Bildgruppe

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Die Bildgruppe ist eine Besonderheit von {{metigo|3D}, um Stereoprojekte für einen Bildverband zu generieren und Verknüpfungspunkte in diesem Bildverband zu finden. The Image group is a special features of metigo® 3D to generate stereo projects for a image sequence and find tie points in that image sequence.

Generate Image Groups

To generate an image group go to the node Source Images in the orientations tree and right-click on the node. Select the function Generate Image Groups... from the prompted context menu.

  1. Select the source image to join an image group by highlighting them. Open the context menu and select Generate Image Groups... there.
  2. The Automatic Point Search dialogue will be opened. Tie points in the images are now found automatically. For more detail on parameters and setting, compare Automatic Point Search.
  3. The image pairs used for stereo models are gathered automatically in an image group.

The context menu

The node for an individual image group

  • Rename Image Group...

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  • Remove Image Group...

Remove the selected image group either only from the project or from both project and harddisc.

  • Model Analysis
Analyse all models in the image group and puts them in a sequence for processing the absolute orientation.
  • Relative Orientation of all Models
Recalculates the relative orientation of all models in the image group.
  • Orientate all Models automatically
Calculates the absolute orientation of all models automatically base either on GCPs or a known distance or the camera base.
  • Save all orientated Models
Saves all models which have been orientated. They now will appear below the model management node of the models tree.
  • Sort Models...
Sorts the images of the stereo models to a sequence.

Model Analysis

Tables analysing the models within the image group.

Right-click on an image group at the orientation tree and select Model Analysis to open a table comparing the models.

A sequence of models at the bottom of the prompted dialogue is suggested. This sequence is created by a simple mathematical approach, solely depending on the number of identical tie points in the models and and the area ratio of the convex hull.

The upper part of the table gives the following values:

  • the number of identical tie points in two models are compared. The green number indicates the number of tie points in one model.
  • to create the percentages given, a simple mathematical ansatz is followed. It is based on the following operation:
    • all image point IDs contained in all four images are collected,
    • the area of the convex hull of these points is calculated for each images and its ratio in each image computed;
    • the first percentage value given is the average area ratio of the convex hull of the identical tie points in all four images;
    • for the second percentage value is the overlap of the convex hull in all four stereo image pairs as well as in stereo model are computed; it therefore is a measure of the similarity of the models - the closer 100% the value is the more similar the models are.
Merke Merke: Based on the calculated sequence image coordinates for GCPs only have to be measured for the first image in a sequence. The absolute orientation then will be calculated for that model first and transfered automatically to the other models within the image sequence (compare Absolute Orientation).

However, the overlap of the models one to each other has to be sufficient. For the first model you still have to measure GCPs in both images and recalculate the relative orientation for the selected model where required.

In the example given the image coordinates of GCPs have to be measured twice:

  • in model 3 for the sequence of models 3 - 7
  • in model 2 for the sequence of models 1 -2.

Orientate Image Groups

The relative orientation of the models in an image group is based on the automatic measurement of tie points.

  1. After the tie points have been created you are going to measure a minimum of 3 known points in the first image of each sequence.
  2. Thereafter open the context menu and conduct the following operations to orientate the stereo image pairs:
    • First, redo the relative orientation of the images by selecting the option Relative Orientation of all models. At the relative orientation table you will recognise that several tie points now will be switched off. They were filtered with one standard deviation of the mean.
    • Next, select Orientate all Models automatically and the absolute orientation of all models will be conducted. The algorithm will start with those image pairs where the image coordinates were measured and is transferring the resulting absolute 3D-coordinates of the tie points in this stereo image pair to the others. Now, in the remaining stereo image pairs the absolute orientation will be calculated.
  3. As soon as you have orientated the models absolutely you are going to save them by selecting Save all orientated Models from the context menu. By saving the following nodes will be created:
    • a stereo model below the node model management of the models tree for every stereo image pair. The file *.STM contains all relevant information about the stereo model.
    • an orientated image below the node orientated individual images for all images involved in the stereo models. Those images used for more than model are contained several times for every model. The computed parameters of exterior orientation are saved in the file *.ORI. Those of the transformation parameter of the image coordinate system in the file *.ATR.
    • a base model for further point cloud generating. This is saved below the node seed growing matching of the DSM tree. An binary file *.EAS containing the base information such as the tie points used for relative orientation and which will now be used as seed points for the seed grown matching algorithm.

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Achtung Achtung: Always check if the resulting coordinates for absolute orientation are within the range of object coordinates. If not you might have to redo the absolute orientation. This problem might have been caused by several errors.
  • The image coordinates for one of the GCPs have been measured faultily in one or both images.
  • A common error which might occur when conducting the final absolute orientation of an image group is that it has been previously orientated absolutely without using GCPs, but the camera base given in tool window Relative Orientation or distances at the Absolute Orientation tool window.
    In this case metigo® 3D will set the object coordinate system to the value calculated for the model coordinates in the first image of each sequence and uses the preset 1 m as camera base.

Merke Merke: To analyse the problem open the list of GCPs by pressing the button 3DIconDisplay Coord.png on the symbol bar and scroll down to the end of the list. Those point numbers with a prefixed C belong to tie points. If they are off the range of the GCPs they have to be removed. Thereto go to tie points node of the orientation tree and select Delete resulting 3D-Coordinates from its context menu. The 3D-coordinates now should be deleted from the GCP-list.

Now, you can continue and redo the absolute orientation by selecting from the context menu of you image group first Relative Orientation of all Models and then Orientate all Models automatically.