updated install_dependencies.sh and readme for python api (#43)

* use absolute path in readme for python api
* Update install_dependencies.sh

inference-engine/ie_bridges/python/README.md

@@ -30,7 +30,7 @@ Then build generated solution INFERENCE_ENGINE_DRIVER.sln using Microsoft\* Visu
 ```shellscript
   mkdir -p build
   cd build
-  cmake -DInferenceEngine_DIR=../../../build -DPYTHON_EXECUTABLE=`which python3.6` \
+  cmake -DInferenceEngine_DIR=`realpath ../../../build` -DPYTHON_EXECUTABLE=`which python3.6` \
   	-DPYTHON_LIBRARY=/usr/lib/x86_64-linux-gnu/libpython3.6m.so \
   	-DPYTHON_INCLUDE_DIR=/usr/include/python3.6 ..
   make -j16

inference-engine/install_dependencies.sh

-#!/bin/bash -x
+#!/bin/bash
 # Copyright (C) 2018 Intel Corporation
 # SPDX-License-Identifier: Apache-2.0
 #
 
+params=$@
+
+function yes_or_no {
+    if [ "$params" == "-y" ]; then
+        return 0
+    fi
+
+    while true; do
+        read -p "Add third-party Nux Dextop repository and install FFmpeg package (y) / Skip this step (N)" yn
+        case $yn in
+            [Yy]*) return 0 ;;
+            [Nn]*) return 1 ;;
+        esac
+    done
+}
+
 # install dependencies
 if [[ -f /etc/lsb-release ]]; then
     # Ubuntu
@@ -66,16 +82,43 @@ else
             libstdc++.i686 \
             libgcc.i686 \
             libusbx-devel \
-            openblas-devel
+            openblas-devel \
+            libusbx-devel \
+            gstreamer1 \
+            gstreamer1-plugins-base
 
-    wget https://cmake.org/files/v3.12/cmake-3.12.3.tar.gz --no-check-certificate
+    # Python 3.6 for Model Optimizer
+    sudo -E yum install -y rh-python36
+    source scl_source enable rh-python36
+
+    wget https://cmake.org/files/v3.12/cmake-3.12.3.tar.gz
     tar xf cmake-3.12.3.tar.gz
     cd cmake-3.12.3
     ./configure
     make -j16
     sudo -E make install
 
-    # FFmpeg and GStreamer for OpenCV
-    sudo -E rpm -Uvh http://li.nux.ro/download/nux/dextop/el7/x86_64/nux-dextop-release-0-1.el7.nux.noarch.rpm
-    sudo -E yum install -y ffmpeg libusbx-devel gstreamer1 gstreamer1-plugins-base
+    echo
+    echo "FFmpeg is required for processing audio and video streams with OpenCV. Please select your preferred method for installing FFmpeg:"
+    echo
+    echo "Option 1: Allow installer script to add a third party repository, Nux Dextop (http://li.nux.ro/repos.html), which contains FFmpeg. FFmpeg rpm package will be installed from this repository. "
+    echo "WARNING: This repository is NOT PROVIDED OR SUPPORTED by CentOS."
+    echo "Once added, this repository will be enabled on your operating system and can thus receive updates to all packages installed from it. "
+    echo
+    echo "Consider the following ways to prevent unintended 'updates' from this third party repository from over-writing some core part of CentOS:"
+    echo "a) Only enable these archives from time to time, and generally leave them disabled. See: man yum"
+    echo "b) Use the exclude= and includepkgs= options on a per sub-archive basis, in the matching .conf file found in /etc/yum.repos.d/ See: man yum.conf"
+    echo "c) The yum Priorities plug-in can prevent a 3rd party repository from replacing base packages, or prevent base/updates from replacing a 3rd party package."
+    echo
+    echo "Option 2: Skip FFmpeg installation."
+    echo
+
+    if yes_or_no; then
+        sudo -E rpm -Uvh http://li.nux.ro/download/nux/dextop/el7/x86_64/nux-dextop-release-0-1.el7.nux.noarch.rpm
+        sudo -E yum install -y ffmpeg
+    else
+        echo "FFmpeg installation skipped. You may build FFmpeg from sources as described here: https://trac.ffmpeg.org/wiki/CompilationGuide/Centos"
+        echo
+    fi
+
 fi
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