iOS相机：手动曝光时间但自动ISO？

Question

我正在使用相机视频源进行一些图像处理，并希望优化以获得最快的快门速度。我知道你可以手动设置曝光时间和 ISO 使用

setExposureModeCustomWithDuration:ISO:completionHandler:

但这需要手动设置这两个值。有没有一种方法或巧妙的技巧可以让您手动设置曝光持续时间但让 ISO 句柄本身尝试正确曝光图像？

Answer 1

我不确定这个解决方案是否是最好的，因为我和你一样在这个问题上苦苦挣扎。我所做的是聆听曝光偏移的变化，并从中调整 ISO，直到达到可接受的曝光水平。此代码大部分取自 Apple 示例代码

所以，首先，您要监听 ExposureTargetOffset 的变化。添加你的类声明：

static void *ExposureTargetOffsetContext = &ExposureTargetOffsetContext;

然后，一旦您正确完成了设备设置：

[self addObserver:self forKeyPath:@"captureDevice.exposureTargetOffset" options:NSKeyValueObservingOptionNew context:ExposureTargetOffsetContext];

（而不是 captureDevice，使用您的设备属性） 然后在你的类中实现 KVO 的回调：

- (void)observeValueForKeyPath:(NSString *)keyPath ofObject:(id)object change:(NSDictionary *)change context:(void *)context{

if (context == ExposureTargetOffsetContext){
        float newExposureTargetOffset = [change[NSKeyValueChangeNewKey] floatValue];
        NSLog(@"Offset is : %f",newExposureTargetOffset);

        if(!self.device) return;

        CGFloat currentISO = self.device.ISO;
        CGFloat biasISO = 0;

        //Assume 0,3 as our limit to correct the ISO
        if(newExposureTargetOffset > 0.3f) //decrease ISO
            biasISO = -50;
        else if(newExposureTargetOffset < -0.3f) //increase ISO
            biasISO = 50;

        if(biasISO){
            //Normalize ISO level for the current device
            CGFloat newISO = currentISO+biasISO;
            newISO = newISO > self.device.activeFormat.maxISO? self.device.activeFormat.maxISO : newISO;
            newISO = newISO < self.device.activeFormat.minISO? self.device.activeFormat.minISO : newISO;

            NSError *error = nil;
            if ([self.device lockForConfiguration:&error]) {
                [self.device setExposureModeCustomWithDuration:AVCaptureExposureDurationCurrent ISO:newISO completionHandler:^(CMTime syncTime) {}];
                [self.device unlockForConfiguration];
            }
        }
    }
}

使用此代码，快门速度将保持不变，并且将调整 ISO 以使图像不会过曝或曝光过度。

不要忘记在需要时移除观察者。希望这适合你。

干杯！

Answer 2

@khose 在 swift 上提供的代码示例：

private var device: AVCaptureDevice?
private var exposureTargetOffsetContext = 0

override func observeValue(forKeyPath keyPath: String?, of object: Any?, change: [NSKeyValueChangeKey : Any]?, context: UnsafeMutableRawPointer?) {
    if device == nil {
        return
    }
    if keyPath == "exposureTargetOffset" {
        let newExposureTargetOffset = change?[NSKeyValueChangeKey.newKey] as! Float
        print("Offset is : \(newExposureTargetOffset)")

        let currentISO = device?.iso
        var biasISO = 0

        //Assume 0,01 as our limit to correct the ISO
        if newExposureTargetOffset > 0.01 { //decrease ISO
            biasISO = -50
        } else if newExposureTargetOffset < -0.01 { //increase ISO
            biasISO = 50
        }

        if biasISO != Int(0) {
            //Normalize ISO level for the current device
            var newISO = currentISO! + Float(biasISO)
            newISO = newISO > (device?.activeFormat.maxISO)! ? (device?.activeFormat.maxISO)! : newISO
            newISO = newISO < (device?.activeFormat.minISO)! ? (device?.activeFormat.minISO)! : newISO

            try? device?.lockForConfiguration()
            device?.setExposureModeCustom(duration: AVCaptureDevice.currentExposureDuration, iso: newISO, completionHandler: nil)
            device?.unlockForConfiguration()
        }
    }
}

用法：
device?.addObserver(self, forKeyPath: "exposureTargetOffset", options: NSKeyValueObservingOptions.new, context: &exposureTargetOffsetContext)

Answer 3

如果照明条件发生较大变化，则接受的答案将需要很长时间才能提高/降低 ISO。这是一个示例（Swift 4），它根据曝光偏移量的比例改变 ISO 的值​​。

fileprivate var settingISO = false
@objc func exposureTargetOffsetChanged(notification: Notification) {
    guard !settingISO, self.device.exposureMode == .custom, let exposureTargetOffset = notification.userInfo?["newValue"] as? Float else {
        return
    }
    var isoChange = Float(0.0)
    let limit = Float(0.05)
    let isoChangeStep: Float
    if abs(exposureTargetOffset) > 1 {
        isoChangeStep = 500
    } else if abs(exposureTargetOffset) > 0.5 {
        isoChangeStep = 200
    } else if abs(exposureTargetOffset) > 0.2 {
        isoChangeStep = 50
    } else if abs(exposureTargetOffset) > 0.1 {
        isoChangeStep = 20
    } else {
        isoChangeStep = 5
    }

    if exposureTargetOffset > limit {
        isoChange -= isoChangeStep
    } else if exposureTargetOffset < -limit {
        isoChange += isoChangeStep
    } else {
        return
    }
    var newiso = self.device.iso + isoChange
    newiso = max(self.device.activeFormat.minISO, newiso)
    newiso = min(self.device.activeFormat.maxISO, newiso)

    guard newiso != self.device.iso, (try? self.device.lockForConfiguration()) != nil else { return }
    self.settingISO = true
    Camera.log("exposureTargetOffset=\(exposureTargetOffset), isoChange=\(isoChange), newiso=\(newiso)")

    self.device.setExposureModeCustom(duration: self.customDuration ?? AVCaptureDevice.currentExposureDuration, iso: newiso) { (_) in
        self.settingISO = false
    }
    self.device.unlockForConfiguration()
}