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3.3.5.1.1.1 Construction of a Primary Drawing Order

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All primary drawing orders MUST conform to the structure and rules defined in section 2.2.2.2.1.1.2.

To efficiently construct a primary drawing order, the server MUST use a Primary Drawing Order History (section 3.2.1.1) store. This store holds three pieces of information:

  • Last primary order type constructed.

  • Current bounding rectangle.

  • Per-order record of the last value used in each field.

These stored records allow the server to use the minimum amount of data when constructing an order; if a field is unchanged from the value that it had when the order type was last sent, it SHOULD NOT be included in the order being constructed. Hence, only fields that have new values are required to be sent to the client. The fields that are present in the order MUST be indicated by the fieldFlags field.

If all of the Coord-type fields (see section 2.2.2.2.1.1.1.1) in an order can be represented as a signed delta in the range -127 to 128 from the previous field value, the size of the order SHOULD be optimized by using delta-coordinates (see sections 2.2.2.2.1.1.1.1 and 2.2.2.2.1.1.2). In that case, all of the fields SHOULD be represented using delta-coordinates, and the TS_DELTA_COORDINATES (0x10) flag MUST be used in the primary drawing order header to indicate this fact.

Before a given order is sent, the server MUST also ensure that all of the data required to process the order is accessible to the client. For example, if the order refers to a cached item, that item MUST be present in the client-side cache when the order is processed. Or, if palettized color is being used, the correct palette MUST be applied at the client-side.

Once a primary drawing order has been constructed and transmitted to the client, the server MUST update the records in the Primary Drawing Order History (section 3.3.1.2) to ensure that future encodings use the minimum fields and data required.