docs: Document environment variables

Author: xlz

README.md

@@ -145,7 +145,7 @@ cmake --build . --config RelWithDebInfo --target install
 * Run the test program: `.\install\bin\Protonect.exe`, or start debugging in Visual Studio.
 * Test OpenNI2 (optional)
 
-    Copy freenect2-openni2.dll, and other dll files (libusb-1.0.dll, glfw.dll, etc.) in `install\bin` to `C:\Program Files\OpenNI2\Tools\OpenNI2\Drivers`. Then run `C:\Program Files\OpenNI\Tools\NiViewer.exe`.
+    Copy freenect2-openni2.dll, and other dll files (libusb-1.0.dll, glfw.dll, etc.) in `install\bin` to `C:\Program Files\OpenNI2\Tools\OpenNI2\Drivers`. Then run `C:\Program Files\OpenNI\Tools\NiViewer.exe`. Environment variable `LIBFREENECT2_PIPELINE` can be set to `cl`, `cuda`, etc to specify the pipeline.
 
 ### Mac OSX
 
@@ -189,7 +189,7 @@ make
 make install
 ```
 * Run the test program: `./bin/Protonect`
-* Test OpenNI2. `make install-openni2` (may need sudo), then run `NiViewer`.
+* Test OpenNI2. `make install-openni2` (may need sudo), then run `NiViewer`. Environment variable `LIBFREENECT2_PIPELINE` can be set to `cl`, `cuda`, etc to specify the pipeline.
 
 ### Linux
 
@@ -253,4 +253,4 @@ make install
     You need to specify `cmake -Dfreenect2_DIR=$HOME/freenect2/lib/cmake/freenect2` for CMake based third-party application to find libfreenect2.
 * Set up udev rules for device access: `sudo cp ../platform/linux/udev/90-kinect2.rules /etc/udev/rules.d/`, then replug the Kinect.
 * Run the test program: `./bin/Protonect`
-* Run OpenNI2 test (optional): `sudo apt-get install openni2-utils && sudo make install-openni2 && NiViewer2`.
+* Run OpenNI2 test (optional): `sudo apt-get install openni2-utils && sudo make install-openni2 && NiViewer2`. Environment variable `LIBFREENECT2_PIPELINE` can be set to `cl`, `cuda`, etc to specify the pipeline.

doc/mainpage.dox

@@ -25,16 +25,16 @@ Features
 
 ### Issues and Future Work
 
-- Audio. There is basic access to Kinect v2's audio via ALSA (Linux). However,
-  this is directional audio with intricate calibration, which is probably
-  beyond the scope of this image processing library.
-- Unstable USB and crashes. Due to differences in a vast range of hardware, it
-  is very hard to test them all. Also, the libusb usage in %libfreenect2 may
-  miss a lot of error checking and simply crash. This can be improved.
-- Firmware upload. This is being worked on. Use Windows for this right now.
+- Audio. Raw audio is accessible via Linux USB audio. There is no support for
+  the calibrated directional audio.
+- Unstable USB and crashes. Due to differences in driver support, USB problems
+  can happen a lot. Error handling in %libfreenect2 is not fully verified for
+  production use.
+- Firmware upload. The protocol has been reverse engineered, but use Windows
+  for this right now.
 - Example of multiple Kinects.
 - Example utility of dumping image frames.
-- API for pausing, or on-demand processing.
+- API for on-demand processing.
 - Verification of systematic errors through accurate calibration.
 - Bindings for C, Python, Java, etc.
 
@@ -54,6 +54,20 @@ to implement performance optimizers, you are encouraged to read the source
 code. The source code is the updated and authoritative reference for any
 functionalities.
 
+Environment Variables
+=====================
+
+There are a few environment variables providing controls for both end-users and
+programmers:
+
+* `LIBFREENECT2_LOGGER_LEVEL`: The default logging level if not explicitly set
+  by the code.
+* `LIBFREENECT2_PIPELINE`: The default pipeline if not explicitly set by the
+  code.
+* `LIBFREENECT2_RGB_TRANSFER_SIZE`, `LIBFREENECT2_RGB_TRANSFERS`,
+  `LIBFREENECT2_IR_PACKETS`, `LIBFREENECT2_IR_TRANSFERS`: Tuning the USB buffer
+  sizes. Use only if you know what you are doing.
+
 You can also see the following walkthrough for the most basic usage.
 
 Walkthrough