Architecture, Perspective Correction, and Photoshop

More of a comment than an answer:

Elliot I do not so much of this correction but if I do, the number crunching time bothers me. It helps to do all corrections on a smaller image version and later go through the entire process again with the full size image. With 6x9 you must get 120MB or larger files? I use the "Panorama Tools" plugin in PS. In the panorama tools you can dial in the number of degrees for correction. This again is easy to determine by the measure tool. ( Well, yes you need to substract two numbers if there is no perfect vertical alignment). I like the quality of the output but have never tested if your route will give better results.

BTW: In my images the most pleasing perspective is not always the "perfect square" of windows or other rectangular structures. Often this gives an "overcorrected" impression. On tele shots this is usually not the case or not so severe.

The aspect ratio of a building is preserved if you stretch the top and squeeze the bottom symmetrically (or vice versa). Afterwards, you must crop the image to remove the corners you just clipped.

Walter: My files are very big (400Mb 16 bit Tiffs). I have a fast computer (G5), but nevertheless certain methods of corrrecting perspective are too slow (i.e. Photoshop's Lens Correction filter). But Transform Perspective is fast. I understand what you're saying about the dangers of full correction - that it can look artificial and over-corrected. However my current project is about low level buildings, so none of the corrections I'm making are extreme. The buildings have an abstract quality which is enhanced if all horizontal and vertical lines are parallel to the edges of the photograph.

Edward: Thanks. I had already picked up that tip elsewhere, and that is the method I use. Pull out the top corners, and pull in the bottom corners by the same amount. I was just unsure about whether that was it, or whether further corections along the vertical axis were necessary.

I'm still interested in any further comments about using Photoshop to correct perspective distortion... I'd also like to ask whether there is any fundamental difference between doing it this way and doing it using shift with a large format camera? Some say there is, some say there isn't....

thanks

Elliot, I use the Perspective option with the crop tool, and use sets of ruler guides for alignment. In particular, this is handy for indoor shots: Please see the shot below which I corrected for another photog in the lab.

I might well pick the second; it looks more dynamic, informal, befitting the mood. There are times when parallel lines are not desirable, so it is best to keep an open mind.

'I use the Perspective option with the crop tool, and use sets of ruler guides for alignment'

I'm not sure how guides would help when using the perspective option of the crop tool, as there is no live preview - you just line up the crop box against the lines in your image that you want to be horizontal or vertical, hit return, and wait for Photshop to do its magic.

It's an amazing tool, but I prefer the live preview of Edit > Transform Perspective - you can see what is going to happen before committing to it. In this case, guides are very useful to help get everything lined up.

Now I remember why I don't like Edit > Transform Perspective: It's very jumpy, and is symmetrical.

Generally I apply the Perspective crop using Bicubic Sharper when I perform the main cropping, i.e. I first do a trimming crop (film edges), then global color correct, then final crop (i.e. 5 x 5 x 300 ppi) is done via Bicubic Sharper, with the perspective option to bring it in. Please see the scan from a 4x5 Tri-X sheet below, noting the ceiling line.

[Other photos are at http://users.snip.net/~joe/austin/pages/044.htm ]

Dan, I'm not sure why you're not just using rotate.

Err,

BECAUSE ROTATE DOESN'T CORRECT FOR PRESPECTIVE!

Sure, but the pictures you posted don't really have any perspective distortion (the camera is pointing more or less straight ahead) - they're just shot a bit wonky.

Hi all,

If you are seriously into architectural photography, where converging verticals are a cardinal sin, you are certainly wasting your time trying to correct them with Photoshop.

Neither Filter>Distort>Lens Correction nor Perspective Crop use the correct projective geometry. Assuming the camera has been tipped up, all they will do is to squash the image more at the bottom than the top, whereas, to do it properly, the image needs to be stretched vertically as well. Unfortunately, the stretching is not linear ? it cannot be done by resizing the image vertically ? more vertical stretching is needed at the top than the bottom. As a result, these two tools only give a reasonable result if the amount of correction is very small.

Edit>Transform>Perspective is much better, but, even then, the result is technically wrong. Worse, and this is a serious defect in Photoshop, the operation has to be set up by eye.

Although my main interest is photography is street photography, I do architectural work as well, and I try to get things as perfect as possible. Ideally, I would use a view camera, which is, overall, a lot faster than a digital camera with its fixed lens, but often I have only a digital camera, admittedly with a suitably wide lens. For digital, the software I use is PTGui. It started as a GUI front end for Panorama Tools, written by Helmut Dersch, which stitches images together to form a panorama. Other than an image in the set which is dead centre of the desired panorama, all other images will need perspective transformations. It is also possible to stitch an image to itself. PTGui is now much improved.

The way it all works is that you mark control points between the adjacent images, then the panorama creation applies perspective transformations and affine transformations to register the images so that they can be blended. In the early versions of PTGui it was a bit tedious, but it did mean that you were going into the detail, and not having to set up the perspective transformation from the whole of the images by eye. The latest versions of PTGui have a tool to generate control points automatically, which can then be manually edited. PTGui also has a facility for marking horizontals and verticals between the images. To apply perspective transformations to a single image, horizontals and verticals are marked within the image. Then, when you say ?go? it will rotate the image, and apply the perspective transformation. Also, if you have not corrected the special distortion of you lens already (using, for example DxO Optics Pro), it will correct the distortion of your lens.

There is a problem with PTGui. It does not lower the basic resolution of the image, so it can generate massive image files if there is a lot of stretching to be done, which need to be cropped. Other software might find its output a bit much.

Now for some examples. The first image is the result that we are trying to achieve. It is not digital ? it was shot on a Wista Field, with a 90mm Super Angulon (and I apologise about the tonal range ? the tonal range of Ektachrome, or any silver image, can be way beyond the capabilities of digital technology, in this case a large format scanner). My camera was dead straight (laboriously checked with a large spirit level), so, if you think something is not straight in the image, it is the Cathedral, not me. For example, the roof of the nave has a distinct bend.

The second image is a photograph taken by my daughter in Brugge, as it came from the camera. Like me, she cannot hold a camera accurately vertically, so it needs a bit of rotation first.

The third image is the result from that image obtained from Photoshop?s Filter>Distort>Lens Correction. What it has done is to squash the image horizontally at the bottom, and, when cropped, the resolution turns out to be much lower. But, worse, the image has not been stretched at the top. This is awful.

The fourth image is the result obtained from Photoshop?s Edit>Transform>Perspective. Now there is some vertical stretching, but, unfortunately, there is too much stretching at the bottom. This is bad.

The last image was processed through PTGui, with a little bit of enhancement with Photoshop to lighten the shadows. This is the only one that looks right to me.

http://photo.net/photodb/photo?photo_id=4814809

So, my recommendation to anyone who wants to apply perspective control to an image in post processing is not to use Photoshop (which is crap), but to use a tool that is dedicated to the job (or, much better, get yourself a view camera, so you do not have to do anything at all after you have fired the shutter: just send the film to the lab).

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