Here's one of the easiest capture solutions for getting HDMI data into your computer or Raspberry Pi, really any system with a USB port..
HDMI to USB 2.0 Capture Card Full HD 1080p for Live Video Streaming Recording
1080P 4K HDMI Compatble To USB 2.0 Video Capture Card Board For Game Record Live Streaming Broadcast TV Local Loop Teaching, Video Conference From 6,6 €
Audio Video Capture Card, 4K 1080P 60FPS HDMI USB3.0 Capture Adapter Video Capture Device Portable Video Converter for Video Recording Gaming Streaming Live Broadcast ,Support PS4/PC/OBS/DSLR/Camera
Connectronics HDMI to USB 2.0 Video Capture with Audio Insertion & Extraction
HDMI Video Capture Adapter
HDMI Input to USB 2.0 Video Capture Adapter : ID 4669 : $19.95 : Adafruit Industries, Unique & fun DIY electronics and kits
TechFlo 4K HDMI Video Capture Card HDMI to USB 2.0 Video Capture with – TechFlo Australia
Generic HDMI Video Capture Card Recorder USB 2.0 1080P Capture HDTV Streaming à prix pas cher
Cheap USB 2.0 Video Capture Card Easy Cap Video Audio Converter TV DVD VHS Audio Capture Adapter Card TV Video DVR