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countEachLabel

Counts number of pixel labels for each class

Since R2021a

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    Syntax

    counts = countEachLabel(bimds)
    counts = countEachLabel(___,Name,Value)

    Description

    counts = countEachLabel(bimds) counts the occurrence of each pixel label in all the blocks represented by the blocked image datastore bimds.

    example

    counts = countEachLabel(___,Name,Value) specifies additional name-value arguments.

    If bimds contains categorical data, countEachLabel obtains the class names from the categories specified in the InitialValue property of the first blocked image. In this case, do not specify the Classes and PixelLabelIDs name-value arguments. If bimds contains numeric data, you must provide values for the Classes and PixelLabelIDs name-value arguments.

    example

    Examples

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    Open Live Script

    Create a blocked image from a sample label image.

    bim = blockedImage("yellowlily-segmented.png",BlockSize=[512 512]);
b = bigimageshow(bim);
showlabels(b,bim)

    Figure contains an axes object. The axes object contains an object of type bigimageshow.

    Create a blocked image datastore from the blocked image.

    bimds = blockedImageDatastore(bim);

    Count the labels in the blocked image datastore. Labels 0 and 3 both map to "Background".

    countEachLabel(bimds, ...
      Classes=["Background" "Flower" "Leaf" "Background"], ...
      PixelLabelIDs=[0 1 2 3])
    ans=3×3 table
        Name        PixelCount    BlockPixelCount
    ____________    __________    _______________

    "Background"    2.3706e+06      3.1457e+06   
    "Flower"        4.3349e+05      1.5729e+06   
    "Leaf"          3.4159e+05      2.0972e+06
    Open Live Script

    Load pixel label data.

    load("buildingPixelLabeled.mat");

    Create a blockedImage to manage the pixel label data.

    blockedLabeledImage = blockedImage(label,BlockSize=[200 150]);
bigimageshow(blockedLabeledImage)

    Figure contains an axes object. The axes object contains an object of type bigimageshow.

    Create a blockedImageDatastore that reads blocks of size 200-by-150 pixels at the finest resolution level from blockedLabeledImage.

    blockLabelDS = blockedImageDatastore(blockedLabeledImage);

    Count the number of pixel labels for each class.

    tbl = countEachLabel(blockLabelDS)
    tbl=4×3 table
       Name       PixelCount    BlockPixelCount
    __________    __________    _______________

    "building"    1.8036e+05        4.5e+05    
    "grass"            32983        1.5e+05    
    "sidewalk"         10491        1.5e+05    
    "sky"              81525        1.8e+05

    Balance the classes by using uniform prior weighting.

    prior = 1/height(tbl);
uniformClassWeights = prior ./ tbl.PixelCount
    uniformClassWeights = 4×1
10-4 ×

    0.0139
    0.0758
    0.2383
    0.0307

    Balance the classes by using inverse frequency weighting.

     totalNumberOfPixels = sum(tbl.PixelCount);
 freq = tbl.PixelCount / totalNumberOfPixels;
 invFreqClassWeights = 1./freq
    invFreqClassWeights = 4×1

    1.6931
    9.2580
   29.1067
    3.7456

    Balance the classes by using median frequency weighting.

    freq = tbl.PixelCount ./ tbl.BlockPixelCount;
medFreqClassWeights = median(freq) ./ freq
    medFreqClassWeights = 4×1

    0.7743
    1.4114
    4.4373
    0.6852

    Input Arguments

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    Blocked image datastore, specified as a blockedImageDatastore object.

    Name-Value Arguments

    Specify optional pairs of arguments as Name1=Value1,...,NameN=ValueN, where Name is the argument name and Value is the corresponding value. Name-value arguments must appear after other arguments, but the order of the pairs does not matter.

    Example: countEachLabel(lbimds,Classes=["Background","Flower","Leaf","Background"],PixelLabelIDs=[0,1,2,3])

    Before R2021a, use commas to separate each name and value, and enclose Name in quotes.

    Example: countEachLabel(lbimds,"Classes",["Background","Flower","Leaf","Background"],"PixelLabelIDs",[0,1,2,3])

    Class names, specified as a string array or a cell array of character vectors.

    Example: ["Background","Flower","Leaf"]

    Data Types: char | string | cell

    Values for each label, specified as a numeric array of values with the same length as Classes. This name-value argument provides the mapping from numeric values to the label class.

    Example: [0 1 2 3]

    Data Types: single | double | int8 | int16 | int32 | int64 | uint8 | uint16 | uint32 | uint64

    Use new or existing parallel pool, specified as a numeric or logical 0 (false) or 1 (true). If no parallel pool is active, then countEachLabel opens a new pool based on the default parallel settings. This argument requires Parallel Computing Toolbox™.

    Data Types: logical

    Output Arguments

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    Counts of the occurrence of each pixel label in all blocks represented by the blocked image datastore, returned as a table that contains three variables.

    Pixel Count VariablesDescription
    NamePixel label class name
    PixelCountNumber of pixels of a given class in all blocks
    BlockPixelCountTotal number of pixels in blocks that have an instance of the given class

    Tips

    You can use the label information returned by countEachLabel to calculate class weights for class balancing. For example, for labeled pixel data information in tbl:

    • Uniform class balancing weights each class such that each contains a uniform prior probability:

      numClasses = height(tbl)
prior = 1/numClasses;
classWeights = prior./tbl.PixelCount

    • Inverse frequency balancing weights each class such that underrepresented classes are given higher weight:

      totalNumberOfPixels = sum(tbl.PixelCount)
frequency = tbl.PixelCount / totalNumberOfPixels;
classWeights = 1./frequency

    • Median frequency balancing weights each class using the median frequency. The weight for each class c is defined as median(imageFreq)/imageBlockFreq(c), where imageBlockFreq(c) is the number of pixels of a given class divided by the total number of pixels in image blocks that had an instance of the given class c. 

      imageBlockFreq = tbl.PixelCount ./ tbl.BlockPixelCount
classWeights = median(imageBlockFreq) ./ imageBlockFreq

    Version History

    Introduced in R2021a

    See Also

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