Halcon 20.11: precautions for use of calibration assistant
2021-08-08 00:42:05 【Machine vision 001】
HALCON 20.11： Precautions for using the calibration assistant
- The camera needs to be held during calibration ( aperture 、 The focal length 、 Postures ) Fix .
- There are two types of calibration plates ： Hexagonal alignment calibration plate (.cpd file ) And rectangular alignment calibration plate (.descr file ).
- The transparent calibration plate can be used in applications requiring backlight .
- The hexagonal alignment calibration plate should fill the entire image , The rectangular alignment calibration plate should fill a quarter of the image .
- During calibration , The first thing that needs to be improved is the image quality .
- Calibration assistant , Camera parameters refer to camera internal parameters , Like focal length , Pixel size ( wide / high ) etc. ; External parameters refer to the position and rotation angle of the camera relative to the world coordinate system .
- At initial calibration , Full calibration can be selected directly (Full Calibration). When fully calibrated , Telecentric lens does not need to set focal length size , Just set the magnification ; The standard lens needs to set the focal length size .
- The reference image refers to the image positioning world coordinate system , Its origin is located at the origin of the calibration plate in the reference image . The origin of the calibration plate is the center of the center mark of the first addressing mode of the hexagonal alignment calibration plate ; Or the center of the rectangular alignment calibration plate . By default , The first calibration image is used as a reference image . however , You can also select other images in any calibration sequence .
- Calibration assistant , A standard lens is a transmission projection lens similar to the human eye , That is, the farther away the object is , The smaller in the image ; A telecentric lens is a parallel projection lens , That is, the size of objects in the image is independent of their distance from the camera .
- Hexagonal alignment calibration plates are usually marked with light color on a dark background . however , Calibration plates with dark markings on a light background are also available , At this point, just select a file name to “_dark_on_light” The description file at the end .
- Rectangular alignment calibration plates are always printed with dark marks on a light background .
- Function of calibration plate thickness parameter ： The world coordinate system and the position of the measurement plane located below the surface of the calibration plate can be modified by referring to the thickness value in the image .
- Face scan in camera model ( division )(Area Scan (Division)) You can usually return good results , You can also use face scan ( polynomial )(Area Scan( polynomial )) Camera models improve accuracy and reduce error rates . therefore , If the average error of the calibration state is too high , We recommend that you use face scan ( polynomial )(Area Scan( polynomial )) Camera model .
- Use face scan ( polynomial )(Area Scan( polynomial )) Camera model , It must be ensured that the calibration plate image completely covers the field of view and does not miss the image edge . The higher average error may be caused by improper calibration images .
- The main work of the calibration process is to obtain the images of different positions and directions of the calibration plate relative to the vision system . Please note that ： The more you change your position and direction , The better the calibration results will be . therefore , When placing the calibration plate , Make it appear in different corners ( For rectangular alignment calibration plates ; The hexagonal alignment calibration plate covers the whole image with a single image ), Appear at different distances from the camera , Appear on different planes （ That is, tilt some images ）. Be careful , For rectangular alignment calibration plates , It is not only necessary to place the calibration plate in the center of the field of view , It also needs to be moved to corners and edges . A good calibration image will significantly improve your calibration results .
- The calibration assistant currently supports 8 Bit and 16 position (' byte ' and ' uint2 ') Images .
- Calibration uses a single channel image . For color RGB Images , The red channel will be used . Color transformation can be done by the operator trans_from_rgb To execute .
- The attitude of the bid plate can be determined according to the reference image , Specify the world coordinate system and the survey plane , For subsequent three-dimensional measurements . therefore , In a calibration image ( Usually the first ) in , You should place the calibration plate at the top of the measurement plane . If not , Place the calibration plate in a position parallel to the measuring plane , And by adjusting the parameter thickness “ Move ” Coordinate system .“ calibration ” The asterisk on the left side of the window indicates the reference image . It is the default setting in the first image . however , You can do this by clicking “ Set as reference pose (Set Reference)” Button , Select another image as the reference image .
- Are you using a very wide-angle lens ? Distortion close to the image boundary will lead to higher average error , It may even cause calibration failure .
- Is the size of your camera chip compatible with the lens ? Using a lens that is incompatible with the camera chip size will reduce the image quality .
- We should always pursue high-quality images . However, if for some reason , Unable to get high quality images , You can adjust some parameters under calibration plate extraction parameters ：
Hexagonal alignment calibration plate ： If the image is blurred or contains strong noise , Should be “ smooth (Sigma)” Set to a higher value .
Rectangular alignment calibration plate ： If the calibration plate tilts strongly ,“ Spacing tolerance (Gap Tolerance)” Should be set to a higher value ; If the image is blurred ,“ smooth (Alpha)” Should be set to a smaller value ; Besides , The maximum marking diameter can be changed .
- Average error （Mean Error） It refers to the average error in pixels during calibration . Once the system has been calibrated , The ideal center of the calibration mark will be calculated , And compare with the real marking Center . The average error is the deviation between the ideal mark center and the real mark center .0.1 Or lower values are considered a good result .
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