Accessing iOS 6 new APIs for camera exposure and shutter speed

问题

On Apple's iOS 6.0 feature page, it used to say

Take advantage of the built-in camera’s advanced features. New APIs let you control focus, exposure, and region of interest. You can also access and display faces with face detection APIs, and leverage hardware-enabled video stabilization.

This text has since been removed, and I can't find new methods in the API for controlling exposure. In class AVCaptureDevice under "Exposure Settings" there is no new property/method for iOS 6.0. Do you know where i can find new features for exposure in API?

回答1:

It's true that there is an -exposureMode property on AVCaptureDevice, but that's only for setting the mode (off/auto/continuous) and not the actual f-stop, SS, or ISO. Camera apps that provide "exposure" control all seem to do it through post-processing.

However, it seems there are undocumented APIs in the framework to do this. Check out the full headers for AVCaptureDevice.h (via a class-dump) and note the following methods:

- (void)setManualExposureSupportEnabled:(BOOL)arg1;
- (BOOL)isManualExposureSupportEnabled;

- (void)setExposureGain:(float)arg1;
- (float)exposureGain;

- (void)setExposureDuration:(struct { long long x1; int x2; unsigned int x3; long long x4; })arg1;
- (struct { long long x1; int x2; unsigned int x3; long long x4; })exposureDuration;

- (void)setExposureMode:(int)arg1;
- (int)exposureMode;

- (BOOL)isExposureModeSupported:(int)arg1;

My guess is gain is equivalent f-stop (fixed aperture), and duration is shutter speed. I wonder if these are used for the iPhone 5's low-light boost mode.

You can also use otool to poke around and try to piece together the symbols. There's likely a new constant in exposureMode for enabling manual control, and exposureDuration seems like it has flags too. When calling these, make sure to use the new -isExposureModeSupported: and also call -respondsToSelector: to check compatibility.

As always, using private APIs is frowned upon by Apple and is cause for rejection from the App Store. There might be ways around this, such as hiding the calls using -performSelector: or obc_msgsend with rot13 strings or something, as I'm pretty sure they only do static analysis on the app binary.

回答2:

I've managed to 'trick' the camera into running a shorter exposure time, but I suspect it will only be of use to those doing similar (macro) image acquires. I first set up AVCaptureDevice to use AVCaptureExposureModeContinuousAutoExposure and set the flash to TorchMode. I then UnlockForConfiguration and set up a key-value observer to watch for adjustingExposure to finish. I then re-lock the device, flip to AVCaptureExposureModeLocked, and turn off the Torch. This has the effect of brute-force setting a shorter shutter speed than what the camera would select on the un-illuminated scene. By playing with the Torch level I can set any relative shutter speed value I want (it would be best of course to leave the torch on, but in my application it produces glare on the subject). Again this only really works when your object distance is very close (less than say 6 inches), but it's allowed me to eliminate hand shake blurring in my close-up images. The down side is that the images are darker since I don't have a way of spoofing the camera gain, but not a problem in my particular application.

回答3:

It looks like they've updated that linked text—there's no mention of new APIs for exposure:

Use powerful new features of the built-in camera. New APIs support real-time video stabilization, an improved LED flash, and face detection and display. You can get reports of dropped frames during capture and leverage new utilities to map UI touches to focus and exposure commands. And apps that support iPhone 5 can take advantage of low light boost mode.

There is an opt-in low-light boost mode for iPhone 5, detailed here by Jim Rhoades (and in this developer forum post, log-in required).

回答4:

As a follow-up to Michael Grinich's excellent information, I found that there is an order dependency on some of the calls in the private API. To use "manual" exposure controls, you have to enable them before you set the mode, like so:

#define AVCaptureExposureModeManual     3
NSError*    error = nil;
if ([captureDevice lockForConfiguration:&error]) {
    captureDevice.manualExposureSupportEnabled = YES;
    if ([captureDevice isExposureModeSupported:AVCaptureExposureModeManual]) {
        captureDevice.exposureMode = AVCaptureExposureModeManual;
        captureDevice.exposureGain = ...;
        captureDevice.exposureDuration = {...};
    }
    [captureDevice unlockForConfiguration];
}

All of this is demonstrated in iOS-ManualCamera.