analysis.m

- (void)analyzeImageWithCompletion:(void (^)())block {
    NSParameterAssert(block);
    
    dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
        CFDataRef data = CGDataProviderCopyData(CGImageGetDataProvider(self.image.CGImage));
        
        UInt8 *buf = (UInt8 *)CFDataGetBytePtr(data);
        size_t width = CGImageGetWidth(self.image.CGImage);
        size_t height = CGImageGetHeight(self.image.CGImage);
        
        NSMutableArray *amps = [NSMutableArray array];
        for(NSInteger x=0;x<width;++x) {
            for(NSInteger y=0;y<(height/2);++y) {
                UInt8 red, green, blue, alpha;
                
                red =       buf[(x * 4) + ((width * 4) * y) + 0];
                green =     buf[(x * 4) + ((width * 4) * y) + 1];
                blue =      buf[(x * 4) + ((width * 4) * y) + 2];
                alpha =     buf[(x * 4) + ((width * 4) * y) + 3];
                
                if(!red && !green && !blue && !alpha) {
                    [amps addObject:[NSNumber numberWithFloat:1.f - (float)y/(height/2)]];
                    break;
                }
            }
        }
        
        self.amplitudes = amps.copy;
        CFRelease(data);
        
        block();
    });
}