ce picture as "unused for reference" as specified in clause 8.2.5.4.2 is invoked. For this invocation of the process specified in clause 8.2.5.4.2, all pictures that are marked as "reference base picture" are considered as not present. NOTE 2 - Long-term reference pictures that are marked as "reference base pictures" cannot be marked as "unused for reference" by a nemory_management_control_operation equal to 2. Rec. ITU-T H.264 (01/2012) Otherwise, if memory_management_control_operation is equal to 3, the following ordered steps are specified: 1. The assignment process of a LongTermFrameIdx to a short-term reference picture as specified in clause 8.2.5.4.3 is invoked. For this invocation of the process specified in clause 8.2.5.4.3, all pictures that are marked as "reference base picture" are considered as not present. The variable picNumX is set equal to the value picNumX that is derived during the invocation of the process specified in clause 8.2.5.4.3. Depending on whether there exists a picture that is marked as "reference base picture" and "used for short-term reference" and has a value of PicNum equal to picNumX, the following applies: If there exists a picture that is marked as "reference base picture" and "used for short-term reference" and has a value of PicNum equal to picNumX, the assignment process of a LongTermFrameIdx to a short-term reference picture as specified in clause 8.2.5.4.3 is invoked again. For this second invocation of the process specified in clause 8.2.5.4.3, all pictures that are not marked as "reference base picture" are considered as not present. NOTE 3 - When the marking of a decoded picture (not marked as "reference base picture") is changed from "used for short-term reference" to "used for long-term reference" and there exists a reference base picture (marked as "reference base picture") that has the same value of PicNum as the decoded picture (before the marking is modified), the marking of this reference base picture is also changed from "used for short-term reference" to "used