Velocity Reviews > how to rotate image so that image is perfectly level

# how to rotate image so that image is perfectly level

bucky3
Guest
Posts: n/a

 07-11-2009
When hand shooting, it is not possible to have the camera perfectly
level all the time. Often, the images may be 1-2 degrees off, so I
need to correct the rotation of the image. (If you use the pitch/roll/
yaw analogy, I'm only correcting for roll.)

Sometimes, it's pretty easy to figure out exactly how much to rotate
it, because you can rotate the image so that all the vertical edges
are perfectly straight up and down, or all the horizontal edges are
perfectly straight across. But sometimes, due to perspective, vertical
or horizontal edges should NOT be straight up/down or across, or the
edges aren't parallel. Then I struggle with the rotation correction.

Are there any rules/techniques to figure out the proper rotation
correction needed? I don't have much grasp of perspective, so I was
trying to learn about it in hopes that it would help:
http://wiki.panotools.org/Perspective_correction
http://wiki.blender.org/index.php/Do...ra/Perspective

Ray Fischer
Guest
Posts: n/a

 07-11-2009
bucky3 <(E-Mail Removed)> wrote:
>When hand shooting, it is not possible to have the camera perfectly
>level all the time. Often, the images may be 1-2 degrees off, so I
>need to correct the rotation of the image. (If you use the pitch/roll/
>yaw analogy, I'm only correcting for roll.)
>
>Sometimes, it's pretty easy to figure out exactly how much to rotate
>it, because you can rotate the image so that all the vertical edges
>are perfectly straight up and down, or all the horizontal edges are
>perfectly straight across. But sometimes, due to perspective, vertical
>or horizontal edges should NOT be straight up/down or across, or the
>edges aren't parallel. Then I struggle with the rotation correction.
>
>Are there any rules/techniques to figure out the proper rotation
>correction needed?

Yep. Adjust until it looks right.

In my book, if the photo _is_ correct but _looks_ crooked then that's
a problem. I'm interested in the esthetics more than then technical
accuracy.

> I don't have much grasp of perspective, so I was
>trying to learn about it in hopes that it would help:
>http://wiki.panotools.org/Perspective_correction
>http://wiki.blender.org/index.php/Do...ra/Perspective

Urg. Well, it depends on the tools you have. I use photoshop and it
lets you draw a quadrilateral that it uses to correct the perspective.
Line up the edges with straight lines in the scene and it does the
rest. Other tools tend to work similarly. When you don't have
straight edges in the scene then you have to resort to some guesswork.

--
Ray Fischer
http://www.velocityreviews.com/forums/(E-Mail Removed)

Scott W
Guest
Posts: n/a

 07-11-2009
On Jul 11, 8:16*am, bucky3 <(E-Mail Removed)> wrote:
> When hand shooting, it is not possible to have the camera perfectly
> level all the time. Often, the images may be 1-2 degrees off, so I
> need to correct the rotation of the image. (If you use the pitch/roll/
> yaw analogy, I'm only correcting for roll.)
>
> Sometimes, it's pretty easy to figure out exactly how much to rotate
> it, because you can rotate the image so that all the vertical edges
> are perfectly straight up and down, or all the horizontal edges are
> perfectly straight across. But sometimes, due to perspective, vertical
> or horizontal edges should NOT be straight up/down or across, or the
> edges aren't parallel. Then I struggle with the rotation correction.
>
> Are there any rules/techniques to figure out the proper rotation
> correction needed? I don't have much grasp of perspective, so I was
> trying to learn about it in hopes that it would help:http://wiki.panotools.org/Perspectiv...ra/Perspective

Vertical edges in the center of the image should be vertical in the
photo. The
edges to the sides will slope in or out depending on if the camera was
pointed up or down.

Scott

bucky3
Guest
Posts: n/a

 07-11-2009
On Jul 11, 12:10 pm, Scott W <(E-Mail Removed)> wrote:
> Vertical edges in the center of the image should be vertical in the
> photo. The
> edges to the sides will slope in or out depending on if the camera was
> pointed up or down.

Thanks, that's the kind of rule I was looking for.

Does it also apply that horizontal edges in the center of the image
should be straight across in the photo too?

PatM
Guest
Posts: n/a

 07-11-2009
On Jul 11, 3:31*pm, bucky3 <(E-Mail Removed)> wrote:
> On Jul 11, 12:10 pm, Scott W <(E-Mail Removed)> wrote:
>
> > Vertical edges in the center of the image should be vertical in the
> > photo. *The
> > edges to the sides will slope in or out depending on if the camera was
> > pointed up or down.

>
> Thanks, that's the kind of rule I was looking for.
>
> Does it also apply that horizontal edges in the center of the image
> should be straight across in the photo too?

The rule of thumb is to just make it look right, but sometimes you
need precision. In that case, things should go out straight and level
from the center of the picture (unless you don't want that for
artistic reasons). If you need precision, you need to use something
that you know is level and/or straight. That leaves the edge of the
box the photo is in or the edge of the monitor. They are about the
only "true" reference points because they are generated independently
of the image. Sometimes you can align with the edge by moving the
picture and sometimes you need to break out an (old school) ruler and
measure from the edge of something.

Good luck with it.

bucky3
Guest
Posts: n/a

 07-11-2009
On Jul 11, 12:31 pm, bucky3 <(E-Mail Removed)> wrote:
> Does it also apply that horizontal edges in the center of the image
> should be straight across in the photo too?

OK, I think I figured out the answer to this question. The answer is
no, horizontal edges to do not need to be level in the center of the
image because I am allowing for the camera to be tilted up or down.

The only time that horizontal edges will be straight in the center of
the image is if the camera is not tilted up or down (i.e. the camera
is at the same height as the edge).

Ray Fischer
Guest
Posts: n/a

 07-11-2009
Savageduck <savageduck@{REMOVESPAM}me.com> wrote:
>On 2009-07-11 12:06:10 -0700, (E-Mail Removed) (Ray Fischer) said:
>
>> bucky3 <(E-Mail Removed)> wrote:
>>> When hand shooting, it is not possible to have the camera perfectly
>>> level all the time. Often, the images may be 1-2 degrees off, so I
>>> need to correct the rotation of the image. (If you use the pitch/roll/
>>> yaw analogy, I'm only correcting for roll.)
>>>
>>> Sometimes, it's pretty easy to figure out exactly how much to rotate
>>> it, because you can rotate the image so that all the vertical edges
>>> are perfectly straight up and down, or all the horizontal edges are
>>> perfectly straight across. But sometimes, due to perspective, vertical
>>> or horizontal edges should NOT be straight up/down or across, or the
>>> edges aren't parallel. Then I struggle with the rotation correction.
>>>
>>> Are there any rules/techniques to figure out the proper rotation
>>> correction needed?

>>
>> Yep. Adjust until it looks right.
>>
>> In my book, if the photo _is_ correct but _looks_ crooked then that's
>> a problem. I'm interested in the esthetics more than then technical
>> accuracy.
>>
>>> I don't have much grasp of perspective, so I was
>>> trying to learn about it in hopes that it would help:
>>> http://wiki.panotools.org/Perspective_correction
>>> http://wiki.blender.org/index.php/Do...ra/Perspective

>>
>> Urg. Well, it depends on the tools you have. I use photoshop and it
>> lets you draw a quadrilateral that it uses to correct the perspective.
>> Line up the edges with straight lines in the scene and it does the
>> rest. Other tools tend to work similarly. When you don't have
>> straight edges in the scene then you have to resort to some guesswork.
>> Adjust until it looks right.

>
>In Photoshop one of the easiest ways to deal with this issue is to use
>the Lens Correction filter.

There are several ways to do anything in Photoshop. I cited that
method because it corresponds to a mention on the wiki page he cited.

Which is moot if he doesn't have Photoshop.

But a useful tip regardless. I'm still learning about how to do
things in Photoshop.

>This is found in filters-->distort-->Lens Correction.
>Here you have tools for leveling, correcting barrel & pincushion
>distortion, CA, Vignetting, Vertical & Horizontal perspective.

--
Ray Fischer
(E-Mail Removed)

nospam
Guest
Posts: n/a

 07-11-2009
In article <4a58e2a2\$0\$1652\$(E-Mail Removed)>, Ray Fischer
<(E-Mail Removed)> wrote:

> >Are there any rules/techniques to figure out the proper rotation
> >correction needed?

>
> Yep. Adjust until it looks right.
>
> In my book, if the photo _is_ correct but _looks_ crooked then that's
> a problem. I'm interested in the esthetics more than then technical
> accuracy.

agreed.

> Urg. Well, it depends on the tools you have. I use photoshop and it
> lets you draw a quadrilateral that it uses to correct the perspective.
> Line up the edges with straight lines in the scene and it does the
> rest. Other tools tend to work similarly. When you don't have
> straight edges in the scene then you have to resort to some guesswork.
> Adjust until it looks right.

another way in photoshop is to use the ruler tool and drag it along an
edge that should be horizontal or vertical, then in the image menu,
pick rotate canvas/arbitrary and the angle to make it level will be
pre-filled.

Jürgen Exner
Guest
Posts: n/a

 07-11-2009
Oh what the hell ... <(E-Mail Removed)> wrote:
>I'm SO sick to death

That would be great. Unfortunaly I'm afraid that's only an empty promise
again.

jue

Paul Furman
Guest
Posts: n/a

 07-11-2009
bucky3 wrote:
> bucky3 wrote:
>
>> Does it also apply that horizontal edges in the center of the image
>> should be straight across in the photo too?

>
> OK, I think I figured out the answer to this question. The answer is
> no, horizontal edges to do not need to be level in the center of the
> image because I am allowing for the camera to be tilted up or down.
>
> The only time that horizontal edges will be straight in the center of
> the image is if the camera is not tilted up or down (i.e. the camera
> is at the same height as the edge).

Google '3-point perspective' to get a better sense of how it works. If
the horizontal is the edge of a building viewed on a diagonal, then you
can't/don't need to straighten it.

Here's a 4-point perspective just to confuse you:
http://www.termespheres.com/mh-page2.html
-figure 2

Scott's advice is good: hold vertical in the center. Sometimes that's
still hard to judge if you have a building viewed at an angle and no
windows in the middle. In that case visualize sketching more windows on
the wall, all going to the same vanishing point as the outside edges...
or even go ahead & sketch that on another layer. Then use vertical guide
lines, in photoshop: control-click-drag guides off the rulers to match
against subject geometry. It helps to have a couple of these guides
dragged out near the available verticals, then match & balance till it
makes sense with the vanishing point.

Another approach in photoshop is free transform. Control-a then
control-t and control-click & drag the corner grips one by one till it
looks right. That can be used to fake things and avoid cropping too much
off or to make it 'look right' for odd situations where you align
properly & it still looks wrong. That also potentially gets you into
perspective correction territory <g>.

--
Paul Furman
www.edgehill.net
www.baynatives.com

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