3rdpartyDOC: note about version compatibility between eigen3 and CUDA (…

…CMU-Perceptual-Computing-Lab#1302)

doc/prerequisites.md

@@ -59,7 +59,7 @@ git submodule update --init --recursive --remote
     - Caffe prerequisites: By default, OpenPose uses Caffe under the hood. If you have not used Caffe previously, install its dependencies by running `sudo bash ./scripts/ubuntu/install_deps_and_cuda.sh` (if Ubuntu 16 or 14 and for Graphic cards up to 10XX) or run `sudo bash ./scripts/ubuntu/install_deps.sh` after installing your desired CUDA and cuDNN versions.
     - OpenCV must be already installed on your machine. It can be installed with `apt-get install libopencv-dev`. You can also use your own compiled OpenCV version.
 7. **Eigen prerequisite** (optional, only required for some specific extra functionality, such as extrinsic camera calibration):
-    - If you enable the `WITH_EIGEN` flag when running CMake. You can either:
+    - If you enable the `WITH_EIGEN` flag when running CMake, you must have Eigen already installed in your system. Note that [Eigen <= 3.3.6 is not supported by CUDA >=9.1](https://bitbucket.org/eigen/eigen/commits/034b6c3e101792a3cc3ccabd9bfaddcabe85bb58?at=default). In order to install it, you can perform any of the 3 following options (but only 1 of them!), while making sure that Eigen version is compatible with CUDA:
         1. Do not do anything if you set the `WITH_EIGEN` flag to `BUILD`, CMake will automatically download Eigen. Alternatively, you might prefer to download it manually:
             - [Eigen3](http://posefs1.perception.cs.cmu.edu/OpenPose/3rdparty/eigen_2018_05_23.zip): Unzip as `3rdparty/eigen/`.
         2. Run `sudo apt-get install libeigen3-dev` if you prefer to set `WITH_EIGEN` to `FIND`.