The Querying Interface

Text-based querying is provided by the Multimedia Object Query Language (MOQL), which is an extension of the standard OQL language. We also defined an equivalent visual query language (VisualMOQL [OÖX99]) and a translator to translate a visual query into an MOQL query.

VisualMOQL has these particular features:

• It is a declarative visual query language with a step-by-step construction of queries, close to the way people think in natural languages.
  
• It has a clearly defined semantics based on object calculus. This feature can be used to conduct a theoretical study of the language, involving concepts such as expressive power and complexity.
  
• It combines several querying approaches: semantic-based (query image semantics using salient objects), attribute-based (specify and compare attribute values), and similarity-based (query by example). A user can start a query using the semantic and/or attribute-based approach and then choose an image for a similarity-based query.

Using the VisualMOQL startup window (Figure ) the user specifies a query by choosing the image class s/he wants to query and the salient objects s/he wants to see in the images. Several levels of refinement are offered depending on the type of query and also on the level of precision the user wants the query result. The startup window consists of the following components:

• A chooser to select the image classes. Images stored in the database are categorized into user-defined classes.
• A salient object class browser which allows the user to choose the objects that he/she wants.
• A horizontal slider to specify the maximum number of images that will be returned as the result of the query. This is a quality of service parameter used by the query result presentation interface.
• A horizontal slider to specify the similarity threshold between the query image and the target images stored in the database. It is also used for color comparison. This is also a quality of service parameter for the presentation interface.
• A working canvas where the user constructs queries step by step.
• A query canvas where the user can construct compound queries based on simple queries (sub-queries) defined in the working canvas using AND, OR, and NOT operators.