25.1. Introduction ↩

The sdscatter command may run user-defined function each time a data sample is clicked. This allows us to visualize data related to a specific sample depending on our application.

Say, our problem concerns the pose-recognition application, discussed in this video. The person in the video frame is detected by fitting a torso model. Parameters of the model are used for classifying the human pose. This data set was provided by TU Delft researchers Drs.Feifei Huo and Prof.Emile Hendricks.

The data set contains 2800 examples of nine poses gathered from 13 persons:

>> a
2809 by 6 sddata, 9 classes: [572  1022    24    46   206   114   201   220   404]

We may see the available meta-data using:

>> a'
2809 by 6 sddata, 9 classes: [572  1022    24    46   206   114   201   220   404]
sample props: 'lab'->'class' 'class'(L) 'frame'(N) 'person'(L)
feature props: 'featlab'->'featname' 'featname'(L)
data props:  'data'(N)

Apart of class label, each example contains the frame number and person label.

We me access the meta-data similarly to structure fields

>> a.lab
sdlab with 2809 entries, 9 groups

>> a.lab'
 ind name               size percentage
   1 pose1               572 (20.4%)
   2 pose2              1022 (36.4%)
   3 pose3                24 ( 0.9%)
   4 pose4                46 ( 1.6%)
   5 pose5               206 ( 7.3%)
   6 pose6               114 ( 4.1%)
   7 pose7               201 ( 7.2%)
   8 pose8               220 ( 7.8%)
   9 pose9               404 (14.4%)

>> a.person
sdlab with 2809 entries, 13 groups

The frame and person label for a specific data sample:

>> a(1).frame

ans =

   299

>> a(1).person
sdlab with one entry: 'Berend'

We would like to visualize the frame image corresponding to a sample clicked in sdscatter figure. To do so, we need to write a Matlab function that loads the image corresponding to a sample and shows it in a separate figure.

function show_frame(data)

frame=data.frame;
person=strtrim(+data.person);

% image path is 'poses/Name/NameXX.bmp' where XX is a frame number
image_path=fullfile('poses',person,[person num2str(frame) '.bmp']);

im=imread(image_path); 

figure(10);
imagesc(im);
title(image_path)

The show_frame.m function will receive from sdscatter the data set object data containing the clicked example. We extract the frame number and person name from data. The unary plus operator converts the sdlab object into string. Next, we compose image_path variable and load the image file. Finally, we show the image in a specific figure.

To use this custom function, we only need to call sdscatter with the 'callback' function:

>> sdscatter(a,'callback',@show_frame)

ans =

 1

When we click on a sample, sdscatter calls show_frame.m which opens the respective image in figure 10.

Scatter plot showing custom visualization using a callback function

25.1.1. Passing parameters to the callback function ↩

Note, that our show_frame function hard-codes the figure used for image visualization. It would be more practical to decide what figure to use at the moment we call sdscatter. We may do just that by passing an additional parameter after the callback function handle.

We may modify our function to accept a figure number as the second parameter:

function show_frame2(data,figh)

frame=data.frame;
person=strtrim(+data.person);

% image path is 'poses/Name/NameXX.bmp' where XX is a frame number
image_path=fullfile('poses',person,[person num2str(frame) '.bmp']);

im=imread(image_path); 

figure(figh);
imagesc(im);
title(image_path)

We can now start the sdscatter providing the handle of the specific figure:

>> figh=figure

figh =

 2

>> sdscatter(a,'callback',@show_frame2,figh)

ans =

 3

Interactive scatter plot showing video frame in user-defined figure.

Note, that the additional parameter, passed to sdscatter, may be only one and it cannot be string. If we need to pass multiple parameters, we may wrap them into a structure or a cell array.

We may use custom callbacks to understand why are certain observations misclassified or why are they outliers.