What is Object-Based Image Analysis?

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Kirk Benell

What is an object?

• An object is a region of interest with spatial, spectral (brightness and color), and/or texture characteristics that define the region

Object-Based Image Analysis

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region

• Pixels are grouped into objects, instead of single pixel analysis

• May provide increased accuracy and detail for classification purposes

Object-Based Image Analysis

Traditional pixel-based classification• Based on reflectance values of pixels• Works for low and medium resolution imagery• Works for mass area-based features• Multispectral or hyperspectral imagery

Limitations of pixel-based analysis

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Limitations of pixel-based analysis• Only spectral, seldom spatial and contextual• Results with inconsistent salt-and-pepper noise • Inaccurate borders for texture computation• Limited extraction of small-scale objects

Pixel-based Classification

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Group materials based on their reflectance response per pixel

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Object-Based Image Analysis

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• Group contiguous pixels into objects

• Objects are classified into feature classes based on their spatial, textural and spectral attributes

ImagePixels

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Objects

• Greater accuracy from input: tone, color, texture, shape, size, orientation, pattern, shadow, situations

• Advanced visualizations: Computer vision technique using image segmentation

• Use homogeneous regions as basic analysis elements

• Additional spatial, contextual and semantic information

Object-Based Image Analysis

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• Additional spatial, contextual and semantic information

ENVI Feature Extraction• Uses an object-based approach to classify imagery

• The ENVI tool provides an easy to use method for extracting information from panchromatic, multispectral, hyperspectral, and elevation data

• Vehicles

• Buildings

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• Buildings

• Transportation

• Natural Features

Needs for Feature Extraction• Increased availability of high-

resolution images• Manual digitization, labor intensive• Semi-automated solution is highly

desired

ENVI Feature Extraction

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desired

Applications• Defense and Intelligence• Geographic Information Systems• Transportation• Urban planning and mapping

Workflow:• Spectral/spatial/texture attributes• Object-based fuzzy rule-based

classification• Object-based supervised

classification

ENVI Feature Extraction

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Preview Window for instant feedback prior to processing an entire image

Post-Classification Vector Tool• Centerline extraction• Snapping, smoothing• Vector editing

ENVI Feature Extraction

Input Data

Image Segmentation

Attribute Computation for Object Primitives

Object Generation

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Rule Base

Decision Making

Feature Selection

Supervised Classification

Extracted Features/Classes

Object-Based Classification

Image Segmentation Scale Level

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A low scale level provides moresegments in the final processed image

A high scale level provides fewersegments in the final processed image

The Preview Window provides on-the-fly feedback for the selected Scale Level

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Segmentation scale level = 50

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Under segementedscale level = 70

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Over segmentedscale level = 30

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Merge to aggregate adjacent segments

Select Classification Method• Select Classify by

Selecting Examples to select training data and perform a supervised classification

• Select Classify by Creating Rules to select attribute

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select attribute parameters to perform a classification

• Select Export Vectorsto export without performing a classification

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View attributes to characterize feature of interest

Create rules to define features of interest

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ENVI Feature ExtractionSpatial Attributes

• Region area, length, compactness, convexity, solidity, form factor, rectangular fit, roundness, elongation, main axis direction, axes length, number of holes, hole/solidity ratio

Spectral Attributes• Band minimum, maximum, average and standard deviation

Texture Attributes

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Texture Attributes• Variance, range, mean, and entropy

Color Space and Band Ratio• Hue, saturation, intensity, NDVI, NDWI, other ratios

Preview classification results and adjust training data on-the-fly

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Export features as one or individual vectors

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View Feature Extraction Report

• View parameters used and statistics of exported vectors

• Save as a text report to share with colleagues

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colleagues

• Edit vector properties

• View Attribute Information

• Square-up building sides

• Smooth vectors

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• Push data into ArcMap for further analysis and vector editing

• Add imagery and new vector layer to GIS database

Thank You

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