Update image_apis.md

This adds an extra section to the image type documentation about the new image types added in #3938

docs/image_apis.md

@@ -217,7 +217,9 @@ To change resolution, FOV etc, you can use [settings.json](settings.md). For exa
   DisparityNormalized = 4,
   Segmentation = 5,
   SurfaceNormals = 6,
-  Infrared = 7
+  Infrared = 7,
+  OpticalFlow = 8,
+  OpticalFlowVis = 9
 ```                
 
 ### DepthPlanar and DepthPerspective
@@ -287,5 +289,8 @@ The `simGetSegmentationObjectID` API allows you get object ID for given mesh nam
 ### Infrared
 Currently this is just a map from object ID to grey scale 0-255. So any mesh with object ID 42 shows up with color (42, 42, 42). Please see [segmentation section](#segmentation) for more details on how to set object IDs. Typically noise setting can be applied for this image type to get slightly more realistic effect. We are still working on adding other infrared artifacts and any contributions are welcome.
 
+### OpticalFlow and OpticalFlowVis
+These image types return information about motion perceived by the point of view of the camera. OpticalFlow returns a 2-channel image where the channels correspond to vx and vy respectively. OpticalFlowVis is similar to OpticalFlow but converts flow data to RGB for a more 'visual' output.
+
 ## Example Code
 A complete example of setting vehicle positions at random locations and orientations and then taking images can be found in [GenerateImageGenerator.hpp](https://github.com/Microsoft/AirSim/tree/master/Examples/DataCollection/StereoImageGenerator.hpp). This example generates specified number of stereo images and ground truth disparity image and saving it to [pfm format](pfm.md).