Leveling and matching exposure using CoreImage / GPUImage - objective-c

Question

I was wondering if its possible to level exposure across a set of images using either CoreImage or GPUImage. And how would I go about that?
Example:
Say you have 4 images, but the exposure is different on the third one. How could you level the exposure so all 4 images have the same exposure?
One idea I had was measuring and matching the exposure using AVCapture, i.e. if the input image is -2.0, then simply add 2.0 using CoreImage.
Another idea is to implement histogram equalization..
Has anyone ever dealt with the same task before? Any insights?

Answer 1

You can use CoreImage (#import ) and ImageIO (CGImage) and extract exif metadata with EXIF Dictionary Keys keys:kCGImagePropertyExifExposureTime, kCGImagePropertyExifExposureMode, kCGImagePropertyExifExposureProgram, kCGImagePropertyExifExposureBiasValue, kCGImagePropertyExifExposureIndex, kCGImagePropertyExifWhiteBalance
- (NSDictionary*) exifData: (NSString*) path {
NSDictionary* dic = nil;
NSURL* url = [NSURL fileURLWithPath: path];
if ( url )
{
CGImageSourceRef source = CGImageSourceCreateWithURL ((CFURLRef) url, NULL);
if (source != nil)
{
CFDictionaryRef metadataRef =
CGImageSourceCopyPropertiesAtIndex (source, 0, NULL);
if (metadataRef)
{
NSDictionary* immutableMetadata = (NSDictionary *)metadataRef;
if (immutableMetadata)
{
dic = [NSDictionary dictionaryWithDictionary : (NSDictionary *)metadataRef];
}
CFRelease ( metadataRef );
}
CFRelease(source);
source = nil;
}
}
return dic;
}
Usage:
NSDictionary* dic = [self exifData: path];
if (dic)
{
NSString *s = [dic valueForKey: kCGImagePropertyExifExposureTime];
NSLog(#"Image : %# - ExposureTime : %.2f", path, [s floatValue]);
}
And then changing the exposure or white balance with:
- (UIImage *) changeImageExposure:(NSString*) imagename exposure:(float) exposure {
CIImage *inputImage = [[CIImage alloc] initWithImage:[UIImage imageNamed: imagename]];
CIFilter *exposureAdjustmentFilter = [CIFilter filterWithName:#"CIExposureAdjust"];
[exposureAdjustmentFilter setValue:inputImage forKey:kCIInputImageKey];
[exposureAdjustmentFilter setValue:[NSNumber numberWithFloat:exposure] forKey:#"inputEV"];
CIImage *outputImage = exposureAdjustmentFilter.outputImage;
CIContext *context = [CIContext contextWithOptions:nil];
return [UIImage imageWithCGImage:[context createCGImage:outputImage fromRect:outputImage.extent]];
}