Ios 如何改变图像某些部分的饱和度？

我有一个UIImageView和一些pointsCGPOint值。这样我就可以创建这样的路径

var mutablePath = CGPathCreateMutable()
    var point1 = patternImageView.someFeature.rightPoints.objectAtIndex(1).CGPointValue()
    CGPathMoveToPoint(mutablePath, nil, point1.x, point1.y)
    point1 = patternImageView.someFeature.rightPoints.objectAtIndex(2).CGPointValue()
    CGPathAddLineToPoint(mutablePath, nil, point1.x, point1.y)
    point1 = patternImageView.someFeature.rightPoints.objectAtIndex(3).CGPointValue()
    CGPathAddLineToPoint(mutablePath, nil, point1.x, point1.y)
    point1 = patternImageView.someFeature.rightPoints.objectAtIndex(4).CGPointValue()
    CGPathAddLineToPoint(mutablePath, nil, point1.x, point1.y)
    CGPathCloseSubpath(mutablePath)
    var pathForRight:CGPathRef = mutablePath

var rectRight = CGPathGetBoundingBox(pathForRight)

这样我就可以用路径中的所有点创建rect，如下所示

var mutablePath = CGPathCreateMutable()
    var point1 = patternImageView.someFeature.rightPoints.objectAtIndex(1).CGPointValue()
    CGPathMoveToPoint(mutablePath, nil, point1.x, point1.y)
    point1 = patternImageView.someFeature.rightPoints.objectAtIndex(2).CGPointValue()
    CGPathAddLineToPoint(mutablePath, nil, point1.x, point1.y)
    point1 = patternImageView.someFeature.rightPoints.objectAtIndex(3).CGPointValue()
    CGPathAddLineToPoint(mutablePath, nil, point1.x, point1.y)
    point1 = patternImageView.someFeature.rightPoints.objectAtIndex(4).CGPointValue()
    CGPathAddLineToPoint(mutablePath, nil, point1.x, point1.y)
    CGPathCloseSubpath(mutablePath)
    var pathForRight:CGPathRef = mutablePath

var rectRight = CGPathGetBoundingBox(pathForRight)

或者我可以将NSValue添加到数组中

var arrayRect = NSMutableArray()
    arrayRect.addObject(NSValue(CGRect: CGPathGetBoundingBox(pathForRight)))

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是否可以更改rect rectRight中所有像素的饱和度，或者更改某个rect中图像的饱和度？

如何仅对图像上的某个rect应用CiFilter？CiFilter包括遮罩。或者，提取图像的一个矩形，应用CiFilter，然后将其放回原处。