讲义文稿视觉blob

1、Session 6BlobObjectivesThe student will correctly:Identify applications where a Blob tool may be part of a vision solutionCreate and configure a blob tool thatFinds blobs in a designated grey-level rangeFilters blobs based on given criteriaBlob OverviewBlob analysis is the detection and analysis of

2、two-dimensional shapes within an imageBlob finds objects by identifying groups of pixels that fall into a user-defined grey-scale rangeBlob reports many properties:AreaCenter of MassPerimeterPrincipal Axescenter of mass (CM)principal axes (PA)extremaCMPAextremaWhen to Use BlobBlob analysis is well-s

3、uited for applications where:Objects vary greatly in size, shape, and/or orientation (Difficult or impossible to train a model)Objects are of a distinct shade of grey not found in the backgroundObjects are not overlapping or touching Sample applications:Inspect for number, size, and shape of dispens

4、ed epoxy dotsInspect for correct position and size of ink dots indicating bad wafer diesInspect for fragmentation and size of pharmaceutical tabletsSort or classify objects according to their size, shape, or positionSegmentationThe first thing Blob does when it runs is image segmentation, determinin

5、g which pixels are blob pixels and which are background pixelsThere are several modes to specify what separates blob from background pixelsBlob pixelsBackground pixelsSegmentationMost segmentation modes will require:PolarityDark blobs on lightLight blobs on darkThresholdThe value(s) that separate bl

6、ob pixels from background pixels Fixed ThresholdingIn Fixed Thresholding, the division between blob pixels and background pixels is determined by grey values.Set a grey-level threshold:blobbackgroundgrey value threshold= 140pixelsgrey-values0255Relative ThresholdingRelative thresholds are expressed

7、as percentages of the total pixels between the left and right tailsTails represent noise-level pixels that lie at the extremes of the histogramImage:5% of pixels with5% of pixels withHistogram:lowest valueshighest valuesLeft tail pixel valueRight tail pixel valueThreshold pixel value40% ofUsing Rela

8、tive ThresholdsRelative thresholds adjust for linear lighting changesThreshold = 30Threshold = 100Threshold = 14040% of40% of40% ofFixed vs. Relative ThresholdingFixed grey-level thresholds do not modate linear lighting changes16080Pixelvalue200120Pixelvalue9010PixelvalueDark image:Average image:Lig

9、ht image:Threshold = 100Threshold = 100Threshold = 100Fixed vs. Relative ThresholdingFixed is faster than relative because the grey levels corresponding to the percentages do not have to be computedFixed thresholding can test for absence of a feature in a scene, whereas relative thresholding will al

10、ways find a blob in the scene0backgroundobject255grey level with weight 1.0left tailgrey level with weight 0right tailHard ThresholdingThe examples so far have all used Hard ThresholdingOne value (grey level or percentage) divides blob pixels from background pixelsApply threshold value =/p>

11、010080PixelvalueExamine a histogram to determine the threshold grey valueblobbackgroundgrey value thresholdHard ThresholdingSpecify single percentage & tailsThreshold dynamically chosen; good for images with bimodal distribution of grey valuesSpecify single grey valueSpatial Quantization ErrorOccurs

12、 with hard thresholding when the object falls differently on the pixel grid from image to imageMay result in erroneous results for blob size, perimeter, and locationError es more pronounced as the perimeter of the object increases Pixels in object = 64 Pixels in object = 81 Pixels in object = 44 Pix

13、els in object = 25Pixel WeightingSpatial Quantization Error can be eliminated by applying pixel weightingAs the blob moves relative to the pixel grid, the total weight remains the sameSoft ThresholdingCreate a pixel weighting scheme by using soft thresholdingSoft thresholding uses a range of thresho

14、ldsThresholding for dark object on light backgroundThresholding for light object on dark backgroundSoft ThresholdingSoft Thresholding exampleLow Threshold = 50HighThreshold = 65Softness = 301.00.250.500.7550556065Grey ValuesWeightingThresholdHigh ThresholdSoft ThresholdingUses grey values for thresh

15、oldsUses percentages for thresholds and tailsUsing a Subtraction ImageUse a Subtraction Image when the image consists of similar background and blob grey valuesThe threshold image contains only background informationEvery pixel in the image that differs from the corresponding pixel in the threshold

16、image by a specified amount is a blob pixelSubtraction ImageImage to SegmentSegmented ImagePixel MappingUse a pixel map (lookup table) for images that cannot be segmented with hard or soft binary thresholdsRequires a scaling factor which gets applied to the pixel map valuesPixel MappingSupply an out

17、put value for each grey valueConnectivity AnalysisAfter segmenting the image, Blob performs Connectivity AnalysisWhole Image blob analysis returns one result for all blob pixels in the imageGrey Scale analysis identifies discrete, connected blobsWhole Blob AnalysisGrey Scale AnalysisConnected-Blob A

18、nalysisObject pixels must be eight-connectedConnected vertically, horizontally, or diagonallyBackground pixels are four-connectedConnected vertically or horizontally onlyHow many blobs are in this Image?Applying Morphological FiltersFirst choose the filter(s) from the pull-down listOrder matters!To

19、reorder or delete an operation, use the buttons in the dialogPruning and FillingPruning ignores, but does not remove, features which are below a specified sizeFilling fills in pruned features with grey values from neighboring pixels on the leftHoles still exist,but are not reportedHoles are filled i

20、nRegionBy default, the blob analysis is done on the entire imageTo only detect blobs in a portion of the acquired image use a Region ShapeMay graphically position and size on the Input ImageMeasurementsAllows you to specify measurements calculated on each blobMeasurementsFor each selected measuremen

21、t, choose:GridRuntimeFilterMeasurementsUse Filter to exclude blobs outside a certain range for any propertyOr include only in a certain rangeMeasurementsResults may be sorted in order for any of the selected measurementsAscending or descending orderGraphicsChoose to display Result or Diagnostic graphicsRemember that graphics add timeResultsNIndex of the blobIDA unique blob identification number independent of sorting criter