National Instruments NI myRIO, Essentials Manual

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CHAPTER 15. MOTION DETECTOR 109
15.1 Technical Approach
NI Vision Assistant Script
The vision script merely needs to extract the luminance image from the color
webcam image because the optical flow VIs are not available in Vision Assistant;
see Extract Luminance Plane
1
for details.
Optical Flow VIs
The LabVIEW IMAQ Optical Flow (HS) VI computes the optical flow (velocity
flow) from two successive grayscale image frames using the Horn and Schunck
algorithm. Optical flow is a vector field, therefore the VI calculates two single-
precision floating point images ( [SGL] ) in either Cartesian or polar form. The
two images are generically called Velocity Component 1 and Velocity Compo-
nent 2 , and contain the X and Y vector components with “Cartesian” velocity
representation selected and the magnitude and phase with “Polar” represen-
tation selected. The VI includes additional inputs such as stopping criteria,
smoothing parameter, and the option to use the previously-calculated output
as a starting point for the new calculation.
The IMAQ Overlay Motion Vectors VI accepts the pair of velocity component
images generated above and generates a vector field overlay for a suitable image,
i.e., the original webcam image. This VI must use the same selected velocity
representation as that calculated by
IMAQ Optical Flow (HS) , of course, and the
VI provides the ability to select the velocity threshold at which flow vectors are
drawn, the length of the arrows (scale) as well as their color, and a step size to
set the density of arrows.
LabVIEW Techniques
The following LabVIEW techniques are required for this project:
•
(qsm) initialize: Image Buffer
2
– Use the IMAQ Create VI to create image
buffers for the previous webcam frame ( [Grayscale (U8)] ), the two velocity
components ( [Grayscale (SGL)] ), and the detected motion (also [Grayscale
(SGL)] ). Place these VIs in the (qsm) initialize state.
•
The usual “mode” and “action” buttons are not required for this project.
Instead of deleting the controls, simply right-click them in the block di-
1
http://youtu.be/1mmzsd46_Gs
(1:02)
2
http://youtu.be/suCkeB_afOc
(2:02)