Find bottom non-transparent pixels in a UIImage

gistfile1.m

float bottomNonAlphaPixel(UIImage* inputImage){
    
    float bottomNonAlphaPixel = 0;
    float const neighboringPixelThreshold = 4;
    
    CFDataRef imageData = CGDataProviderCopyData(CGImageGetDataProvider(inputImage.CGImage));
    const UInt8 *pixels = CFDataGetBytePtr(imageData);
    UInt8 alphaThreshold = 20;
    int bytesPerPixel = 4;
    
    for(int x = 0; x < inputImage.size.width; x++) {
        for(int y = bottomNonAlphaPixel; y < inputImage.size.height; y++) {
            int pixelStartIndex = (x + (y * inputImage.size.width)) * bytesPerPixel;
            UInt8 alphaVal = pixels[pixelStartIndex];
            
            if (alphaVal>alphaThreshold && y > bottomNonAlphaPixel){
                
                int transparentNeighbors = 0;
                
                //naive neighbor lookup to find outliers
                for (int rY = -1; rY<2; rY ++){
                    for (int rX = -1 ; rX<2; rX ++){
                        
                        if (rX==0 && rY ==0)continue;
                        
                        int neighborPixelStartIndex = ((x+rX) + ((y+rY) * inputImage.size.width)) * bytesPerPixel;
                        UInt8 alphaVal = pixels[neighborPixelStartIndex];
                        if (alphaVal < alphaThreshold){
                            transparentNeighbors ++;
                        }
                    }
                }
                
                if (transparentNeighbors >=neighboringPixelThreshold){
                    //outlier, disqualify!
                }else{
                    bottomNonAlphaPixel= y;
                }
            }
        }
    }
    
    return bottomNonAlphaPixel;
}