PS2 Perspective Correction

[Digi-T]

I am trying to correct the perspective in an image using the Perspective Correction filter in PS2. All I want to do is widen the top of the image to make all the vertical lines perpendicular to each other. I want to keep all the elements in the image at their exact same locations vertically but to simply get wider towards the top of the image. However, when I apply the perspective correction in PS it distorts the image elements by shifting them downward increasingly as the image gets narrower, in the case of my image, so that the upper elements are longer and the lower elements are narrower or more squashed. I do not want this to happen but I don't know how to pevent it in PS. I don't want to crop any of the top or bttom of the image either. I simply want to widen the top of the image while keeping all of the elements in their same locations vertically. Does this make sense? And can this be done simply in PS? This should be an easy task in PS but I just can't figure out how to do it in this program. If what I am asking for is not clear I will try to post examples but I have not done that before and I would rather not take the time to learn that right now.

Thanks,
Tim

[Tim Gray]

maybe Edit Transform Distort would be better, if not post a sample.

[Digi-T]

The Distort tool does the same thing.

Here is an image that shows what I am talking about. Notice in the "After" image how the gazebo has been distorted so that it is now sqashed and is lower in the image. I want to keep all of the elements at the same locations vertically but I want to widen the top of the image to correct the perspective distortion. Does this make sense now? I want to keep the top and bottom edges in the image without cropping but I don't want the image to be distorted in the way that it is. Thanks for any help you can give me.

Tim


[attachment=1149:attachment]

[Tim Gray]

[attachment=1150:attachment]This is probably as good as it gets - distort and stretch...

[Digi-T]

Thanks Tim for trying to help me. On this particular photo I want to make sure I don't crop any of the top or bottom portions. In this next sample I have added grid lines to help show the distortion that is occuring with the perspective correction. It should help other readers understand my problem a little better. You can see how the lower half of the image gets sqashed while the upper half gets stretched. This is not acceptable to me for this particular photo. I don't think it is a good idea create one kind of distortion while correcting for another kind of distortion. There must be other perspective correcting plugins that avoid this problem or a least allow some control over it.

Tim

[attachment=1151:attachment]

[Tim Gray]

maybe the gazebo is really as wide as it is high

[Digi-T]

maybe the gazebo is really as wide as it is high
[a href=\"index.php?act=findpost&pid=83662\"][{POST_SNAPBACK}][/a]

Ha, ha. Maybe.

Actually I played with the perspective tool some more as well as Flo's Undistort plugin and learned some more things. I found it helpful to increase your canvas size first by about 130% or so so your image does not get cropped when you apply the filter. Then you should maintain the aspect ratio or just slightly shorter than that for a good compromise. And finally, crop the image to your desire ratio and you are done. I understand why it distorts the image the way it does (see above images with the grid lines for example) to correct for perspective but I wish there was an additional setting to adjust that effect. The problem with PS is that even the Distort tool works to correct for perspective and you may not want this tool to function in this way. There should be a tool where you can widen the top or bottom of an image where all of the pixels are moved only along their horizontal axis not also on their vertical axis. There are lots of uses for this feature but I can't find it in any plugins or filters.

Tim

[elliot_n]

Interesting question.

The way I use Photoshop's Transform Perspective tool is to pull in the bottom corners the same amount as I pull out the top corners.

Doing it this way seems to preserve a structure's aspect ratio.

I've tried it with your gazebo, and it no longer appears squat as a result of the transformation.

But it still drops lower in the frame.

Elliot

[Tim Gray]

Ha, ha. Maybe.

Actually I played with the perspective tool some more as well as Flo's Undistort plugin and learned some more things. I found it helpful to increase your canvas size first by about 130% or so so your image does not get cropped when you apply the filter. Then you should maintain the aspect ratio or just slightly shorter than that for a good compromise. And finally, crop the image to your desire ratio and you are done. I understand why it distorts the image the way it does (see above images with the grid lines for example) to correct for perspective but I wish there was an additional setting to adjust that effect. The problem with PS is that even the Distort tool works to correct for perspective and you may not want this tool to function in this way. There should be a tool where you can widen the top or bottom of an image where all of the pixels are moved only along their horizontal axis not also on their vertical axis. There are lots of uses for this feature but I can't find it in any plugins or filters.

Tim
[a href=\"index.php?act=findpost&pid=83671\"][{POST_SNAPBACK}][/a]

I think you've identified the problem.  If you tilt the camera when making a shot you're basically vertically compressing the image to fit the capture area of the sensor.  The real, undisotrted, aspect ratio would be more extreme eg 2:1 instead of 1.5:1.  So if you want to get the undistorted image back to the 1.5:1 aspect ratio you're going to have to crop, or if not crop you have to squeeze, and then it's distorted.

To steal a tag line on another forum:  "The only unbreakable rule in photography: TANSTAAFL"  (There ain't no such thing as a free lunch.)

[attachment=1152:attachment]

I can't imagine how any plugin could get this back to the original aspect ratio without inducing distortion.

[Digi-T]

I understand that if I maintain the aspect ratio then there will be distortion of some kind but it is the kind of distortion that I am concerned with. The tools allow me to make an image shorter or longer, that is easy enough, but they don't allow me to control the amount that is corrected for distance. As it is now if you widen the top of the image, as I did in my image, a specific amount is applied to how much the image gets stretched progressively to comensate for the distance you are correcting for. With the distort or perspective correction tools it is automatically assumed that you also want to correct for distance scaling and that is not always the case. In my example with the gazebo I don't want to fool the viewer into thinking that they are seeing the scene straight on I simply want to make the vertical lines more vertical.

Here is a new example. Notice how I made the correction in the third image. I widened the top of the image in a way that moved the picture elements only along their horizontal axis. Then I stretched the image slightly so that it looked correct. I had to do this correction crudely without proper tools but hopefully this will better illustrate what I am trying to do.

[attachment=1153:attachment]

Tim