Calculate luminosity of an image.

gistfile1.m

NSArray* dayArray = [NSArray arrayWithObjects:@"Image One",@"Image Two",@"Image Three",nil];
for(NSString* day in dayArray)
{
	for(int i=1;i<=2;i++)
	{
		UIImage* image = [UIImage imageNamed:[NSString stringWithFormat:@"%@%d.png",day,i]];
		unsigned char* pixels = [image rgbaPixels];
		double totalLuminance = 0.0;
		for(int p=0;p<image.size.width*image.size.height*4;p+=4)
		{
			totalLuminance += pixels[p]*0.299 + pixels[p+1]*0.587 + pixels[p+2]*0.114;
		}
		totalLuminance /= (image.size.width*image.size.height);
		totalLuminance /= 255.0;
		NSLog(@"%@ (%d) = %f",day,i,totalLuminance);
	}
}

totalLuminance /= 255.0;

Why this?

If the values are in the 0-255 range you have to divide. (Usally are 0-1, then you dont need to divide by 255)