Android CameraX运行时获取当前曝光、对焦、感光度等参数及自动控制锁定问题

Absolutely, you can pull real-time camera parameters like exposure time, ISO, focus state, and more using Android’s Camera2 API. Let’s walk through how to do this, plus a tailored solution for your cross-device compatibility issue.

Step 1: Grab CaptureResult from the Camera Callback #

To access live camera parameters, you need to listen for CaptureResult objects in your camera session’s capture callback. These results hold the current state of all active camera controls every time a frame is captured.

Here’s how to set this up:

// Configure your camera session's capture callback
CameraCaptureSession.CaptureCallback captureCallback = new CameraCaptureSession.CaptureCallback() {
    @Override
    public void onCaptureCompleted(@NonNull CameraCaptureSession session, @NonNull CaptureRequest request, @NonNull TotalCaptureResult result) {
        super.onCaptureCompleted(session, request, result);
        
        // Extract parameters from the latest result
        Long exposureTime = result.get(CaptureResult.SENSOR_EXPOSURE_TIME);
        Integer iso = result.get(CaptureResult.SENSOR_SENSITIVITY);
        Float focusDistance = result.get(CaptureResult.LENS_FOCUS_DISTANCE);
        
        // Store these values—you'll need them when switching to manual controls
    }
};

// Start repeating the preview request with the callback attached
session.setRepeatingRequest(previewRequest.build(), captureCallback, backgroundHandler);

Step 2: Capture Parameters Before Disabling Auto Controls #

Your goal is to avoid the "black screen" issue on Pixel 4a when using CONTROL_MODE_OFF_KEEP_STATE. The fix is to capture the auto-adjusted parameters first, then manually apply them after disabling auto controls:

1. Let the camera’s auto controls stabilize (e.g., wait 5 seconds as you originally did)
2. Save the latest CaptureResult values for exposure time, ISO, etc.
3. Disable auto controls, then manually set these saved values in your CaptureRequest

Here’s a code snippet for this flow:

// After auto controls stabilize, fetch the latest parameters
Long currentExposure = latestCaptureResult.get(CaptureResult.SENSOR_EXPOSURE_TIME);
Integer currentIso = latestCaptureResult.get(CaptureResult.SENSOR_SENSITIVITY);

// Build a new manual capture request
CaptureRequest.Builder manualRequestBuilder = cameraDevice.createCaptureRequest(CameraDevice.TEMPLATE_PREVIEW);
manualRequestBuilder.set(CaptureRequest.CONTROL_MODE, CaptureRequest.CONTROL_MODE_OFF);
// Apply the captured auto-adjusted values
manualRequestBuilder.set(CaptureRequest.SENSOR_EXPOSURE_TIME, currentExposure);
manualRequestBuilder.set(CaptureRequest.SENSOR_SENSITIVITY, currentIso);
// Lock other required parameters
manualRequestBuilder.set(CaptureRequest.CONTROL_AWB_LOCK, true);
manualRequestBuilder.set(CaptureRequest.BLACK_LEVEL_LOCK, true);

// Update the session with this manual request
session.setRepeatingRequest(manualRequestBuilder.build(), captureCallback, backgroundHandler);

Step 3: Fix Cross-Device Compatibility #

Your Pixel 4a vs Galaxy S7 issue stems from how different OEMs implement the Camera2 API. Here’s a targeted approach:

• Galaxy S7 (with flash enabled): CONTROL_AE_MODE_OFF works, but you must capture the auto-adjusted exposure/ISO first before disabling AE mode.
• Pixel 4a: CONTROL_AE_LOCK is reliable, but if you need to use CONTROL_MODE_OFF, manually apply the captured parameters to avoid black screens.

Add device-specific checks to apply the right strategy:

String manufacturer = Build.MANUFACTURER.toLowerCase();
String model = Build.MODEL.toLowerCase();

if (manufacturer.contains("samsung") && model.contains("s7")) {
    // Galaxy S7 logic: capture params first, then disable AE mode
    manualRequestBuilder.set(CaptureRequest.CONTROL_AE_MODE, CaptureRequest.CONTROL_AE_MODE_OFF);
} else {
    // Pixel 4a and other devices: use AE lock or manual params with CONTROL_MODE_OFF
    manualRequestBuilder.set(CaptureRequest.CONTROL_AE_LOCK, true);
}

Key Parameters You Can Retrieve #

Here are the most useful parameters for your use case:

• Exposure time: CaptureResult.SENSOR_EXPOSURE_TIME (in nanoseconds)
• ISO sensitivity: CaptureResult.SENSOR_SENSITIVITY
• Focus distance: CaptureResult.LENS_FOCUS_DISTANCE (0 = infinity, 1 = closest focus)
• AWB gains: CaptureResult.CONTROL_AWB_GAINS (for manual white balance tuning)
• AE stabilization state: CaptureResult.CONTROL_AE_STATE (check if auto exposure has settled)

Important Notes #

• Always retrieve parameters after auto controls stabilize (e.g., wait 5 seconds) to get valid, usable values.
• Some parameters may return null on older or less capable devices—always add null checks before using them.
• When setting manual parameters, ensure they fall within the device’s supported ranges (you can get these from CameraCharacteristics, like SENSOR_INFO_EXPOSURE_TIME_RANGE).

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