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Changes between Version 12 and Version 13 of Tutorial/GeometricCalibration

Timestamp:: Apr 14, 2014, 2:06:26 PM (10 years ago)
Author:: sommeria
Comment:: --

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Tutorial/GeometricCalibration

-                      v12
+                      v13
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 = [wiki:Tutorial] / Geometric calibration =
 == Simple scaling ==
 Open again a test image in 'UVMAT_DEMO01_pair'.(accessible on http://servforge.legi.grenoble-inp.fr/pub/soft-uvmat/)
+We shall use the diameter of the half cylinder visible on the upper let of the image to set the calibration. Its physical diameter
+is 2cm. The corresponding diameter in pixels can be obtained with the ruler displayed by the menu bar '''Tools/ruler''' of uvmat.
+We shall use the diameter of the half cylinder visible on the upper let of the image to set the calibration. Its physical diameter  is 2cm. The corresponding diameter in pixels can be obtained with the ruler displayed by the menu bar '''Tools/ruler''' of uvmat.
 First zoom on the cylinder to optimize the precison. Select '''zoom on''', press the left mouse button and adjust the field with the directional key board arrows. Then unselect '''zoom on''' to allow for other mouse actions (otherwise zoom has priority). It is also useful to increase the contrast at the cylinder edge by setting '''MaxA''' to 100 in the frame '''Scalar''' (right side of uvmat).
 Then select the menu bar '''Tools/ruler''', press the left hand mouse button on the cylinder edge, draw a diameter keeping the mouse pressed, release it on the opposite edge. The length in pixels, 140, is displayed, so the scaling factor is 140/2=70 pixels/cm.
+Then select the menu bar '''Tools/ruler''', press the left hand mouse button on the cylinder edge, draw a diameter keeping the mouse pressed, release it on the opposite edge. The length in pixels, 140, is displayed, so the scaling factor is 140/2=70 pixels/cm.
 Open the menu bar '''Tools/geometric calibration'''. A new GUI '''geometry_calib''' appears on the right side. Activate the upper menu bar '''!Tools/Set scale''' on this GUI and introduce the value 70 in the edit box which pops up, and validate with '''OK'''. A set of calibration point coordinates appears in the table '''[!ListCoord]''' of the GUI.
+Open the menu bar '''Tools/geometric calibration'''. A new GUI '''geometry_calib''' appears on the right side. Activate the upper menu bar '''!Tools/Set scale''' on this GUI and introduce the value 70 in the edit box which pops up, and validate with '''OK'''. A set of calibration point coordinates appears in the table '''[!ListCoord]''' of the GUI.
 To see the calibration points on the image, first display the whole image by unselecting '''fix''' (tag '''[!CheckFixLimits]''') in the frame '''Coordinates''' of uvmat. Then press '''PLOT PTS''' in '''geometry_calib'''.
+To perform the calibration, press '''APPLY''', first with the default option 'rescale' in '''calib_type'''.  The image is now displayed in phys coordinates. A xml file 'images.xml', containing the calibration parameters and reference point coordiantes, has been created in the folder 'UVMAT_DEMO01_pair' (it should be identical with the file
+'images.ref.xml' put for reference).
+To perform the calibration, press '''APPLY''', first with the default option 'rescale' in '''calib_type'''.  The image is now displayed in phys coordinates. A xml file 'images.xml', containing the calibration parameters and reference point coordinates, has been created in the folder 'UVMAT_DEMO01_pair' (it should be identical with the file 'images.ref.xml' put for reference). [[BR]]Note that the xml file name reproduces the name of the folder containing the images, so that images from different cameras should not be put in the same folder.
+To apply the same calibration to a set of image folders, use the press button '''REPLICATE''' instead of '''APPLY'''.
 == Translating the coordinates ==
 The origin of the phys coordiantes is now arbitrary. It is more convenient to have it for instance at the centre of the cylinder, now at (x,y)=(1.69, 4.05).
 To get the precise position of the cylindre, it is useful to introduce a projection circle, using in '''uvmat''' the menu bar command Projection '''object/ellipse'''. In '''set_object''', select '''!ProjMode=''''none', so that the line is just used as a marker, without projection operation. Set !XMax=1, !YMax=1, the radius of the circle, and the coordinates (1.69, 4.05) of the centre in the table '''Coord'''. Then press '''REFRESH'''. To optimise the visualisation, zoom in and increase the image contrast to MaxA=100.
+To get the precise position of the cylindre, it is useful to introduce a projection circle, using in '''uvmat''' the menu bar command Projection '''object/ellipse'''. In '''set_object''', select '''!ProjMode=''''none', so that the line is just used as a marker, without projection operation. Set XMax=1, YMax=1, the radius of the circle, and the coordinates (1.69, 4.05) of the centre in the table '''Coord'''. Then press '''REFRESH'''. To optimise the visualisation, zoom in and increase the image contrast to MaxA=100.
+To shift the coordinates, activate in geometry_calib the menu bar command Tools/Translate points, and fills  (x=-1.69, y=-4.05) in the edit box which pops up. the phys coordinates of the calibration points are then shifted, and a new calibration put the cylindre centre at the origin (0,0).
+To shift the coordinates, activate in geometry_calib the menu bar command '''!Tools/Translate points''', and fills  (x=-1.69, y=-4.05) in the edit box which pops up. the phys coordinates of the calibration points are then shifted, and a new calibration put the cylindre centre at the origin (0,0).
 == Calibration with reference points ==
+An alternative method of calibration consists of using a set of reference points whose physical coordinates are known. Open with '''uvmat''' an image in 'UVMAT_DEMO06_PIVconvection/Dalsa1' (accessible on http://servforge.legi.grenoble-inp.fr/pub/soft-uvmat/)
+The more general method of calibration consists in using a set of reference points whose physical coordinates are known. Open with '''uvmat''' an image in 'UVMAT_DEMO06_PIVconvection/Dalsa1' (accessible on http://servforge.legi.grenoble-inp.fr/pub/soft-uvmat/)
 Select in the menu bar '''Tools/geometric calibration'''. Mark the four box corners of the box with the mouse (left hand button). Their coordinates in pixels are displayed in the two last column of the table '''!ListCoord''' in the GUI '''geometry_calib'''. To clear the table for corrections push the button '''CLEAR_PTS''', or for a single line, use the key board backward arrow. To improve the position on the image, use the zoom and directional arrows. We find the coordinates of the four calibration points in pixels:
 …
 (x,y)=(0,0),(58.8,0),(58.8,55.1),(0,55.1), with an origin (0,0) taken at the lower left (and z=0).
 Introduce those in the two first columns of the table [!ListCoord]. This can be conveniently done by copy-paste Matlab vector x=[0 58.8 58.8 0] in the upper line of the x column, and y=[0 0 55.1 55.1] in the y column (use carriage return to validate the input).
+Introduce those in the two first columns of the table '''[!ListCoord]'''. This can be conveniently done by copy-paste Matlab vector x=[0 58.8 58.8 0] in the upper line of the x column, and y=[0 0 55.1 55.1] in the y column (use carriage return to validate the input).
 To perform the calibration, press '''APPLY''', first with the default option 'rescale' in '''calib_type'''. The image is now displayed in phys coordinates. We observe that the rectangular frame is slightly rotated. furthermore the displayed precision, about 3 pixels, is not excellent.
+To perform the calibration, press '''APPLY''', first with the default option 'rescale' in '''calib_type'''. The image is now displayed in phys coordinates. We observe that the rectangular frame is slightly rotated. Furthermore the displayed precision, about 3 pixels, is not excellent.
+To improve the precision we then apply the option 'linear' in '''calib_type''', which seeks a general linear transform, including rotation. The precision is indeed improved to about 1 pixel. The previous xml file has been saved with a ~, ('Dalsa1.xml~') so it can be reverted in case of error.
+To improve the precision we then apply the option 'linear' in '''calib_type''', which seeks a general linear transform, including rotation. The precision is indeed improved to about 1 pixel. The previous xml file has been saved with a ~, ('Dalsa1.xml~') so it can be reverted in case of error.
+== Calibration with a target grid ==
+Most precise and general calibration relies on the use of a target grid. As an example, open in uvmat the image ima_6 in 'UVMAT_DEMO07_GeometryCalibration/Dalsa1' (accessible on http://servforge.legi.grenoble-inp.fr/pub/soft-uvmat/). Open the menu bar '''Tools/geometric calibration '''and''' '''pick four corner points ABCD with the mouse define the periphery of the phys grid selected for calibration. The first point A will define the phys axis origin while AB defines the x axis and AD the y axis. AB and DC should be parallel on the phys grid (see fig). Then select''' Tools/Detect grid '''on the upper menu bar of''' geometry_calib: '''you get a new GUI '''detect_grid''' in which you define (in phys units) the grid mesh and the positions of the  first and last points on each axis. A z position can be defiend as well, do not fill it in this example. The option white markers is selected (by default) indicating that the grid is white (the opposite option would be needed for a grid made of black crosses on a white background). After validation by '''OK''', the detected grid appears on uvmat (see fig).
+If a  point is not correct, select the option '''[!CheckEnableMouse]''' in '''geometry_calib'''. Then you can adjust the point marker by selecting it with the (left button) mouse and moving it while keeping the mouse pressed (when adjustement is finished, nselected the option  '''[!CheckEnableMouse]''' to avoid spurious point creation with the mouse).
+If the grid image is of poor quality, it is alternatively possble to mark all the points by the mouse, using the '''Tools/Create''' grid instead of '''Tools/Detect grid''' in geometry_calib (not convenient in general).
+Once the grid has been marked, the calibration can be performed by the press button '''APPLY'''. We observe that the simple option 'rescale' is not appropriate in this case: a perspective effect is clearly visible, together with a non-linear deformation (grid lines are curved on the image). Therefore select the option '3D_quadr' which applies a 3D projection and quadratic correction. The grid image now appears of good quality in phys coordinates.
+== 3D calibration :  ==
+The previous calibration corrects for 3D projection effects and gives some 3D indication. This is however made more precise by introducing different views of the same grid with different orientations, like shown in mages 1 to 5.
+z position
+'''Merging the images of several cameras:'''
+'''Setting laser slices: '''