iOS Question [SOLVED] A solution for getting ambient light brightness

JackKirk · Feb 3, 2016

I have driven Erel to distraction looking for a means to automatically turn the torch on when scanning barcodes in poor light conditions, see:

https://www.b4x.com/android/forum/threads/equivalent-of-b4a-phone-phonesensors-type_light.62181/

One of my many shortcomings is an abject lack of any Objective C expertise (one of the reasons I'm using B4A/B4I) - however that does not mean I'm not up for a challenge in it.

I have managed to dig up some Objective C code that looks like it might solve the problem:

https://gist.github.com/JoppeSchwartz/72f3efd4ac0b5456dd8a#file-measurebrightness-m-L1

I have spent the last 2 days toying with this and seem to have managed to get it working inside a minimal B4I life support system:

B4X:

#Region  Project Attributes
    #ApplicationLabel: Brightness
    #Version: 1.0.0
    'Orientation possible values: Portrait, LandscapeLeft, LandscapeRight and PortraitUpsideDown
    #iPhoneOrientations: Portrait, LandscapeLeft, LandscapeRight
    #iPadOrientations: Portrait, LandscapeLeft, LandscapeRight, PortraitUpsideDown
#End Region

Sub Process_Globals
    Public App As Application
    Public NavControl As NavigationController
    Private Page1 As Page
    Private Label1 As Label
End Sub

Private Sub Application_Start (Nav As NavigationController)
    NavControl = Nav
    Page1.Initialize("Page1")
    NavControl.ShowPage(Page1)
End Sub

Private Sub Page1_Resize(Width As Int, Height As Int)
    Page1.Title="Brightness"
    Page1.RootPanel.RemoveAllViews
    Page1.RootPanel.Color=Colors.White
    Label1.Initialize("")
    Page1.RootPanel.AddView(Label1,Width/2,Height/2,10,10)
    Label1.Text="Click screen to get brightness"
    Label1.SizeToFit
    Label1.Left=Page1.RootPanel.Width/2-Label1.Width/2
End Sub

Private Sub Page1_Click
    Dim no As NativeObject = Me
    Dim bright As Object = no.RunMethod("sampleBrightness", Array ())
    Label1.Text=bright
    Label1.SizeToFit
    Label1.Left=Page1.RootPanel.Width/2-Label1.Width/2
End Sub

#If OBJC

//Following code is based on:
//https://gist.github.com/JoppeSchwartz/72f3efd4ac0b5456dd8a#file-measurebrightness-m-L1

#import <AVFoundation/AVFoundation.h>
#import <ImageIO/ImageIO.h>

-(NSString *)sampleBrightness
{

//Initialize variables
    AVCaptureSession *_videoSession = nil;
    AVCaptureDevice *_cameraDevice = nil;
    AVCaptureStillImageOutput *_imageOutput = nil;
    AVCaptureConnection *_avConnection = nil;
    NSError *err = nil;
   
//Set up video capture session
    _videoSession = [[AVCaptureSession alloc] init];
    if ([_videoSession canSetSessionPreset:AVCaptureSessionPresetLow]) {
        _videoSession.sessionPreset = AVCaptureSessionPresetLow;
    } else {
        return [NSString stringWithFormat:@"Error - could not set AVCaptureSession preset."];
    }

//Find front camera device
    NSArray *devices = [AVCaptureDevice devicesWithMediaType:AVMediaTypeVideo];
    for (AVCaptureDevice *device in devices) {
        if ([device position] == AVCaptureDevicePositionFront) {
            _cameraDevice = device;
        }
    }

//Add front camera device to session
    AVCaptureDeviceInput *input = [AVCaptureDeviceInput deviceInputWithDevice:_cameraDevice error:&err];
    if (err) {
        return [NSString stringWithFormat:@"Error instantiating capture input device: %@", err.description];
    }
    if ([_videoSession canAddInput:input]) {
        [_videoSession addInput:input];
    }

//Create and add still image output device to the session
    _imageOutput = [[AVCaptureStillImageOutput alloc] init];
    if ([_videoSession canAddOutput:_imageOutput]){
        [_videoSession addOutput:_imageOutput];
    }

//Find AV connection
    _avConnection = nil;
    for (AVCaptureConnection *connection in _imageOutput.connections) {
        for (AVCaptureInputPort *port in [connection inputPorts]) {
            if ([[port mediaType] isEqual:AVMediaTypeVideo]) {
                _avConnection = connection;
                break;
            }
            if (_avConnection) break;
        }
    }
    if (!_avConnection) {
        return [NSString stringWithFormat:@"Error - could not find AVCaptureConnection for capture session."];
    }
    [_videoSession startRunning];

//Extract a brightness reading
    if (_imageOutput) {
        __block NSString *myResult = nil;
        [_imageOutput captureStillImageAsynchronouslyFromConnection:_avConnection completionHandler:^(CMSampleBufferRef imageSampleBuffer, NSError *error) {
            if (error) {
                myResult = [NSString stringWithFormat:@"Error taking picture: %@", error.description];
                return;
            }
            CFDictionaryRef exifAttachments = CMGetAttachment(imageSampleBuffer, kCGImagePropertyExifDictionary, NULL);
            if (exifAttachments) {
                // Do something with the attachments.
                bool containsBrightnessKey = CFDictionaryContainsKey(exifAttachments, kCGImagePropertyExifBrightnessValue);
                if (!containsBrightnessKey) {
                    myResult = [NSString stringWithFormat:@"Error - EXIF dictionary doesn't contain brightness key"];
                    return;
                }
                CFStringRef brightness = CFDictionaryGetValue(exifAttachments, kCGImagePropertyExifBrightnessValue);
                myResult = (__bridge NSString*)brightness;
                return;
            } else {
                myResult = [NSString stringWithFormat:@"Error - no exifAttachments."];
                return;
            }
        }];
        [_videoSession stopRunning];
        return myResult;
    }
    [_videoSession stopRunning];
    return [NSString stringWithFormat:@"Error - no image output."];
}
#End If

When I say "seem to have managed to get it working" what I really mean is that it doesn't fall over, makes an appropriate camera shutter sound when you use it and gives some sort of reading.

What I am not so confident about is the actual readings - they seem to generally reflect the light conditions but can produce identical readings in slightly different light conditions and in some cases can be negative.

I would really appreciate anyone with a good Objective C background to have a look at this - if they also understand AVFoundation and AVCapture... stuff that would probably be useful.

I have appended the code as a zip below.

Thanks...

JackKirk · Feb 4, 2016

Oh this is embarrassing...

Experimenting with incorporating the above into an embryonic barcode reader app I realised that the code was using the front camera (the one on the same side as the screen) not the back camera (the one on the same side as the torch).

So the inconsistent/weird/negative brightness readings were because I was taking brightness readings of my head - quite ironic actually - this is what you get for blindly following someone else's code.

The corrected code is below and it now seems to work beautifully - I'd still like an Objective C / AV... expert to look it over though...

B4X:

#Region  Project Attributes
    #ApplicationLabel: Brightness
    #Version: 1.0.0
    'Orientation possible values: Portrait, LandscapeLeft, LandscapeRight and PortraitUpsideDown
    #iPhoneOrientations: Portrait, LandscapeLeft, LandscapeRight
    #iPadOrientations: Portrait, LandscapeLeft, LandscapeRight, PortraitUpsideDown
#End Region

Sub Process_Globals
    Public App As Application
    Public NavControl As NavigationController
    Private Page1 As Page
    Private Label1 As Label
End Sub

Private Sub Application_Start (Nav As NavigationController)
    NavControl = Nav
    Page1.Initialize("Page1")
    NavControl.ShowPage(Page1)
End Sub

Private Sub Page1_Resize(Width As Int, Height As Int)
    Page1.Title="Brightness"
    Page1.RootPanel.RemoveAllViews
    Page1.RootPanel.Color=Colors.White
    Label1.Initialize("")
    Page1.RootPanel.AddView(Label1,Width/2,Height/2,10,10)
    Label1.Text="Click screen to get brightness"
    Label1.SizeToFit
    Label1.Left=Page1.RootPanel.Width/2-Label1.Width/2
End Sub

Private Sub Page1_Click
    Dim no As NativeObject = Me
    Dim bright As Object = no.RunMethod("sampleBrightness", Array ())
    Label1.Text=bright
    Label1.SizeToFit
    Label1.Left=Page1.RootPanel.Width/2-Label1.Width/2
End Sub

#If OBJC

//Following code is based on:
//https://gist.github.com/JoppeSchwartz/72f3efd4ac0b5456dd8a#file-measurebrightness-m-L1

#import <AVFoundation/AVFoundation.h>
#import <ImageIO/ImageIO.h>

-(NSString *)sampleBrightness
{

//Initialize variables
    AVCaptureSession *_videoSession = nil;
    AVCaptureDevice *_cameraDevice = nil;
    AVCaptureStillImageOutput *_imageOutput = nil;
    AVCaptureConnection *_avConnection = nil;
    NSError *err = nil;

//Set up video capture session
    _videoSession = [[AVCaptureSession alloc] init];
    if ([_videoSession canSetSessionPreset:AVCaptureSessionPresetLow]) {
        _videoSession.sessionPreset = AVCaptureSessionPresetLow;
    } else {
        return [NSString stringWithFormat:@"Error - could not set AVCaptureSession preset."];
    }

//Find back camera device (one on opposite side to screen and on same side as torch)
    NSArray *devices = [AVCaptureDevice devicesWithMediaType:AVMediaTypeVideo];
    for (AVCaptureDevice *device in devices) {
        if ([device position] == AVCaptureDevicePositionBack) {
            _cameraDevice = device;
        }
    }

//Add back camera device to session
    AVCaptureDeviceInput *input = [AVCaptureDeviceInput deviceInputWithDevice:_cameraDevice error:&err];
    if (err) {
        return [NSString stringWithFormat:@"Error instantiating capture input device: %@", err.description];
    }
    if ([_videoSession canAddInput:input]) {
        [_videoSession addInput:input];
    }

//Create and add still image output device to the session
    _imageOutput = [[AVCaptureStillImageOutput alloc] init];
    if ([_videoSession canAddOutput:_imageOutput]){
        [_videoSession addOutput:_imageOutput];
    }

//Find AV connection
    _avConnection = nil;
    for (AVCaptureConnection *connection in _imageOutput.connections) {
        for (AVCaptureInputPort *port in [connection inputPorts]) {
            if ([[port mediaType] isEqual:AVMediaTypeVideo]) {
                _avConnection = connection;
                break;
            }
            if (_avConnection) break;
        }
    }
    if (!_avConnection) {
        return [NSString stringWithFormat:@"Error - could not find AVCaptureConnection for capture session."];
    }
    [_videoSession startRunning];

//Extract a brightness reading
    if (_imageOutput) {
        __block NSString *myResult = nil;
        [_imageOutput captureStillImageAsynchronouslyFromConnection:_avConnection completionHandler:^(CMSampleBufferRef imageSampleBuffer, NSError *error) {
            if (error) {
                myResult = [NSString stringWithFormat:@"Error taking picture: %@", error.description];
                return;
            }
            CFDictionaryRef exifAttachments = CMGetAttachment(imageSampleBuffer, kCGImagePropertyExifDictionary, NULL);
            if (exifAttachments) {
                // Do something with the attachments.
                bool containsBrightnessKey = CFDictionaryContainsKey(exifAttachments, kCGImagePropertyExifBrightnessValue);
                if (!containsBrightnessKey) {
                    myResult = [NSString stringWithFormat:@"Error - EXIF dictionary doesn't contain brightness key"];
                    return;
                }
                CFStringRef brightness = CFDictionaryGetValue(exifAttachments, kCGImagePropertyExifBrightnessValue);
                myResult = (__bridge NSString*)brightness;
                return;
            } else {
                myResult = [NSString stringWithFormat:@"Error - no exifAttachments."];
                return;
            }
        }];
        [_videoSession stopRunning];
        return myResult;
    }
    [_videoSession stopRunning];
    return [NSString stringWithFormat:@"Error - no image output."];
}
#End If

The changes are in this bit:

B4X:

//Find back camera device (one on opposite side to screen and on same side as torch)
    NSArray *devices = [AVCaptureDevice devicesWithMediaType:AVMediaTypeVideo];
    for (AVCaptureDevice *device in devices) {
        if ([device position] == AVCaptureDevicePositionBack) {
            _cameraDevice = device;
        }
    }
//Add back camera device to session

principally AVCaptureDevicePositionFront is changed to AVCaptureDevicePositionBack

Have fun...

iOS Question [SOLVED] A solution for getting ambient light brightness

JackKirk

Well-Known Member

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JackKirk

Well-Known Member

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