Part 3: Fine-tuning with PTGui

The PTGui Project Assistant has now created some control points for you. We now have to check if the control points are correctly set.
Before we begin, we will define an anchor image. I hope you are a little familiar with setting a correct horizon. If not, you may read the German article "Entzerrung und die unverstandene Horizontlinie" (sorry - no English - anyone who wants to translate it?)

Defining an anchor image

Luckily, our example is a panorama inside a church - We have enough vertical lines for our needs. We will use two columns to tell PTGui that they are perpendicular:
Click on the tab "Control Points" and let both control point windows show the image number 0. This will also tell PTGui that you want to define vertical lines!
Here is a screenshot of PTGui how it may look after defining a vertical line:

Change to the [Optimizer}-tab.
Click on "Advanced" and unset all checkmarks except "Pitch" and "Roll" for image 0, also "Use control points of Image 0".

Click the button "Run Optimizer".
After a short time you will see a message box telling you the optimization would be "Too good to be true".

We now have our "anchor image". Do NOT move/rotate this image any more unless you know what you're doing! (even the optimizer MUST NOT touch the position of this image any more!)

To understand the positioning of the control points, I use the comparison of a "lever force": both defined vertical will try to "pull" your image to a position where they are as perpendicular as possible.
Optimizing the rest of the control points

We will now optimize the rest of the images as well as the "Lens Parameters"

To achieve a mathematical correct optimization, you will need at least one pair of control points for every image pair of your panorama.
Also you should not optimize "FOV", "a", "b" and "c" separately.
We have 7 checkmarks (Yaw, Pitch, Roll, FOV, a, b, c), so we need at least 7 control points per image pair.

Now you have to kind of "invert" your checkmarks. Don't forget: don't move image 0 any more, but you must adapt the rest of the images now.

We will do the optimization step by step. So we won't touch the lens parameters now (with a good reason).

The optimizer will finish very fast, but the result will be "very bad" and it will definitely look very bad:

What happened?

PTGui calculates control points at identifiable structures. In our case we have a lot of architectural elements which look very similar. We have to remove the control points on similar - but not same - structures.
They can be found very easily: Go back to "Control Points". Look at images that do not overlap (e.g. image 0 and 2). Delete the control points there.
To see all other control points that are questionable, there is a useful little feature of PTGui: In the "Control Points" tab you will find a button "Table" in the lower area of the window. This will list all points in your panorama and the current distance:

Double-click on the respective line to see the control points in your image. There you can decide if the control point is set correctly or not. Repeat these steps until you think you have found most of the wrong control points.

Now you can restart the optimizer and recheck the above table. You may repeat the steps until you are content with the optimization.

If you get strange results from the optimizer at a later step - come back here and recheck your control points. Most problems are the result of a wrong control point!

We will now add some points manually in important areas of the image and critical details. Also we will add 1 or 2 control points at the top and bottom of the images. This area is normally somewhat neglected by the Project Assistant...

Finally you can add the checkmarks for the lens parameters "FOV, a, b and c" and optimize again.

Check the result in the "Editor Window" and add some points where the images won't fit nicely.

If you took your photos without obeying the nodal point rule, I strongly discourage setting control points in areas with near and far objects. Because of the parallax a good solution will be impossible!

A good strategy is setting control points only at objects that are at the same distance from the camera.

Now we can try our first preview of the panorama. In my case i use the FSPViewer"PixScreen Lite" as equirectangular viewer. This is something you can determine in the options of PTGui:

A minimum size for the preview of 1200x600 is recommended. The new "fast transform" option of pano12 and a fast computer will also allow fast previews up to 3000x1500.

After some calculation time the "Image Properties"-window of PixScreen will appear. Choose the "Scene Type" Spherical and activate the box "Full Revolution"

You can now judge the final quality of your panorama for the first time. If you are not content, go back to PTGui and try to add/change/delete some control points for a better result.

Don't be too anxious about small image errors. We have one tool remaining: Enblend/Smartblend.
The only important features in the image are straight lines which at least have to cross themselves when spanning over more than one image.

The correct optimization lays the ground for a good panorama. Take some time for this step as it will help you saving a huge amount of time when assembling the images.

Special Case Zenith-Image:

If you don't have a tiltable panorama-rig and you have shot the single images freehand, the Zenith-image may heavily deviate from the rest of the images. In this case, we may achieve better results by using an advanced function of PTGui: individual lens parameters for a single image.

Change back to [Lens Settings] and activate the Option "Use individual lens parameters for: Image 4 (our zenith-image)". Go back to the optimizer and re-optimize. Now the zenith-image won't influence the correct alignment that much than before...

If you have no identifyable features in your image: After dragging the image upwards, enter the (rough) yaw of the image in the image property window. This is the degree reading of your panoramic head (if you used one). A rough alignment will be sufficient. You can do a smooth blending with Enblend or with the layer brush in Photoshop.
If you need further assistance in using Enblend to stitch a smooth zenith or nadir, you may look at this article in the Panotools Wiki: How to use enblend for patching zenith and nadir images


Final Step: Creating the Panorama and exporting to QTVR