在Imagick中双线性失真中的参数是什么意思

I was referring example of Imagick::distortImage

Now look at Example 1 which generates following image

I got confused while looking at below part of code.

/* Control points for the distortion */
$controlPoints = array( 10, 10, 
                        10, 5,

                        10, $im->getImageHeight() - 20,
                        10, $im->getImageHeight() - 5,

                        $im->getImageWidth() - 10, 10,
                        $im->getImageWidth() - 10, 20,

                        $im->getImageWidth() - 10, $im->getImageHeight() - 10,
                        $im->getImageWidth() - 10, $im->getImageHeight() - 30);

Now can anybody tell me what this 16 points exactly means ?

The control point arguments are helpfully organised into this layout:

x1  , y1  ,
x1' , y1' ,

x2  , y2  ,
x2' , y2' ,

x3  , y3  ,
x3' , y3' ,

x4  , y4  ,
x4' , y4'

For each set of coordinates, x_ and y_ are the source points, and x_' and y_' are the targets.

So it's taking the top-left corner and moving it up a bit, the bottom-left is being moved down, the top-right is being moved down and the bottom-right is being moved up, producing the perspective result.