/usr/lib/pd/extra/pdp/pdp_conv_sobel_hor.pd is in pd-pdp 1:0.14.1-3build1.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 | #N canvas 222 642 591 202 10;
#X obj 29 55 inlet;
#X obj 29 132 outlet;
#X obj 29 93 pdp_conv;
#X obj 108 9 loadbang;
#X msg 108 37 vmask 0.5 1 0.5;
#X msg 108 60 hmask -1 0 1;
#X text 108 101 this uses pdp_conv to produce the vertical sobel edge
detector with convolution mask (-.5 0 0.5 \; -1 0 1 \; -0.5 0 .5);
#X connect 0 0 2 0;
#X connect 2 0 1 0;
#X connect 3 0 4 0;
#X connect 4 0 2 0;
#X connect 4 0 5 0;
#X connect 5 0 2 0;
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