Methods and apparatus to count people

A method comprising:

• deleting, by executing an instruction with a processor, existing characteristic datasets from a list;
• obtaining, by executing an instruction with the processor, first characteristic datasets for corresponding faces detected in a first plurality of images representative of an environment over a first period of time of a media presentation;
• populating, by executing an instruction with the processor, the list with the first characteristic datasets;
• limiting, by executing an instruction with the processor, a number of the first characteristic datasets stored in the list for a particular location represented in the first plurality of images to an upper limit, the limiting to reduce an amount of memory used to store the list;
• and
• after the first period of time expires:
• comparing, by executing an instruction with the processor, the first characteristic datasets to each other to determine a uniqueness of respective ones of the first characteristic datasets, the comparing limited to comparisons of the respective first characteristic datasets to other ones of the first characteristic datasets obtained during the first period of time;
• deleting, by executing an instruction with the processor, the first characteristic datasets from the list;
• and
• storing, in a memory, a first people count for the first period of time based on the comparing of the first characteristic datasets to each other.

A tangible computer readable storage medium comprising instructions that, when executed, cause a machine to at least:

• delete existing characteristic datasets from a list;
• populate the list with first characteristic datasets obtained from a first plurality of images representative of an environment during a first period of time of a media presentation, respective ones of the first characteristic datasets representative of respective faces detected in the environment during the first period of time;
• limit a number of the first characteristic datasets stored in the list for a first location in respective ones of the first plurality of images to not exceed an upper limit;
• compare the first characteristic datasets to each other to determine a first number of unique faces in the environment during the first period of time, the comparing limited to comparisons of the respective first characteristic datasets to other ones of the first characteristic datasets obtained during the first period of time;
• delete the first characteristic datasets from the list after the first period of time has ended;
• and
• re-populate the list with second characteristic datasets obtained from a second plurality of images representative of the environment during a second period of time of the media presentation, the second period of time subsequent to the first period of time.