LVDS signal is used to drive the LCD panel and it's also a major source of leakage.
* Exposing Computer Monitor Side-Channel Vulnerabilities With TempestSDR