Scan Cleaning Tools Tutorial - Detailed description

Index

• General
• The Box Selector
• Selection Options / Mode
• Erasor
• Add Noise
• MLS Smoother
• MLS Spray Can
• Point Relaxation
• Outlier Removal
• Remove false normals
• Resize Object
• Recolor Surfels

General

This tool includes a undo / redo mechanism which is accessible for all actions INSTEAD for the outlier removal. You can perform up to 4 undo's.
Undo key shortcut is "Ctrl + Z", redo is "Ctrl + Y".

All other shortcuts are accessed via "Ctrl + Alt +".

You can activate all the tools with "Ctrl + F2" up to "Ctrl + F11" or simply switch to the corresponding tab without activating that tool with "Ctrl + Alt + F2" up to "Ctrl + Alt + F11",, which is helpful when in Selection Mode.

IMPORTANT NOTE: you can use all the features of the navigation tool while holding the CTRL button pressed. That means, CTRL + Left Mouse Button + Mouse Movement rotates the scene, CTRL + Mid Mouse Button + Mouse Movement moves the scene away from screen and to screen, CTRL + Right Mouse + Mouse Movement translates the scene.

The Box Selector

The box selector is to select, used as basis for all actions. It has 3 modi:

• Align to object (Ctrl + Alt + O): the box selector follows the object's surface, using the z buffer of the OpenGL. If you don't have hardware support in vertex programs it could be necessary to select the "constant point size" with adjusting the point size in the OpenGL Renderer configuration. With the Right Mouse Button pressed and the mouse moved in +/- y direction you can resize the box.
• Free Navigation (Ctrl + Alt + F): the box moves parallel to the screen plane. If you press the Middle Mouse Button (or press the Wheel) and move the mouse in +/- y direction you can move the box orthogonal to the screen plane. Also you can resize the box with the Right Mouse Button, like in Align to Object.
• Picking (Ctrl + Alt + I): here the box doesn't move and cannot be moved. The only purpose of this option is to resize the box and rotate the box. To resize the box along the face normal of a side, click on that side and move the mouse in the desired direction. To rotate the box click on an edge and move the mouse, the edge specifies the axis of the rotation which is parallel to this edge and goes through the center of the box.

Selection Options / Mode ("Ctrl + F2" and "Ctrl + Alt + F2")

While in this mode you can make selections using the box selector by pressing the Left Mouse Button on the regions you want to select.

There are 2 available box shapes to select: box (Ctrl + Alt + 8) and sphere (Ctrl + Alt + 9).

You have also 3 different selection indices to mark with, this can be useful if you want to keep your current selection but do something else on another part of the object. With Ctrl + Alt + 1 until Ctrl + Alt + 3 you can specify your current selection . (You can, of course, also use your mouse).

Also three different mouse positioning modes are available, as described in section "The Box Selector". The shortcuts for the 3 options are Ctrl + Alt + O for Align to Object, Ctrl + Alt + I for Free Navigation, Ctrl + Alt + P for Picking (= modifying the box shape).

With Select All (Ctrl + Alt + A) you can select all surfels, with Deselect (Ctrl + Alt + D) you can deselect all surfels tagged with the Current Selection index.

Note: With the "selected objects box" you can select the objects on which the selection (and other operations) should be applied. By default all objects are selected, but it may occur that you want to modify only a subset of the available objects.



While in Selection Mode, you can perform most operations on the current selection by switching to the desired tab (by using the mouse or pressing the corresponding "Ctrl + Alt + F??" command) and then clicking on the "run on current selection" (or similar) button.

A picture of the tool window:

Erasor ("Ctrl + F3" and "Ctrl + Alt + F3")

In this mode you can delete surfels directly using the Box Selector. Or in the Selection Mode via first selecting a region, then switching to the this tab (and pushing the delete current selection button).

A picture of the tool window:

Add Noise ("Ctrl + F4" and "Ctrl + Alt + F4")

In this mode you can add noise directly using the box selector to the surfels, surfels are translated along their normals. Also possible to perform on current selection.

A picture of the tool window:

MLS Smoother ("Ctrl + F5" and "Ctrl + Alt + F5")

Here you can smooth surfaces (either directly or using the box selector). Smoothing is done via "Moving Least Squares Projection", for technical details refer for example to "Computing and Rendering Point Set Surfaces" of Alexa, Behr, Cohen-Or, Fleishman, Levin & Silva.

The parameters are as follows:

• k next neighbors defines how much neighbours influence the smoothing of the current point
• gauss smoothing factor defines the amount of influence of the neigbours. The weight of the influence of one point is exp(- distance^2 / gauss_smoothing_factor^2). The bigger the factor, the more (global) smoothing is applied.
• alpha defines the amount of projection, that means how much we move to the new computed point. This is done via alpha * new_point + (1 - alpha) * old_point. The alpha value can also take negative values to emphasize the surface instead of smoothing it.
• degree of polynom defines the degree of the fitted polynom. Degree must be > 0. For most applications degree 3 is okay, but it depends on the surface. The bumpier it should be, the greater the degree; the more plane like it should be, the smaller the degree.
• use only planefit for smoothing this equals to a polynom degree of 0, can be used to smooth more.

MLS Spray Can("Ctrl + F6" and "Ctrl + Alt + F6")

This can be used to scatter points on the surface to increase sampling density or to fill holes (and regions where sampling density is too low).
point count defines the number of points scattered (or tried to be scattered, in case of fill holes only).

MLS gauss parameter defines the parameter used for the MLS projection. Refer to MLS Smoother for more information.

It's also recommended to use the MLS Smoother itself on the new points after inserting a certain amount of points. Sometimes the new inserted surfels are going to raise (due to the condition of border of the hole), so you must lower them using the MLS smoother with the planefit option on.

A picture of the tool window:

Point Relaxation ("Ctrl + F7" and "Ctrl + Alt + F7")

This tool is used to redistribute the surfels on the surface, can be used to fill holes by flowing the surfels inside the hole.

The idea is: each surfel is considered as particle, which will repel other particles. So every time step forces acting on each particle are considered, then the particle is translated with respect to this forces. Now the surfel is (probably) away from the surface, so it is projected back to the surface and a new iteration begins. At the end of all iterations, MLS can be applied directly.

It is used as follows: Mark the surfels you want to redistribute (can be done either in Selection Mode and in Point Relaxation mode), but be aware that there must remain surfels unmarked around the selection, which can be used as border surfels. Otherwise the surfels will fly around in space (and time) and move away from the object's surface.

The max. query distance and the force scale are extremly sensitive parameters and vary from object to object and from surface to surface. My suggestion: set the iterations count low, try it. If you experience troubles, undo the operation (Ctrl + Z), adjust the parameters, retry. Detailed paramter description:

• max. neighbours defines the maximal number of neighbours which could be used. Doesn't need to be changed in general (only if you experience performance lacks).
• max. query distance defines the maximal radius of influence on each particle. Should be kept low, is a source of errors if the surfels are goint to fly around.
• iteration number of iterations where the repulsion is applied.
• force scale Should also be kept low, another source of error if surfels are moving too fast or away from the specified region.
• use MLS for projection if checked, MLS is applied at the end of all iterations. (refer to MLS Smoother
• (MLS) use onlu planefit for projection if checked, MLS with only Planefit - Projection is applied at the end of all iterations.

Outlier Removal ("Ctrl + F8" and "Ctrl + Alt + F8")

With this tool you can classify surfels (either the selection or using the box selector) to determine which are outliers, that means that they don't belong to the objects surface. First step is always building the knext graph, which is computed over all surfels of all selected objects. This graph needs to be recalculated if object selection changed, objects added or removed, a lot of surfels where deleted or an undo operation was applied since the last build. Note that actions performed by this tool CANNOT BE UNDONE.

After building, you can classify the desired surfels either using the box selector or on the current selection.

After classifying, adjust the weights of the 3 method and either adjust the percentage of surfels to delete (adjust values automatically) or adjust the values manually.

To adjust the values manually: in the title (i.e. Miniball, max = 1.5) the maximum found value is set. This represents the biggest value found during classification. The lower the cutoff value you specify in the edit box below the title, the more surfels will be deleted. This cutoff value is usually a lot smaller than the found biggest value (i.e. for Miniball with max = 1.5 use cutoff = 0.1).

After adjusting the weights and cutoff value, click on the Preview button to preview the surfels which will be deleted. This surfels will appear in red, all surfels which will not be deleted in white. To decide if the chosen setting is a good one, also use the Main Renderer (with Renderer Option "Two sided normals" on), because sometimes here you see better which surfels are going to be deleted.

If your not satisfied with the settings, readjust them and click again on the Preview Button.

If your satisfied with the settings and want to delete the surfels, click on the Delete Button. (Colors will be restored after deletion).

IMPORTANT!! Before you rebuild the graph or do classification on another region or do another action like deletion be sure to click the Restore Colors button.

Another note: if you switch to another tab, do nothing and switch back to the Outlier tab you can click the Skip Rebuild button - but you have to be shure that only a few surfel changed.

The methods itself

• Miniball computes the smallest enclosing sphere of the knext neigbhours, discarding the 20% nearest ones. Then it compares the diameter of this sphere to the distance of the testing point to the center of the sphere.
• Planefit fits a plane throung knext neighbours, compares the distance of the testing point to the distances of the knext neighbours. Also uses the variation as confidence for this classification (the greater the variation the worse the planefit)
• Bidirectional compares the number of neighbours of the testing point, which have the testing point also as neighbour to them that haven't.

Remove false normals ("Ctrl + F9" and "Ctrl + Alt + F9")

This tool removes bad normals. For this select a small area which doesn't contain false normals, then adjust knext and allowed angle deviation and remove them. Note that this action is performed on all surfels of the selected objects.

To determine wether false normals are there or not, switch from preview renderer to main renderer and back, because the opengl renderer shows also normals pointing away from screen (in black) which the surfel renderer doesn't.

The more left the slider, the more surfels will be removed. Try to adjust the slider incrementally to the left, so you don't delete to much surfels.

A picture of the tool window:

Resize Object ("Ctrl + F10" and "Ctrl + Alt + F10")

This tool readjusts the center and the bounding box edge length of all objects and all surfels.

Is necessary when the object isn't exactly positioned in the middle or the edgelength is too big / too small.

A picture of the tool window:

Recolor Surfels ("Ctrl + F11" and "Ctrl + Alt + F11")

With this tool you can recolor surfels (either directly or using the box selector).

With the first option you can assign the desired surfels a uniform color, specified in RGB from (0..1). With the second option you can filter the colors. The knext neighbour's colors are inspected and weighted with a gaussian exp(-distance^2 / coefficient^2) weight. If the coefficient is 0.0 all surfels are weighted equally.

A picture of the tool window: