Tuesday, 28 October 2014

eCognition tutorial: Almost connected components for clumps identification in eCognition

This post is inspired from a post by Steve Eddins, who works in Math works, a company that build MATLAB. He is a software development manager in the MATLAB and one of the co-author of a book " Digitial image processing with MATLAB".I use both MATLAB and eCognition, so I ponder if this can be done in eCognition. eCognition has basic Morphological Operators like dilation and erosion . Advance MM operators like by opening by reconstruction, connected component labeling or skeleton and many others are not available in eCognition.

The problem of almost connected components:

There is simple synthetic image containing a number of circular blobs.  How can we label and measure the three clumps instead of the smaller circles? Two circles are almost connected if circles are within 25 pixels unit.

Binary circles
Connected components labeling
Almost connected components labeling

Of course it can be solved in eCognition. But for this, you have to  be familiar with many concepts in eCognition. Concepts such as PPO, object variables, multi-level representation and temporary layers are required. My workflow for  the solution is as follows:

  • Use multi-threshold segmentation to get circles
  • Use distance map algorithm to get binary distance map
  • Use chessboard segmentation to get pixel level unclassified objects
  • Use  multi-thresholding segmentation based on the distance map to get  clump
  • Copy level above
  • Use object variable concept to assign each clump a unique ID
  • Convert to sub-objects to get original circle at upper level

I will post rule-set after some time. I have given you enough hint how to proceed. Get your hands dirty !

Almost connected components labelling within eCognition
The concept of “almost connected components” can be applicable in remote sensing for clustering buildings detected in remote sensing images for analyzing of micro-climate of urban areas. There can be various other applications. Can you think of any ?

Monday, 27 October 2014

eCognition tutorial: Image object fusion in eCognition: Example of water bodies classification

eCognition is a powerful software for analysis for remote sensing images. Many people have this false impression that eCognition is all about segmentation. That’s not true. Segmentation is a just a part of it big chain that may involve segmentation, temporary classification, fusion, exploitation of contextual information etc. Many people believe that segmentation should be perfect at first time which is a fallacy. You can modify your segments as more information becomes available during the analysis. Typically in my any project I use segmentation at least 10 times. Yes at least 10 times .One of  the underutilized feature of eCognition is “Image Object Fusion” algorithm.The algorithm is an essential part of "iterative segmentation and classification" approach of GEOBIA. In this blog, I will show an example of image object fusion for classification of water bodies in a small remote sensing image. The step involves:
  1. Segmentation
  2. Initial classification of water
    1. Establishment of customized feature RationNIR 
      • RationNIR = (meanNIR/(meanR+meanB+meanG+meanNIR))*100
    2. Classify objects that satisfy property
      • RationNIR  less than 15 
      • Area greater than 10 pixels ( to avoid small shadows)
  3. Image object fusion using PPO ( Parent Process Object) to get whole water body
    1. Starting from water classified in step 2, check neighboring water objects and if difference between water object and neighboring objects is less than 5 in RatioNIR. Run in a loop.
    2. Process all water objects. 
Original image
Initial segmentation
Initial water classification
Final segmentation after image object fusion
Final water classification using image object fusion
Very powerful image object fusion algorithm

Step 3 is a one liner algorithm using "Image object Fusion".  Notice various parameters like Class filter, candidates classes, fitting function threshold, use absolute fitting value and weighted sum.To help you understand, I have made a "gif" to elaborate what is going on.

Blue: Initial water
Yellow: Active object
Red: Fused water after image object fusion

Here is the whole Rule set for water classification with image object fusion.

Friday, 10 October 2014

QGIS tutorial: Display with color symbology from data defined properties of shape files

How many of you have this problem: you classify your images in ecognition or any other software and exported classified image as shape file. Now you want to view the shape file in a GIS environment and with the same color for each class that was used in eCognition. Manually matching color RGB values from eCognition to shape colors for each class is a tedious task, specially when  you have many classes.

This problem can be solved with a little trick.

  • Do your classification in eCognition. Assign class colors as you wish.

  • Export classification result as shape file with Class attributes.

  • Export classification result as thematic raster. By doing so, you will get a CSV file in which you can get RGB color codes for each class.

  • Load your shape file in GIS (I use QGIS which is free. I am a poor guy therefore I always love free stuffs)

  • Load the CSV file in QGIS

  • Perform attribute Join of shape file and CSV file with key attribute join based on Class. With this you will get RGB values attached in attribute table of shape file.

  • Double click color symbology of shape file and use data defined properties of  color attributes  of shape file.

  • Build an expression for color selecting respective color attribute column from shape file.
import CSV file into QGIS

Attribute of shape file after performing join
Data defined properties
data defined properties for color

data defined properties for color
Normal display without color symbology

Display with color symbology

Wednesday, 8 October 2014

eCognition tutorial: Exporting eCognition features as images with array functionalities

There are instances when one would like to export different features from eCognition as a images to perform some tasks outside eCognition. eCognition doesn't provide a way to export many object features as images but it is only possible to export as a thematic raster in which each object has a unique ID, and features values are stored in a separated CSV file. To convert it into images, one has to mapped ID raster tiff file and data from CSV file.

If you want to get eCogntion features as images in an automatic way that can export any numbers of features as images in one go, then here is a way. For the purpose, we are going to utilize array handing capabilities of eCogntion and export each feature as separate tiff (My_1.tiff, My_2.tiff …. and so on) file. The rule set can be downloaded here. The rule set is flexible in the sense that you just have to update an array to store your feature list of interest. Number features can be any number (10, 20 or even 100 features). We will merge it afterwards in open source QGIS.

  • Perform a segmentation
  • Create a array and store features you want to export as images
  • Loop over the array
    1. For each feature, create a temporary image file
    2. Export the temporary file as a unique name
    3. Repeat until all features in the array are executed
  • Now in QGIS, we use GDAL to stack individual images into one single image. We will use merge function of GDAL (Raster>Miscellaneous> Merge).
    Complete ruleset within eCognition
Ruleset  for exporting features as images 
GDALmerge function within QGIS
Color composite of three merge features
Thus produced merge features image now can be used for classification  in ENVI, ERDAS IMAGINE or writing custom script in Python or MATAB. Personally I use such images within Python using scikit-learn library.