ios - 如何使用 captureStillImageAsynchronouslyFromConnection 实现多重拍摄(iOS AVFoundation)

Question

我正在尝试在循环中使用 captureStillImageAsynchronouslyFromConnection 捕获连续(多镜头)高分辨率图像，但它偶尔会暂停以重新聚焦。我锁定了对焦模式(如其他 stackoverflow 帖子中所述)，但这并没有阻止相机偶尔重新对焦。我的代码片段是:

// [self.session beginConfiguration];
if ([device lockForConfiguration:nil] == YES) {
    if ([device isFocusModeSupported:AVCaptureFocusModeLocked]) {
        [device setFocusMode:AVCaptureFocusModeLocked];
        NSLog(@"focus locked");
    }
    if ([device isExposureModeSupported:AVCaptureExposureModeLocked]) {
        [device setExposureMode:AVCaptureExposureModeLocked];
        NSLog(@"exposure locked");
    }
    if ([device isWhiteBalanceModeSupported:AVCaptureWhiteBalanceModeLocked]) {
        [device setWhiteBalanceMode:AVCaptureWhiteBalanceModeLocked];
        NSLog(@"white balance locked");
    }
}
// [self.session commitConfiguration];

for (int n = 0; n < 5; n++) {
    [self.stillImageOutput captureStillImageAsynchronouslyFromConnection:[self.stillImageOutput connectionWithMediaType:AVMediaTypeVideo] completionHandler:^(CMSampleBufferRef imageDataSampleBuffer, NSError *error) {

        if (imageDataSampleBuffer) {
            NSData *imageData = [AVCaptureStillImageOutput jpegStillImageNSDataRepresentation:imageDataSampleBuffer];
            UIImage *image = [[UIImage alloc] initWithData:imageData];
            [[[ALAssetsLibrary alloc] init] writeImageToSavedPhotosAlbum:[image CGImage] orientation:(ALAssetOrientation)image.imageOrientation completionBlock:nil];
        }
    }];
}

[device unlockForConfiguration]

输出日志报告:

focus locked
exposure locked
white balance locked

表示focus等应该已经成功锁定

我尝试用 [device unlockForConfiguration] 和 [device unlockForConfiguration] 包装锁码，但这并没有解决问题。

有人可以识别我的代码中的错误或我遗漏的步骤吗？ (我意识到我可以选择使用视频捕获而不是静态捕获来实现这一点，但我需要 AVCaptureSessionPresetPhoto 分辨率图像。)任何帮助将不胜感激。谢谢。

Answer 1

好的，我想出了问题。由于线程和 GCD 任务的计时，[device unlockForConfiguration] 在所有 captureStillImageAsynchronouslyFromConnection 调用完成之前执行。一个快速的解决方案是添加自旋锁，例如:

if ([device lockForConfiguration:nil]) {
    if ([device isFocusModeSupported:AVCaptureFocusModeLocked])
        [device setFocusMode:AVCaptureFocusModeLocked];
    if ([device isExposureModeSupported:AVCaptureExposureModeLocked])
        [device setExposureMode:AVCaptureExposureModeLocked];
    if ([device isWhiteBalanceModeSupported:AVCaptureWhiteBalanceModeLocked])
        [device setWhiteBalanceMode:AVCaptureWhiteBalanceModeLocked];
}

__block int photoCount = 5; 
for (int n = photoCount; n > 0; n--) {
    [self.stillImageOutput captureStillImageAsynchronouslyFromConnection:[self.stillImageOutput connectionWithMediaType:AVMediaTypeVideo] completionHandler:^(CMSampleBufferRef imageDataSampleBuffer, NSError *error) {
        @synchronize(self) {
            photoCount--;
        }

        if (imageDataSampleBuffer) {
            NSData *imageData = [AVCaptureStillImageOutput jpegStillImageNSDataRepresentation:imageDataSampleBuffer];
            UIImage *image = [[UIImage alloc] initWithData:imageData];
            [[[ALAssetsLibrary alloc] init] writeImageToSavedPhotosAlbum:[image CGImage] orientation:(ALAssetOrientation)image.imageOrientation completionBlock:nil];
        }
    }];
}

while (photoCount > 0); // Spinlock until captureStillImageAsynchronouslyFromConnection captured all photos into memory
[device unlockForConfiguration]

可能有更优雅的解决方案(我很想听听)，但一个简单的自旋锁就可以解决问题。 (另外，由于我的代码在 dispatch_async block 中运行，因此它不会导致 UI 或应用程序响应出现任何问题。)