SilverStripe get half of image width/height - image

Question

how can I get calculate with image height and width? I am trying to center an image like described in the first answer:
How to make an image center (vertically & horizontally) inside a bigger div
That's what I have tried so far:
<% control ProfileImage %>
<% if getOrientation = 1 %>
<img src="$URL" style="margin-left: -{$Width/2)}px" class="portrait"/>
<% else_if getOrientation = 2 %>
<img src="$URL" class="landscape"/>
<% end_if %>
<% end_control %>
The output of the decisive line is:
<img src="/assets/Uploads/picture.jpg" style="margin-left: -{154/2)}px" class="portrait"/>
Then I tried to write an own function:
class Profile extends Page{
//...
function GetHalfWidth(){
return ($this->ProfileImage->Width)/2;
}
//...
}
But I get the following error when I try to view the page in the frontend:
[Notice] Trying to get property of non-object

Answer 1

To get at the width of your image in the model you can change your function to read:
function GetHalfWidth() {
$width = $this->obj('ProfileImage')->Width;
return $width / 2;
}
However, if you are trying to reference that method form within the control loop of ProfileImage you'll need to 'jump out' and use $Top.GetHalfWidth
I would think a better way to handle it would be to define it as a method of your ProfileImage, if ProfileImage was subclassed from Image...
In Profile.php
<?php
class Profile extends Page {
...
public static $has_one = array(
'ProfileImage' => 'Profile_Image'
);
...
}
class Profile_Controller extends Page_Controller {
...
}
class Profile_Image extends Image {
function GetHalfWidth() {
$width = $this->Width;
return $width / 2;
}
}