Value Capability Arrays

A value capability array contains information about the value usages supported by a top-level collection for a specific type of HID report. Information about a collection's value capability arrays is contained in its HIDP_CAPS structure.

A user-mode application or kernel-mode driver uses one of the following HIDClass support routines to obtain button capability information:

  • HidP_GetValueCaps returns a value capability array describing all the values that are contained in a caller-specified report type.

  • HidP_GetSpecificValueCaps filters the value capability information it returns by a caller-specified usage page, usage, link collection, and report type.

A value capability array contains HIDP_VALUE_CAPS structures, each one of which describes the following information about a HID usage or usage range:

  • The usage page for a usage or usage range

  • The report ID of the report that contains the value

  • A usage ID or a usage range

  • Indicates whether a usage is an aliased usage

  • Information about the link collection that contains the usage or usage range

  • The size, in bits, of a value, and the report count (which is the number of individual values described by the structure)

  • Attributes of each value, including: whether it has a null value, its units and exponent, and its logical and physical ranges

  • Information about string descriptors and designators associated with the usage or usage range

  • Information about the data indices that the HID parser assigns a usage or usage range

In general, the following conditions hold for all the usages described by a value capability array:

  • Each capability structure represents a usage, a usage range, or a usage value array that is associated with a variable main item. Array main items are not supported for values.

  • Aliased usages can be used. A usage range cannot be aliased. Aliased values are linked together in a value capability array in the same way as aliased buttons as linked together in a button capability array. See Button Usages in a Variable Main Item.

  • The HID parser uses only the minimum required usages to assign a usage to each value. The parser assigns usages in the order in which they are specified in a report descriptor. Usages in a report descriptor that are not required, are discarded. The value capability array does not contain any information about discarded usages.

  • The HID parser assigns a unique data index to each usage described in the capability array.

For a description of how data indices are assigned to values, see Data Indices.

Usage Value Array

A usage value array is a consecutive set of values specified in a main item, all of which are assigned the same usage. This occurs if only one usage is specified for a main item whose report count is greater than one.

The following figure shows an example of a usage value array that contains five data items, each six bits long.

diagram illustrating a usage value array that contains five data items, each 6 bits long

In the previous example, the value capability structure for such a usage value array would have its IsRange member set to FALSE, its NotRange.Usage member set to 17, its ReportCount member set to 5, and its BitSize member set to 6.

If the report count for a usage is 1, use HidP_GetUsageValue to extract the usage value. If the usage's report count is greater than 1, HidP_GetUsageValue only returns the first data item in a usage value array. To extract all the data items in a usage value array, use HidP_GetUsageValueArray.

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