The below mentioned code uses CIImage library's CICrop filter to crop the desired area from UIImage. The follwing code deoesn't deals with snapshot as CGContextRef rather crops the original image along with maintaining its quality and aspect ratio.

        CGImageRef imageRef = [[UIImage imageNamed@"your_image"] CGImage];

        CIImage *image = [CIImage imageWithCGImage:imageRef];
        CIFilter *scaleFilter = [CIFilter filterWithName:@"CILanczosScaleTransform"];
        int originalHeight=[image extent].size.height;
        int originalWidth=[image extent].size.width;

        float yScale=screenHeight / (float)originalHeight;
        float xScale=screenWidth / (float)originalWidth;
        float scale=fminf(yScale, xScale);

        [scaleFilter setValue:[NSNumber numberWithFloat:scale]
                       forKey:@"inputScale"];
        [scaleFilter setValue:[NSNumber numberWithFloat:1.0]
                       forKey:@"inputAspectRatio"];
        [scaleFilter setValue: image
                       forKey:@"inputImage"];

        UIImage *outputImage = [scaleFilter valueForKey:@"outputImage"];