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InUVC (USB Video Class),Input Terminal,Output Terminal, andCamera Terminalare entities in theVideoControl (VC) Interface. They describe the logical video data path inside a camera device.

Think of the UVC topology as a graph:

Camera Sensor │ ▼ Camera Terminal (Input Terminal) │ ▼ Processing Unit │ ▼ Output Terminal │ ▼ USB Host

1. Input Terminal (IT)

AnInput Terminalrepresents a source of video data entering the UVC function.

Examples:

• Camera sensor

• TV tuner

• HDMI capture input

• Composite video input

Descriptor type:

VC_INPUT_TERMINAL

Key fields:

bTerminalID wTerminalType

Common terminal types:

Example:

Input Terminal ID = 1 Type = Camera

means:

Video originates from a camera sensor.

2. Camera Terminal (CT)

ACamera Terminalis actually aspecialized Input Terminal.

In the descriptor:

wTerminalType = TT_CAMERA (0x0201)

When the terminal type is0x0201, the Input Terminal becomes aCamera Terminal.

Camera-specific controls

Camera Terminal provides camera-related controls:

• Exposure Time

• Focus

• Zoom

• Iris

• Pan

• Tilt

• Roll

Example UVC controls:

CT_EXPOSURE_TIME_ABSOLUTE_CONTROL CT_FOCUS_ABSOLUTE_CONTROL CT_ZOOM_ABSOLUTE_CONTROL

Typical UVC webcam topology:

Camera Terminal │ ▼ Processing Unit │ ▼ Output Terminal

Descriptor example:

Camera Terminal ID = 1 Terminal Type = 0x0201

The host recognizes:

This device contains a controllable camera.

3. Output Terminal (OT)

AnOutput Terminalrepresents where the video stream leaves the internal UVC topology.

Descriptor type:

VC_OUTPUT_TERMINAL

For a USB webcam:

Output Terminal ↓ USB Streaming Interface ↓ Host PC

Typical type:

TT_STREAMING = 0x0101

Meaning:

Video is sent to a USB streaming endpoint.

Example:

Output Terminal ID = 3 Source ID = 2 Type = Streaming

where:

Camera Terminal(ID=1) │ ▼ Processing Unit(ID=2) │ ▼ Output Terminal(ID=3)

Typical Webcam Example

Most UVC cameras use:

Input Terminal (Camera) │ ▼ Processing Unit │ ▼ Output Terminal (USB Streaming)

Descriptor topology

Camera Terminal ID = 1 Processing Unit ID = 2 Source = 1 Output Terminal ID = 3 Source = 2

Graphically:

[Camera Terminal] │ ▼ [Processing Unit] │ ▼ [Output Terminal] │ ▼ USB Video Stream

UVC Capture Dongle Example

For an HDMI capture card:

HDMI Input │ ▼ Input Terminal │ ▼ Processing Unit │ ▼ Output Terminal │ ▼ USB Host

Here the Input Terminal isnota Camera Terminal.

Input Terminal Type != 0x0201

because the source is HDMI, not an image sensor.

How the Host Uses Them

When a PC enumerates a UVC device, it reads the VideoControl descriptors and builds a topology:

Entity 1 : Camera Terminal Entity 2 : Processing Unit Entity 3 : Output Terminal

The host then knows:

1. Where video originates.

2. What controls are available (exposure, focus, zoom, etc.).

3. Where the video stream exits toward the streaming interface.

For example, on Linux:

v4l2-ctl --all

shows controls that come from:

• Camera Terminal (focus, exposure, zoom)

• Processing Unit (brightness, contrast, saturation)

Summary

The most common webcam topology is:

Camera Terminal ↓ Processing Unit ↓ Output Terminal ↓ USB Streaming Interface ↓ Host

where theCamera Terminaldescribes the sensor and camera controls, and theOutput Terminalconnects the processed video stream to the USB host.