title: CIPerspectiveTransform's behaviour
body: <p>I'm currently working on <code>CIPerspectiveTransform</code>, and I can't seem to find it's exact behaviour. At least I've got an idea but I'm not pretty sure of it. I'll expose what I found out here.</p>
  
  <p>As we can see in the <a href="https://developer.apple.com/library/mac/documentation/graphicsimaging/reference/CoreImageFilterReference/Reference/reference.html#//apple_ref/doc/filter/ci/CIPerspectiveTransform" rel="nofollow">Core Image Filter Reference</a>, there are 4 attributes of type <code>CIVector</code> :</p>
  
  <pre><code>inputTopLeft
  A CIVector object whose attribute type is CIAttributeTypePosition and whose display name is Top Left.
  Default value: [118 484]
  
  inputTopRight
  A CIVector object whose attribute type is CIAttributeTypePosition and whose display name is Top Right.
  
  Default value: [646 507]
  
  inputBottomRight
  A CIVector object whose attribute type is CIAttributeTypePosition and whose display name is Bottom Right.
  Default value: [548 140]
  
  inputBottomLeft
  A CIVector object whose attribute type is CIAttributeTypePosition and whose display name is Bottom Left.
  Default value: [155 153]
  </code></pre>
  
  <p>As you can see, the default values are quite arbitrary. But that gives an effect of perspective to the given image. However, that doesn't really help to know how that actually works. So I began picking arbitrary values for these <code>CIVector</code>. And after a few tries, I found out that this :</p>
  
  <pre><code>[perspectiveFilter setValue:[CIVectorWithX:0 Y:200] forKey:@"inputTopLeft"];
  [perspectiveFilter setValue:[CIVectorWithX:200 Y:200] forKey:@"inputTopRight"];
  [perspectiveFilter setValue:[CIVectorWithX:0 Y:0] forKey:@"inputBottomLeft"];
  [perspectiveFilter setValue:[CIVectorWithX:200 Y:0] forKey:@"inputBottomRight"];
  </code></pre>
  
  <p>applied to my image would do nothing on it (my <code>UIImageView</code> is subview to a <code>UIScrollView</code>). So I changed the values to 600x600 and I saw that nothing changed, except for the size of the resulting image. 200x200 in the first case, and 600x600 then.</p>
  
  <p>So here's my question : <strong>Why does my image print a square 200x200 size whereas I'm working on an iPhone device and it is perfectly rectangular ?</strong></p>