README for perception pipeline

Author: Shushman Choudhury

README.md

@@ -197,6 +197,8 @@ adetector = ApriltagsModule(marker_topic='/apriltags_kinect2/marker_array',
                            detection_frame='/head/kinect2_rgb_optical_frame')
 detected_objects = adetector.DetectObjects(robot)
 ```
+IMPORTANT - Most of these methods require some underlying CPP server to be running, before calls can be
+made to the PrPy detector. 
 
 ### Perception Modules
 
@@ -208,11 +210,16 @@ Currently, the following perception routines are supported:
 - `BlockDetector`
 - `ROCK`: Robust Object Constellation and Kinematic Pose
 
+### Underlying Servers
+
+To fill once we have come to a consensus on whether to launch all at start, etc.
+
 
 ### Common Perception Methods
 
 At this point, two methods are common to all perception routines. However, some 
-routine-specific knowledge may be required to make them work.
+routine-specific knowledge may be required to make them work. This is particularly reflected
+in the constructor for the perception module.
 
 - `DetectObjects(self, robot, **kw_args)`: This runs the perception method for all
 objects that the particular routine knows about. Typically, this information is specified
@@ -225,6 +232,23 @@ The return type for both is typically one or more OpenRAVE kinbodies, with the c
 transformation relative to the current environment, if the input `tf`s have been 
 correctly provided.
 
+
+### Caveats
+
+As mentioned above, running the perception routines require a bit of routine-specific knowledge,
+because of differences in the way some of them operate. Some of those caveats, for each routine
+are mentioned here.
+
+- `AprilTags`: This method involves detection of visual fiducial markers. There is a database that maps
+april tag IDs to the objects to which they are attached, along with the relative transform
+of the tag with respect to the object kinbody, in `pr_ordata/data/objects/tag_data.json`.
+- `VNCC`: This is a single-query method and so currently does not support `DetectObjects`, but just
+`DetectObject`, where the object names are obtained from the map in the module's constructor.
+- `SimTrack`: Ask Matt to fill in or do later
+- `BlockDetector`: This is specifically for detecting blocks on a table in front of the camera. Therefore,
+it only has a `DetectBlocks` method.
+- `ROCK`: This is still under development and so does not exactly conform to the underlying API.
+
 ## Environment Cloning
 
 Cloning environments is critical to enable planning with multiple planners in