x1 y1 u1 v1

x2 y2 u2 v2

...

xN yN uN vN

where for each row (x,y) is the position of the vector and (u,v) is its magnitude, use the command

plot "motion_vectors.data" using 1:2:3:4 with vec

plot "motion_vectors.data" every 4:4 u 1:2:($3*4):($4*4) w vec

To overprint the vectors on an image (with gnuplot 4.2). The image must be stored as a binary file image.bin and the size of the image must be known 300x400.

plot "image.bin" binary array=300x400 flipy with image, "motion_vectors.data" every 12:12 u 1:2:($3*8):($4*8) w vec

Sometimes other adjustements are necesarry to the plots, just manipulate: ($1):($2):($3*8):($4*8)

References:

http://t16web.lanl.gov/Kawano/gnuplot/datafile2-e.html#7.6

http://www2.yukawa.kyoto-u.ac.jp/~ohnishi/Lib/gnuplot.html

http://gnuplot.sourceforge.net/demo/image.html

...

xN yN uN vN

where for each row (x,y) is the position of the vector and (u,v) is its magnitude, use the command

plot "motion_vectors.data" using 1:2:3:4 with vec

this plots small vectors at each position (x,y).

If we want to plot only some of the points (because the field is dense) we use the every 4:4 option, and scale the vectors by 4

plot "motion_vectors.data" every 4:4 u 1:2:($3*4):($4*4) w vec

To overprint the vectors on an image (with gnuplot 4.2). The image must be stored as a binary file image.bin and the size of the image must be known 300x400.

plot "image.bin" binary array=300x400 flipy with image, "motion_vectors.data" every 12:12 u 1:2:($3*8):($4*8) w vec

References:

http://t16web.lanl.gov/Kawano/gnuplot/datafile2-e.html#7.6

http://www2.yukawa.kyoto-u.ac.jp/~ohnishi/Lib/gnuplot.html

http://gnuplot.sourceforge.net/demo/image.html

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