How to make a FHD plane to show image on iphone 6+

Hi, all. I am working on an iOS project.

I tried to get resolution of iphone 6+, but ofGetWidth() return 320.
If I turn setting.retina to true, it return 640.
When I make a plane to show ofImage, the plane width cannot be larger than 320 (or 640 in retinal case) or it will exceed the screen range.

The actual points (width) for iphone 6+ is 414 according to this page.
And the actual physical pixels on iphone6+ is FHD.

I have to render each pixels through shader in my project, but it seems OF gives me wrong number of points, therefore I get a bad result.
Is there any way to render FHD images through my shader and display it on a FHD plane without exceed the iphone screen range? I’ve tried many ways but fail…