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Thoughts About Perspective Crop

Community Expert ,
Nov 28, 2021 Nov 28, 2021

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You might remember I started a thread about Perspective Crop a while back asking about adding a border.  Well, something else has just occurred to me after reading a thread in a local (to NZ) forum called Geekzone   A poster asked about cropping and straightening the postcard in this image, and after a bunch of suggestions about various smart phone apps (WTH!), Perspective Crop was suggested...

image.pngexpand image

...and this example shown, but their is a significant change in the aspect ratio.  

image.pngexpand image

 

This got me thinking.  If you zero the grid to the top left corner of the postcard, and box it with guides, we get an area about 1420px by 800px.  

image.pngexpand image

Using Perspective Crop, unsurprisingly, squares up the postcard within the maximum size of the box that contains the original out of perspective image.  We can see from this that the person who uploaded the example in my second image did no in fact use Perspective Crop as they claimed, as Photoshop does not create extra Canvas to address perspective foreshortening

image.pngexpand image

But that is clearly wrong,  The original perspective error means that, in this case, the height is foreshortened.  In fact, it might be possible to use trig to work out the correct aspect ratio, and if that is true, the Photoshop Development team might be able add a feature that corrected the aspect ratio. It wouldn;t be easy as there will invariable be a perspective error in box x and y axis, and if the image is also curled giving it curved edges, then it would be impoosible to work out foreshortening.

 

So who can see the relevant triangle to work out the correct aspect ratio?  If you can could you please explain rather than just give us the answer?  Or do you already make some sort of effort to correct the aspect ratio when using Perspective Crop?  It's certainly something I'll be thinking about from now on.

 

image.pngexpand image

 

 

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Adobe
Community Expert ,
Nov 28, 2021 Nov 28, 2021

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That's a hard one. I'm not sure it can be figured out. Say the angled sides length are foreshortened based on the lens focal length. A telephoto lens would make the angle closer to the length of the height, whereas a wide angle would make them longer.

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