Update cv_opencvdemo.xml (#3272)

conf/modules/cv_opencvdemo.xml

@@ -2,10 +2,51 @@
 
 <module name="cv_opencvdemo" dir="computer_vision">
   <doc>
-    <description>This example shows how opencv can be used on (for example) the Bebop drone.
+    <description>This example shows how OpenCV can be used on (for example) the Bebop drone.
-    Important to know is that sw/ext/opencv_bebop must be downloaded, and made.
+    Important to know is that sw/ext/opencv_bebop must be downloaded, and compiled.
-    After this is done the folder sw/ext/opencv_bebop/install has a opencv.xml file.
+    After this is done the folder sw/ext/opencv_bebop/install has an opencv.xml file.
     The LDFLAGS in this file should be the same as in this conf file.
+
+// Install OpenCV:
+WARNING! During simulation, a native version of OpenCV is needed, while for AP you need a cross-compiled version included in ./sw/ext/opencv_bebop.
+
+The following instructions are for the AP (ARM) version:
+
+1) For OpenCV on the bebop, make sure to use Ubuntu 20.4: This solution was only tested for this version of Ubuntu.
+In NPS, newer versions of OpenCV also work. Then do not follow the instructions below but install (sudo apt-get install) corresponding opencv-dev packages instead, and then skip to step 7) 
+
+2) Navigate to paparazzi/sw/ext/opencv_bebop:
+cd ~/paparazzi/sw/ext/opencv_bebop
+
+3) Install the required OpenCV Libraries:
+sudo apt install libjpeg-turbo8-dev libpng-dev libtiff-dev zlib1g-dev libdc1394-22-dev
+
+4) Do make (inside the current directory): 
+make
+
+5) Then go to the build files for the drone : (will configure the files for OpenCV on the drone)
+command: cd /home/username/paparazzi/sw/ext/opencv_bebop/install_arm 
+command: make install
+
+6) Then go to the build files for the computer: (will configure the files for OpenCV on your on computer)
+command: cd  /home/username/paparazzi/sw/ext/opencv_bebop/install_pc 
+command: make install
+
+7) finally update the paths in this module file (XML file) for your drone (AP) and for your computer (NPS):
+
+For the Bebop, check the paths are: $(PAPARAZZI_SRC)/sw/ext/opencv_bebop/install_arm/include and lib.
+
+Example for NPS, if you followed the steps 2 to 6 above, your opencv will normally be in $(PAPARAZZI_SRC)/sw/ext/opencv_bebop/install_pc/include
+
+But if you did: sudo apt-get install of opencv-dev, then you will need to update all paths to where opencv-dev is on your machine: /usr/bin/...
+
+8) USEFUL BACKGROUND INFORMATION
+OpenCv only works on C++ code, therefore you need to use a special trick to call a C++ function from C:
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+      
     </description>
 
     <define name="OPENCVDEMO_CAMERA" value="front_camera|bottom_camera" description="Video device to use"/>