android - 如何检测相机何时完成曝光初始化？

Question

背景

当我开始相机预览时，尤其是在室内时，预览帧和使用 takePicture 拍摄的照片最初是暗的。下一秒，相机的曝光会自动调整，直到曝光最佳。当我在手机上打开相机应用程序时，也会出现此行为。

问题

一旦曝光合适，我会尽快拍照。最好我可以注册一个回调，以便在曝光良好时调用。我该怎么做？

我尝试过的

• 将相机的曝光补偿配置为最大值。 (没有帮助；预览开始后相机仍需要大约一秒钟才能正确曝光。)
• 自动对焦 API，但由于相机不支持自动对焦，它们无法正常工作。
• 硬编码延迟一秒以等待曝光。这是次优的，因为在室外日光下，不需要这么多时间。

Answer 1

旧相机 API

不幸的是，带有传统相机驱动程序的设备似乎不支持这一点。因此，最好的选择似乎是开始预览，然后等待大约一秒钟。

camera2 API

使用 camera2 API，您可以通过检查 CaptureResult.CONTROL_AE_STATE 的值来检查自动曝光状态。

示例代码

首先开始捕捉:

// Auto-exposure only seems to start after the first picture has been taken
// And on the Pixel 3 XL, it finishes fastest when you make a lot of captures
captureSession.setRepeatingRequest(request, captureCallback, null);

并使用此样板代码来监控自动曝光:

private CaptureResult firstResult;

private final CameraCaptureSession.CaptureCallback captureCallback = new CameraCaptureSession.CaptureCallback() {

    @Override
    public void onCaptureCompleted(@NonNull CameraCaptureSession session, @NonNull CaptureRequest request, @NonNull TotalCaptureResult result) {
        super.onCaptureCompleted(session, request, result);

        if (firstResult == null)
            firstResult = result;

        boolean aeAcquired;
        Integer aeState = lastResult.get(CaptureResult.CONTROL_AE_STATE);
        if (aeState == null) {
            // This camera doesn't support monitoring of auto-exposure, so we'll just have to wait a bit and then assume it's adjusted.
            long nanosSinceFirstResult = result.get(CaptureResult.SENSOR_TIMESTAMP) - firstResult.get(CaptureResult.SENSOR_TIMESTAMP);
            long millisSinceFirstResult = nanosSinceFirstResult / 1000 / 1000;
            aeAcquired = millisSinceFirstResult >= 750;
        } else {
            aeAcquired = aeState == CameraMetadata.CONTROL_AE_STATE_CONVERGED || aeState == CameraMetadata.CONTROL_AE_STATE_FLASH_REQUIRED;
        }

        if (aeAcquired) {
            // Auto-exposure has finished
        }   
    }

};