Check if image is dark-only bottom part - objective-c

Question

I am checking if UIImage is darker or more whiter . I would like to use this method ,but only to check the third bottom part of the image ,not all of it .
I wonder how exactly to change it to check that,i am not that familiar with the pixels stuff .
BOOL isDarkImage(UIImage* inputImage){
BOOL isDark = FALSE;
CFDataRef imageData = CGDataProviderCopyData(CGImageGetDataProvider(inputImage.CGImage));
const UInt8 *pixels = CFDataGetBytePtr(imageData);
int darkPixels = 0;
long length = CFDataGetLength(imageData);
int const darkPixelThreshold = (inputImage.size.width*inputImage.size.height)*.25;
//should i change here the length ?
for(int i=0; i<length; i+=4)
{
int r = pixels[i];
int g = pixels[i+1];
int b = pixels[i+2];
//luminance calculation gives more weight to r and b for human eyes
float luminance = (0.299*r + 0.587*g + 0.114*b);
if (luminance<150) darkPixels ++;
}
if (darkPixels >= darkPixelThreshold)
isDark = YES;
I can just crop that part of the image, but this will be not efficient way, and wast time .

Answer 1

The solution marked correct here is a more thoughtful approach for getting the pixel data (more tolerant of differing formats) and also demonstrates how to address pixels. With a small adjustment, you can get the bottom of the image as follows:
+ (NSArray*)getRGBAsFromImage:(UIImage*)image
atX:(int)xx
andY:(int)yy
toX:(int)toX
toY:(int)toY {
// ...
int byteIndex = (bytesPerRow * yy) + xx * bytesPerPixel;
int byteIndexEnd = (bytesPerRow * toY) + toX * bytesPerPixel;
while (byteIndex < byteIndexEnd) {
// contents of the loop remain the same
// ...
}
To get the bottom third of the image, call this with xx=0, yy=2.0*image.height/3.0 and toX and toY equal to the image width and height, respectively. Loop the colors in the returned array and compute luminance as your post suggests.