A descriptor is a set of parameters associated with a frame. The descriptor defines the value of each parameter. Since each frame can have a unique descriptor, custom descriptors can be used to change camera settings on a frame-by-frame basis for each image in the video stream. If the user wants to receive different images in a video stream, he can set up a descriptor for each frame. The camera will apply each descriptor in turn when capturing images and will output one frame per descriptor. After the last descriptor is used, the next frame in the stream will return to the first descriptor. This allows for rapid changes of camera parameters in a deterministic manner.

The descriptor is tagged onto the last packet of every frame. When multiple descriptors are in use, the camera will encode a synchronization number in the first 16 bytes of image data returned from the camera.  The format of the synchronization code is shown in Video Formats.

When to use

Descriptors are useful for obtaining different views of the same image. Several examples of this are listed below.

  • Zooming in on several smaller regions of interest within a larger image
  • Obtaining images with different exposure times
  • Sequencing through various trigger inputs and General-Purpose outputs (GPOs) settings

How to use

Descriptors can be programmed using Pixelink Capture OEM. Most features that can be controlled via Capture OEM can be modified in a descriptor.  The exceptions are the gamma correction/look-up table feature and Flat Field Correction. 

  • Descriptors are created by increasing the number of descriptors in the descriptor configuration CSR
  • Descriptors can be changed by updating the descriptor values in camera memory
  • To delete a descriptor, reduce the number of descriptors in the descriptor configuration CSR, the highest indexed descriptor will be deleted first
  • In rolling shutter mode, any descriptor changes that affect the sensor will cause the camera to drop the first frame with the new settings and send another one, effectively halving the frame rate. For example changes to the region of interest.

The number of descriptors supported varies between cameras.  Up to a maximum of 16 descriptors can be created and stored in camera memory.