How to understand face detection xml

Question

I have trained faces using opencv_trainedcascade.exe. I have a series of xml files for different stages. For each xml file has internal nodes and leafVlaues and one of them is shown below.

<?xml version="1.0"?>
<opencv_storage>
<stage0>
  <maxWeakCount>3</maxWeakCount>
  <stageThreshold>-1.3019366264343262e+000</stageThreshold>
  <weakClassifiers>
    <_>
      <internalNodes>
        0 -1 2711 -2099201 -2623493 -774797061 -2162625 -827343685
        -5535541 -1163949377 -21761</internalNodes>
      <leafValues>
        -9.2679738998413086e-001 6.0445684194564819e-001</leafValues></_>
    <_>
      <internalNodes>
        0 -1 1533 -252379683 -203697739 1410462197 1435881947 -74449473
        -1147414357 1510080511 -1</internalNodes>
      <leafValues>
        -9.1606438159942627e-001 6.2200444936752319e-001</leafValues></_>
    <_>
      <internalNodes>
        0 -1 917 -42468780 -11479728 -745548289 -2371181 -23070497
        -552607093 -74777633 -536871937</internalNodes>
      <leafValues>
        -9.2716777324676514e-001 5.4092508554458618e-001</leafValues></_></weakClassifiers></stage0>
</opencv_storage>

My queries are (1)What do those stageThreshold, internalNodes and leafValues mean? (2)In actual face detection, how are they used in cascaded classifier, I read a few paper for Adaboost algorithm. But I don't understand quite well. Thanks

Answer 1

After digging the detection_based_tracker.cpp, now I understood what are the internalNodes, leafValues, and stagethreshold and how they are used. When we look at the lbpcascade_frontalface.xml, we see a list of rectangles. These are the rectangles of trained face images (i.e. these areas have distinct features and can be used to differentiate faces from non-face images). For lbpcascade_frontalface.xml has 139 rectangles. Each rectangle's x,y points are multiplied with a constant number to make additional three rectangels, so one rectangle represents four rectangles actually.

Then I will explain what is internalNode.

<internalNodes>
            0 -1 13 -163512766 -769593758 -10027009 -262145 -514457854
            -193593353 -524289 -1</internalNodes>

The first two numbers 0 -1 represents left and right. I think they represent left leafValue and right leafValue. The third one is the feature index. If we put those 139 rectangles into an array, that feature index refers to the array index. That means which rectangle to represent. The last eight numbers represents corner point subtractions from four rectangles. These are calculated from the integral images, so the numbers are quite big.

But I am not quite sure how leafValues are computed, but summation of these leafValues are compared with stageThreshold to make decision of face or non-face.

That is what I understood from debugging the code. If somebody explains how leafValues are computed, that will be a complete solution for my query. Thanks