# Working with a Blob Scene Description

The blob scene description amounts to a collection of chunks of information about blobs. Within the blob scene description each feature (blob or hole) is uniquely identified by an ID. IDs are integers in the range 0 to the number of features in the scene minus 1. Certain kinds of information can be obtained regarding the collection of blobs to which the blob scene description refers. Other kinds of information are available for each blob in the scene.

The Blob tool does not compute any information about any feature until you explicitly or implicitly request particular information about that feature. The Blob tool stores all the information about each blob as it computes that information; no measure is computed more than once.