V1.3 - Geometry Editing
Correcting Image Geometry
Two words: framing and perspective. Reframing the panorama is quite easy, there won’t be much involved in this step. Perspective correction in the other hand will require relocation of the horizon at the right place, then choosing the desired projection type: you can let the software manage these steps automatically and you will, most of the time, obtain decent results; but not always...
There are two ways to approach the perspective issues:
- the trial and error method based on the initial output of Autopano Pro displayed in the Editor Window and choose what you may see fit best.
- gain an understanding of the mechanics driving the process, what are the available choices and the results you can expect from the different methods.
Autopano Pro is very well suited for the “trial and error” method as it will display (as a panorama preview) almost instantly the results of each change you can make in the geometry variables.
What you must know about perspective is quite simple: if you take a photograph of a tiled floor, the individual tiles naturally line-up on the right perspective path, and no trigonometry calculation is necessary to take a picture of a spiral staircase! Since the early stages of photography, comforted by the idea that they did not have to master the perspective concepts, photographers got used to almost ignore the approach to perspective.
On this topic, you can refer to the Understanding Projecting Modes page. This page describes the different projection types available in Autopano Pro and how they work.
- Linear Projection
- Advantages: always a good choice when working with a small FOV (field of view), recommended for architecture as it is the only projection mode that will not curve any line (the other two modes will always, to a certain extend, “bend” curbs and building tops).
- Downside: in theory this mode can only be used if the FOV is smaller than 180°, determined by the diagonal of the image. In real world situation the limitation is actually 90° as the stretching produced on both edges of the image is already visible at 90°, and even more pronounced at the corners. Passed 120°, the results become unacceptable as the stretching will produce a significant and uncomfortable loss of sharpness.
Tip: in certain architecture cases it may be better to use Linear Projection despite the excessive stretching and then cut away those part, leaving only the less deformed center of the panorama.
- Cylindrical Projection
- Advantages: can handle horizontal FOVs up to 360°.
- Downside: all straight lines parallel to the horizon (curbs, building tops) will, to various extents, be bent. The vertical angle is limited, it MUST be smaller than 180°, but a stretching will start to appear at the top and at the bottom of the image passed 45° from the horizon (over and down).
Tip: when stitching long landscape panoramas, you may want to use Cylindrical instead of Spherical projection, in order to have a better width:height ratio (ie a bit taller panoramas). The main problem with long panoramas is that they look "squashed" and Cylindrical projection can help with that.
- Spherical Projection
- Advantages: this is the default choice as it can handle any panorama type. This mode is the default projection mode when opening the Editor Window; this is also the default mode when launching a rendering without going through the Editor Window.
- Downside: for a direct display on a computer screen (without using a special viewer) or to print the panorama, you must first make sure that the amount of curvature of the lines parallel to the horizon stays acceptable. There is no set rule to determine what is acceptable and what’s not, you must use your best judgment. When the vertical field of view is large, the stretching of the top of the image (close to the zenith) and of the bottom of the image (close to the nadir) can vary from very natural to quite unnatural.
The Layer View mode will let you visualize how the original images are arranged in the panorama. Activate the Layer View mode by clicking the button, next run your mouse pointer over the panorama to see the actual picture located under the pointer’s location. If the cursor is on top of an overlap (i.e. stitch seam), use the mouse wheel to scroll through the different pictures overlapping at the current location.
When the mouse pointer is located outside of the panorama area, the whole panorama is shown (i.e. the resulting stitched image with brightness, contrast and colors adjusted on the overlaps).
Another click on the button will exit the Layer View mode.
Tip: The task bar displays information related to the current selection (i.e. the selected source image): number and file name. This is often useful to identify an image we want to exclude from the panorama using the Control Point Editor.
These three buttons let you rotate the panorama when the original pictures in the source group have a wrong landscape/portrait orientation. Note that even if the panorama looks well aligned in the first place, a 90° rotation can result in a wrong panorama alignment: it is then good practice to use the Auto Level tool after a rotation.
As surprising as it may sound this tool can by itself perform all the panorama modifications allowed by the following tools.
- 180° Flip
- 90° Left rotation
- 90° Right rotation
- Set Center Point
- Auto Level
- Set Verticals
It is obviously more reasonable to hand a camera to a person who never took pictures than trying to explain to that person what you see in the viewfinder when orienting the camera in different directions!
So, it is probably a better use of your time to stitch the example pictures provided with Autopano Pro and to alter the values (let’s say 10 then -10) of the three different parameters provided for this tool to get a better "feel" for each parameter. One word of warning however before you proceed: alter only one parameter at a time and never all three of them at once! Try this first with the Spherical Projection, then with the other projection types.
Note that if you do not spend the time to carry out this little tryout you will end up spending much more time trying to fix a panorama without knowing what you are doing, and without even knowing if you even have a chance to achieve what you want.
The idea is to "rotate" the panorama left or right, towards the top or the bottom, or around its center by selecting the Yaw, Pitch and Roll parameters and altering their values.
- Yaw = Right and Left offset (a positive value moves the panorama to the right).
- Pitch = Up and Down offset (a positive value moves the panorama down).
- Roll = Rotation around the set center point (a positive value rotates the panorama clockwise).
The panorama center is materialized by the point formed by the intersection of the vertical and horizontal grey lines displayed on top of the panorama.
The rotation angle is in degrees, the effects of this parameter are cumulative: a -10° rotation cancels a previous 10° rotation.
A random use of this tool makes it difficult to understand as the effects are cumulative, this would be an unfortunate approach as it is a very powerful tool.
This tool centers the panorama on the click location, implying two things:
- it moves the center of the panorama left or right (the vertical grаy line act as a guide).
- it locates the horizon at the click’s height (the horizontal grаy line act as a guide).
Regarding left and right centering; note that when using a Planar Projection the perspective paths are affected by the location of the vertical grаy line.
Note that when setting the vertical center (i.e. placing the horizon at the location of the horizontal grey line), the click location should be located at the camera's height, unless you are looking for a low angle shot (verticals converging up) or a high angle shot (verticals converging down). To move the horizon up or down it is best to use the Set Verticals tool described in the next paragraph.
Autopanо Pro will automatically estimate the location of the horizon before displaying the preview. If you think this estimation was acceptable after you made changes, you can click on Auto Level to go back to the estimated value calculated by Autopano Pro. This being said, it is important to keep in mind that Autopano Pro totally ignores what was photographed. You remain the best judge to determine what is right.
When using a Cylindrical or a Planar projection, the best way to offset the image to the right or to the left without altering anything else is to use the Numerical Transformation tool and acting only on the Yaw parameter.
This tool will allow you to straighten-up a panorama by placing vertical lines on the pictures. How to use the tool:
- Click on the Set Verticals tool, move your pointer over the preview then click/hold the left mouse button and drag the mouse to create and set a line, release the mouse button when done.
- Locate the red lines where you are sure there was a straight line in the subject (building corner, doors and windows, furniture). Two or three lines are generally enough, especially if they are far apart from each other, but you can use more as long as you know there is a straight line underneath (be careful with old building walls and with leaning telephone poles). The software will then find the best possible compromise, taking in account your input and the bonds created between the source photographs.
Hit the Enter key to validate your input, the preview will then refresh to reflect your changes.
You can use the tool with a single line to do a quick rotation; the line you traced will then become a perfect vertical.
The yellow dotted line attached to the red line is used to straighten the horizon. In landscape or nature photography is it quite frequent to shoot a subject with no verticals, when very often this subject will have a horizon. So instead of using a vertical as a guide you can adjust the verticals by placing the yellow dotted line on the horizon. The use of 3 or 4 lines on the horizon is generally enough to correct the panorama.
Note: The red line should be created from top to bottom to apply a transformation to the panorama. If the red line is made horizontally the panorama will rotate a quarter of a turn; if made from bottom to top the panorama will flip 180 degrees.
Even if the original pictures were shot with the lens pointing up or down Autopano Pro will often produce a leveled panorama, to a point where one could think that this tool doesn't work, as there is no visible difference between “before” and “after”!
If, in the other hand, the previous Yaw, Pitch and Roll tools were used to modify the panorama and/or if the center point was relocated, the effect of the Auto Level tool becomes visible.
Although Autopano Pro tries it’s best to produce acceptable results, it does not always succeed:
- if the rotation axis of the camera wasn’t vertical when going from one shot to the other (handheld shots, tripod axis or panohead axis not well set) it is possible that parts of the resulting panorama will not be straight.
- if all the pictures are leaning in the same direction (with a panohead this should not arise, with a tripod it's a difference between two settings), the whole panorama will not be straight.
In both cases the Set Verticals tool should be used to straighten everything up.
This tool will allow you set the framing of the panorama. It works in the very same way as the Adobe Photoshop crop tool except that the original setting is automatically set to the maximum rectangular surface of the panorama. None of the rectangle’s surface is left empty; the automatic framing is quick and precise. Note: there's an exception to this rule (see example), when there's a small empty area near the edge of the panorama. Autopano Pro assumes such "holes" are easy to fix with Clone Stamp or other means and leaves them inside the final crop.
You can move the rectangular marquee by placing the mouse pointer inside the marquee, and with a click/hold action move it over the surface of the image.
You can rotate the rectangular marquee by placing the mouse pointer outside of the marquee area, and with a click/hold action rotate it over the surface of the image.
Hit the Enter key or double click inside the marquee to validate your selection.
Use the Auto Level tool to cancel your changes.
When you press this button, the produced effect is the exact opposite of the one produced by the Crop tool: the panorama is included in a rectangle as big as needed to include all the pixels of the panorama.
If you previously used the Crop tool, then modified the panorama’s geometry, by let’s say applying a rotation, this tool is useful to recover the areas that are now excluded from the image.
As long as this button stays down, the totality of the resulting image will remain visible, whatever you do.