National Instruments NI myRIO, Essentials Manual

Page: 19 / 163

CHAPTER 3. DESIGN PATTERN FOR MACHINE VISION APPLICATIONS 13
3.3 “Machine Vision App” Template
Download the .zip file from the NI myRIO Vision Essentials Guide
4
main page,
extract the “Machine Vision App” folder, open the “machine vision app” Lab-
VIEW project (double-click the .lvproj file), and then double-click Main.vi to
open its block diagram. Take a few moments to study the structure of the block
diagram, and especially scroll through the case-structure subdiagrams to see
the various states and their associated activities.
This template provides a single starting point for each of the projects in this
lab manual. As you complete each project and develop your own library of
machine vision applications you may find it easier to work from a copy of an
existing project that is similar to the requirements of the new project. In either
case, implementing a new machine vision application will become a relatively
straightforward task once you understand the basic principles of the queued
state machine design pattern.
IMPORTANT : Do not be discouraged by the seeming complexity of the
queued state machine template. You will soon discover that the template is
easy to edit and achieves expected and reliable behavior without expending
unnecessary development effort.
QSM Structure
Figure 3.2 on page 20 shows the task diagram for the “Machine Vision App”
(MVA) template project. The task diagram shows only two tasks but the
template includes many empty states that you can adapt to the needs of your
project. The default task applies an edge detector to the webcam image and
displays either this processed image or the original webcam image along with a
nondestructive graphics overlay that shows the system date and time. Figure 3.3
on page 21 shows the state sequence for the default task.
Pressing the front-panel save image action button selects the Save Image
task (Figure 3.4 on page 22); this task saves the displayed image to an image
file on the attached USB flash drive. The action button remains pressed until
released, therefore the task releases the button as its final action. Because all tasks
that respond to action buttons must conclude in a like manner, the button release
behavior is implemented in a single state that can be included in any task.
Figure 3.5 on page 23 shows the state sequence for the start-up task. These
states initialize data values, allocate memory for image buffers, reset front-panel
buttons, and configure on-board and external devices. Figure 3.6 on page 24
4
http://www.ni.com/myrio/vision-guide