Vision Pipeline

They say a picture is worth a thousand words...

click for an enlarged image map
(click for an enlarged image map)

Overview

Tekkotsu's vision system is divided into many separate modules.  Each module communicates by throwing events to the EventRouter, which then sends an event  to any dependent modules.

Data is typically stored in filter banks.  Each filter bank contains images in several resolution layers, forming an image pyramid.  In addition, each filter bank can contain multiple channels, such as Y, U, and V color information.

The image computations are lazy evaluated.  The FilterBankEvents and SegmentedColorFilterBankEvents which are thrown do not contain the actual image data - they reference the generator itself, and the data does not need to be computed until it's actually accessed.  Once computed, the image data is cached so future accesses are cheap, until the generator receives notice its own source has new image data available, which invalidates the cache.

The section marked "CMVision" on the diagram are the modules which use the CMVision library.  The CMVision modules are somewhat intertwined due to the nature of their data structure layouts.  The intermixed data fields allow some short cuts, but the tradeoff is that to replace one of these sections you may need to need to know a bit about what the downstream components are expecting.

It is important to keep in mind that the AIBO's system provides several image channels, each at several resolutions, and this pipeline will allow you access the results of pulling any of these images through the pipeline to any stage.

Information Flow

The following information is for the default layout.  However, all the setup is done in <project>/StartupBehavior_VisionSetup.cc, so you can actually disconnect/reconnect everything as you see fit, or even add new stages.

Data is sent from the system to the Main process.  This data is organized by 3 data layers, each of which represents a different resolution level.  In addition, there are currently 7 data channels available from the system, which store:

  • Y - intensity (brightness information)
  • U - reddish color channel
  • V - bluish color channel
  • Y horizontal derivative - hilights vertical edges
  • Y vertical derivative - hilights horizontal edges
  • Y horiz. & vert. derivative - hilights diagonal edges
  • CDT - Color Detection Table - segmented (indexed) color channel

The symbolic names for these channels are defined in channel_id_t of RawCameraGenerator.

The main process simply wraps a data event around a pointer to the OFbkImageVectorData structure that it is given by the system.  This is received by the RawCameraGenerator and CDTGenerator.

The CDTGenerator will pull out the CDT channel from the system structure and fill out a SegmentedColorFilterBankEvent for it.  Note that by default the CDTGenerator isn't connected to anything, and further, to use it you will need to call the appropriate system functions to define the color region thresholds.  Some modification of the CDTGenerator may be necessary for full functionality - this is just starter code.  By default, CMVision is used for segmentation.  The CDT segmentation is faster since it is done in hardware, but it is less flexible than CMVision because its color regions must be rectangular, which usually leads to less accurate segmentation.

The RawCameraGenerator parses the system structure and fills out a FilterBankEvent.  By default, the RawCameraGenerator also adds a few more resolution levels than is directly available from the system:

  • 5 - double resolution - for the Y channel, this is computed by combining the 4 Y channels to reclaim the full resolution of the camera.  For the other channels, it simply upsamples the pixels.  All of this requires some significant processing, so use it sparingly.
  • 4 - full resolution - direct from system
  • 3 - half resolution - direct from system
  • 2 - quarter resolution - direct from system
  • 1 - eigth resolution - references every other pixel of quarter resolution image
  • 0 - sixteenth resolution - references the every fourth pixel of quarter resolution image

You can use the symbolic names defined in ProjectInterface.h to access these, but since they go in numerical order, it would also make sense to just use the numerical values listed above.

Note that you must use the increment value given by the getIncrement() function to properly access the bottom two layers.  All other layers will have an increment value of 1.

From the RawCameraGenerator the image information goes down several different paths. 

One is through the CMVision library, which will segment the color image into an indexed  image, then run length encode it, and then connect runs into regions.  Some noise removal is performed during RLE compression to achieve better compression as well as cleaner segmentation.

Each of these steps is thrown as a separate event so you can access the intermediate data.  For instance, you may be able to do further processing or detection directly on the RLE compressed image instead of the segmented image itself, which could save some data processing.  Alternatively, other algorithms may be better off accessing the uncompressed image if they do not run through all the pixels in order.  The key here is to use whichever information best suits your algorithm, because it's all available.

Another module which uses the RawCameraGenerator is the JPEGGenerator.  The JPEGGenerator uses the libjpeg library to perform compression.  There are two instantiations of the JPEGGenerator however.  One receives the data directly from the RawCameraGenerator.  This will then JPEG compress each of the individual channels as grayscale images.  The other receives data after it has passed through the InterleavedYUVGenerator first.  This is necessary to pass the data to the libjpeg library in the format it expects in order to produce a single "color" image instead of a collection of individual grayscale iamges.

The main use of the JPEG compressed image is to be streamed over wireless ethernet to a desktop computer for viewing.  However, it is conceivable that some algorithms may be able to directly run off of the data produced by the JPEG compression. For example, a type of edge detection, image summary, or gradient information could be obtained quickly from the compressed image.

Transmission over the network is handled by the RawCamBehavior and SegCamBehavior.  The RawCam will select a resolution layer and channel from either the raw image data or one of the JPEG compressors depending on current settings.  The SegCam will pick from the original segmented image or the RLE compressed version depending on current settings. (These settings are defined in the <project>/ms/config/tekkotsu.cfg file, and can be changed at runtime with the !set command (see FAQ entry))

Object detection is currently handled by a BallDetectionGenerator, which receives the region information and attempts to find square-ish regions which have the same area as a circle would if it were contained in the region's bounding box.

Usage

Since the image data is lazily evaluated, there is very little overhead if your behavior subscribes to image data but never actually calls the getImage() function to get the image data.

However, if your behavior is subscribed to one of the detectors, it will force the detector to access the image information on each frame to perform detection so the detector can decide if it should throw an event.  Thus, you should always be sure to unsubscribe from object detectors whenever you no longer need them to reduce computational load.  If just one behavior is causing the image information to be accessed, the whole pipeline from that generator to the root will have to be evaluated.  However, any further accesses by other behaviors will have trivial overhead.

Also, you should try to have each of your behaviors access the same resolution layer.  Since each layer is handled separately, accessing a layer which has not been computed will cause the entire pipeline to be run for that layer.  (It might be an interesting bit research for someone to write a method of cost minimization that will decide when it's better to resample results from an already available layer instead of recomputing the requested layer from scratch.  It's not always obvious when to do one or the other, although some additional smarts could be hardwired if you find yourself needing two different layers.)

The functions available from the FilterBankEvent give all the information necessary for interpreting image pixels, supporting a variety of memory layouts.  By using the values given in the event appropriately, you can ensure your code will be able to handle image data, even if upstream components change their data layout, such as interleaving pixels instead of rows, or putting each channel in a contiguous memory block.  (Currently, the system interleaves rows from each channel within a large memory block, but most of the Tekkotsu FilterBankGenerators will store each channel in a separate memory block.)

In order to receive image information, you simply need to call addListener() on the EventRouter, just like for any other information source.  Then cast the EventBase you receive to the appropriate type, and call getImage() and other functions as needed.

The generator IDs are used to designate the format of the data returned by getImage().  For instance, if you get an event from visRLEEGID, it's going to be an array of CMVision run structures.  But if it's from visRawCameraEGID, it's just a straight pixel array.

The source IDs are used to differentiate multiple generators which might produce the same data types.  For instance, the CDTGenerator and CMVision SegmentedColorGenerator both produce visSegmentEGID events.  However they will have different source IDs so they can both be active at the same time and behaviors can choose one or the other.  Another example is the JPEG compressors - one source is grayscale individual channels, the other is a color image in only the first channel.

Thus, if you want to make a new image source, all you need to do is start throwing events from a different source ID.  You don't have to turn off or break the other sources to add your own.

For further reading, the Dutch ARchitecture Project for Aibos (DARPA) has written a nice tutorial on adding custom vision processing code which can be found here: visionInDepth.pdf



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Last modified: 2005-07-19