Hugin Control Points tab
Control points are central to Panorama Tools and hugin, because they are used to estimate the position of image position and lens parameters described above. A control point specifies a corresponding point between two images. Using these corresponding points, the optimizer can estimate the image position and lens parameters. It is therefore important that the control points are accurate and usually at least 3 well distributed control points should be used to estimate the image position (yaw, roll and pitch) and maybe the HFOV. For accurate estimation of the a,b,c distortion parameters, many well distributed control points, and a large overlap (up to 50%) are required.
The Tab consists of two image displays and tab bars to switch images to be edited. The bottom contains a list view where Points can be selected and some fields to edit a selected point. Points can also be selected by clicking or dragging on them in the images. It is possible to zoom out to show the full image.
Adding a control point works by selecting one point in the left or right image, and then clicking onto the corresponding point in the other image. If auto add is not set, the points can be moved by clicking at some other place in the images. They are added to the list of control points by pressing the right mouse button, the "a" key or by pushing the Add button. If you press the right mouse button when only one point is selected, the point selection will be aborted. auto add adds the control point as soon as both points have been specified.
If the images are zoomed out (fit to window), the first click zooms to a temporary 100% view to give you the chance to refine your selection. Note that only the second click will trigger the auto estimate.
Often it is hard to select the corresponding point exactly. However, the control points should be as accurate as possible, for good results. Once a point pair has been roughly selected, the fine tune function of hugin can be used to estimate the corresponding point up to one tenth of a pixel. The keyboard short cut for the fine tune function is the f key. Fine tune only search in a small neighbourhood of the currently selected points. The size of this neighbourhood can be controlled by opening the preferences panel and setting the "Local area search width".
Note that the fine tune function estimates the translation of the patch around the point selected in the other image with respect to the current image. This works well if the rotation between the images is small and narrow angle lenses have been used. If wide angle or fisheye images are used, rotation search should be activated in the preferences panel. Then hugin also searches for rotated occurences of the patch around the selected point.
The image can be scrolled by pressing the middle mouse button or the CTRL key while moving the mouse. If the shift key is pressed instead, both images will be scrolled. This is very useful, if control points are set using the 100% zoom level.
Control point creation is also influenced by the following checkboxes:
- auto fine tune hugin helps you to find the second point by looking for it in a search region (shown by a rectange around the cursor). This might not always work, but usually is reliable, if the image distortions are not too big. Try and play with it.
- auto add A control point is automatically added when both points are know. You won't have time to refine the selection before adding the point.
- auto estimate Tries to estimate the position of the second point by estimating the translation between the two images. This is very crude and probably only works for single row panoramas created from rectilinear images.
All these flags can be combined. I typically use auto fine tune and auto estimate at the same time. Then hugin usually automatically selects the second point correctly, at least for normal, rectilinear images that are not rotated too much.
hugin also includes an experimental control point creation algorithm. It can be invoked by pressing the g key. Corners in the currently selected image are detected, and corresponding control points are set, based on the
Here is a summary of the keyboard shortcuts available in the Control Point tab:
- a add a new point that has been selected in both images, and the auto add is switched off.
- cursor keys scroll image under the mouse cursor
- shift + cursor keys' scroll both images at the same time
- f fine tune currently selected control point pair. Same as the Fine Tune button
- g experimental control point generation algorithm.
- Del Remove currently selected control point.
- 0 Zoom out to full view.
- 1 100% view.
Mouse function Function
- control key + mouse movement Scroll image under cursor
- shift key + mouse movement Scroll both images
- left button Use left mouse button to select new points or drag existing points.
- right mouse button Add control point, if auto add is switched off
- middle mouse button Scroll image under cursor
- shift + middle mouse button Scroll both images