AVFoundation image captured is dark - xcode

Question

On osx i use AVFoundation to capture image from a USB camera, all work fine, but the image I get is darker compared to live video.
Device capture configuration
-(BOOL)prepareCapture{
captureSession = [[AVCaptureSession alloc] init];
NSError *error;
imageOutput=[[AVCaptureStillImageOutput alloc] init];
NSNumber * pixelFormat = [NSNumber numberWithInt:k32BGRAPixelFormat];
[imageOutput setOutputSettings:[NSDictionary dictionaryWithObject:pixelFormat forKey:(id)kCVPixelBufferPixelFormatTypeKey]];
videoOutput=[[AVCaptureMovieFileOutput alloc] init];
AVCaptureDeviceInput *videoInput = [AVCaptureDeviceInput deviceInputWithDevice:MyVideoDevice error:&error];
if (videoInput) {
[captureSession beginConfiguration];
[captureSession addInput:videoInput];
[captureSession setSessionPreset:AVCaptureSessionPresetHigh];
//[captureSession setSessionPreset:AVCaptureSessionPresetPhoto];
[captureSession addOutput:imageOutput];
[captureSession addOutput:videoOutput];
[captureSession commitConfiguration];
}
else {
// Handle the failure.
return NO;
}
return YES;
}
Add view for live preview
-(void)settingPreview:(NSView*)View{
// Attach preview to session
previewView = View;
CALayer *previewViewLayer = [previewView layer];
[previewViewLayer setBackgroundColor:CGColorGetConstantColor(kCGColorBlack)];
AVCaptureVideoPreviewLayer *newPreviewLayer = [[AVCaptureVideoPreviewLayer alloc] initWithSession:captureSession];
[newPreviewLayer setFrame:[previewViewLayer bounds]];
[newPreviewLayer setAutoresizingMask:kCALayerWidthSizable | kCALayerHeightSizable];
[previewViewLayer addSublayer:newPreviewLayer];
//[self setPreviewLayer:newPreviewLayer];
[captureSession startRunning];
}
Code to capture the image
-(void)captureImage{
AVCaptureConnection *videoConnection = nil;
for (AVCaptureConnection *connection in imageOutput.connections) {
for (AVCaptureInputPort *port in [connection inputPorts]) {
if ([[port mediaType] isEqual:AVMediaTypeVideo] ) {
videoConnection = connection;
break;
}
}
if (videoConnection) { break; }
}
[imageOutput captureStillImageAsynchronouslyFromConnection:videoConnection completionHandler:
^(CMSampleBufferRef imageSampleBuffer, NSError *error) {
CFDictionaryRef exifAttachments =
CMGetAttachment(imageSampleBuffer, kCGImagePropertyExifDictionary, NULL);
if (exifAttachments) {
// Do something with the attachments.
}
// Continue as appropriate.
//IMG is a global NSImage
IMG = [self imageFromSampleBuffer:imageSampleBuffer];
[[self delegate] imageReady:IMG];
}];
}
Create a NSImage from sample buffer data, i think the problem is here
- (NSImage *) imageFromSampleBuffer:(CMSampleBufferRef) sampleBuffer
{
// Get a CMSampleBuffer's Core Video image buffer for the media data
CVImageBufferRef imageBuffer = CMSampleBufferGetImageBuffer(sampleBuffer);
// Lock the base address of the pixel buffer
CVPixelBufferLockBaseAddress(imageBuffer, 0);
// Get the number of bytes per row for the pixel buffer
void *baseAddress = CVPixelBufferGetBaseAddress(imageBuffer);
// Get the number of bytes per row for the pixel buffer
size_t bytesPerRow = CVPixelBufferGetBytesPerRow(imageBuffer);
// Get the pixel buffer width and height
size_t width = CVPixelBufferGetWidth(imageBuffer);
size_t height = CVPixelBufferGetHeight(imageBuffer);
// Create a device-dependent RGB color space
CGColorSpaceRef colorSpace = CGColorSpaceCreateDeviceRGB();
// Create a bitmap graphics context with the sample buffer data
CGContextRef context = CGBitmapContextCreate(baseAddress, width, height, 8,
bytesPerRow, colorSpace, kCGBitmapByteOrder32Little | kCGImageAlphaPremultipliedFirst);
// Create a Quartz image from the pixel data in the bitmap graphics context
CGImageRef quartzImage = CGBitmapContextCreateImage(context);
// Unlock the pixel buffer
CVPixelBufferUnlockBaseAddress(imageBuffer,0);
// Free up the context and color space
CGContextRelease(context);
CGColorSpaceRelease(colorSpace);
// Create an image object from the Quartz image
//UIImage *image = [UIImage imageWithCGImage:quartzImage];
NSImage * image = [[NSImage alloc] initWithCGImage:quartzImage size:NSZeroSize];
// Release the Quartz image
CGImageRelease(quartzImage);
return (image);
}

Answer 1

Solution found
The problem was in imageFromSampleBuffer
I used this code and the picture is perfect
// Continue as appropriate.
//IMG = [self imageFromSampleBuffer:imageSampleBuffer];
CVImageBufferRef imageBuffer = CMSampleBufferGetImageBuffer(imageSampleBuffer);
if (imageBuffer) {
CVBufferRetain(imageBuffer);
NSCIImageRep* imageRep = [NSCIImageRep imageRepWithCIImage: [CIImage imageWithCVImageBuffer: imageBuffer]];
IMG = [[NSImage alloc] initWithSize: [imageRep size]];
[IMG addRepresentation: imageRep];
CVBufferRelease(imageBuffer);
}
Code found in this answer

Answer 2

In my case, you still need to call captureStillImageAsynchronouslyFromConnection: multiple times to force the built-in camera to expose properly.
int primeCount = 8; //YMMV
for (int i = 0; i < primeCount; i++) {
[imageOutput captureStillImageAsynchronouslyFromConnection:videoConnection completionHandler: ^(CMSampleBufferRef imageSampleBuffer, NSError *error) {}];
}
[imageOutput captureStillImageAsynchronouslyFromConnection:videoConnection completionHandler: ^(CMSampleBufferRef imageSampleBuffer, NSError *error) {
CVImageBufferRef imageBuffer = CMSampleBufferGetImageBuffer(imageSampleBuffer);
if (imageBuffer) {
CVBufferRetain(imageBuffer);
NSCIImageRep* imageRep = [NSCIImageRep imageRepWithCIImage: [CIImage imageWithCVImageBuffer: imageBuffer]];
IMG = [[NSImage alloc] initWithSize: [imageRep size]];
[IMG addRepresentation: imageRep];
}
}];