Windows 10下程序化控制摄像头设置：占用时能否调焦？

Great question! The short answer is yes, it's possible—you just need to work around the exclusive access limitations of UWP's camera API, which is why your initial UWP approach failed when OBS was using the camera. Let's break down how to do this, especially for your Logitech C920 with Windows' generic driver:

Why Your UWP Approach Failed #

UWP's MediaCapture API defaults to exclusive access mode for cameras. Once an app like OBS grabs the camera, no other UWP app can request full access to modify settings. This is a design choice for UWP, but it's not the only way to interact with camera hardware on Windows.

Workable Solutions #

Since you've seen Webcams Pro modify settings while the camera is in use, it's leveraging lower-level Windows APIs that don't require exclusive access. Here are the most reliable approaches:

1. Use DirectShow's IAMCameraControl Interface #

DirectShow (a legacy but still fully supported Windows media framework) allows shared access to camera devices, and includes an IAMCameraControl interface specifically for adjusting settings like focus, exposure, and zoom—even when the camera is active in another app.

You can implement this in C++ directly, or use a .NET wrapper like DirectShow.NET for easier development. Here's a simplified C# example:

// Requires referencing DirectShow.NET
using DirectShowLib;

public void SetCameraFocus(string cameraName, int focusValue)
{
    // Locate the camera device by name
    var devices = DsDevice.GetDevicesOfCat(FilterCategory.VideoInputDevice);
    var targetDevice = devices.FirstOrDefault(d => d.Name.Contains(cameraName));
    
    if (targetDevice == null) return;

    // Create the filter and query for IAMCameraControl
    var filter = (IBaseFilter)new VideoInputFilter();
    var moniker = targetDevice.Moniker;
    moniker.BindToObject(null, null, typeof(IBaseFilter).GUID, out var obj);
    filter = (IBaseFilter)obj;

    var cameraControl = filter as IAMCameraControl;
    if (cameraControl != null)
    {
        // Set manual focus to the specified value (adjust range based on your device)
        cameraControl.Set(CameraControlProperty.Focus, focusValue, CameraControlFlags.Manual);
    }
}

Before setting values, use cameraControl.GetRange(CameraControlProperty.Focus, out min, out max, out step, out default, out flags) to get your C920's supported focus range.

2. Direct WDM Device IO Control #

Windows' generic camera driver follows the Windows Driver Model (WDM), which supports direct communication via IO control codes. You can bypass higher-level media frameworks entirely by opening the camera device with shared read/write permissions and sending IOCTL_VIDEO_SET_CONTROL commands to modify focus.

This is more low-level, but gives you full control. Here's a rough outline of the steps in C++:

// Open the camera device (replace with your device's path)
HANDLE hDevice = CreateFile(
    L"\\\\.\\USB#VID_046D&PID_082D...", // Your C920's device path
    GENERIC_READ | GENERIC_WRITE,
    FILE_SHARE_READ | FILE_SHARE_WRITE,
    NULL,
    OPEN_EXISTING,
    0,
    NULL
);

if (hDevice != INVALID_HANDLE_VALUE)
{
    VIDEO_CONTROL_LOCKED_CONTROL control = {};
    control.ControlId = VideoControlFocus;
    control.Value = 50; // Your desired focus value
    control.Flags = VIDEO_CONTROL_FLAG_MANUAL;

    // Send the IO control to set focus
    DeviceIoControl(
        hDevice,
        IOCTL_VIDEO_SET_CONTROL,
        &control,
        sizeof(control),
        NULL,
        0,
        NULL,
        NULL
    );

    CloseHandle(hDevice);
}

You can find your camera's device path using the Device Manager (look under "Imaging devices" → C920 → Properties → Details → "Device instance path").

3. Leverage Open-Source Libraries #

Some open-source media libraries support shared camera access and setting adjustments:

• OpenCV: The cv::VideoCapture class can set focus via CAP_PROP_FOCUS on Windows, provided you use the correct backend (DirectShow is usually the default).
• libv4l2 (Windows port): While originally for Linux, the Windows port supports similar device control commands for USB cameras.

Key Notes #

• Driver Compatibility: Your Logitech C920 with Windows' generic driver is fully compatible with these methods—since Webcams Pro works, the driver exposes the necessary control interfaces.
• Permissions: Some low-level device access may require running your tool as an administrator. If you get access denied errors, try launching with elevated privileges.
• Avoid UWP: Stick to Win32/.NET Framework (not .NET Core/UWP) for these approaches, as UWP's sandboxing prevents direct device access.

内容的提问来源于stack exchange，提问作者witnessmenow