The overlay input selector before the overlay manager is a n-input to n-output crossbar switch. As such, any input can be connected to any output in the selector. The selection is based on the Z-order layer selection in the overlay configuration through the following register fields:
- DSS0_OVR_ATTRIBUTES_0[4-1] CHANNELIN field for layer 0, Z-order 0
- DSS0_OVR_ATTRIBUTES_1[4-1] CHANNELIN field for layer 1, Z-order 1
- DSS0_OVR_ATTRIBUTES_2[4-1] CHANNELIN field for layer 2, Z-order 2
- DSS0_OVR_ATTRIBUTES_3[4-1] CHANNELIN field for layer 3, Z-order 3
- DSS0_OVR_ATTRIBUTES_4[4-1] CHANNELIN field for layer 4, Z-order 4
The z-order determines the order in which the selected layers are blended together to generate the final output by the overlay manager:
- The lowest enabled order input is the bottom-most layer, just above the background color
- The highest enabled order input is the top-most layer
The OVR input selector also passes the following attributes from the selected input pipeline to the corresponding selector output port (zsel):
- Source pipeline size attributes:
- Number of pixels per line (from DSS0_VID_SIZE[13-0] SIZEX register field)
- Number of lines (from DSS0_VID_SIZE[29-16] SIZEY regiser field)
- Source pipeline global blending level (from DSS0_VID_GLOBAL_ALPHA
register)
Then, the overlay manager uses the above listed attributes as input layer attributes along with POSX and POSY overlay manager configuration parameters. The POSX and POSY can be configured in the [13-0] POSX and [29-16] POSY fields of the following registers:
- For layer 0, Z-order 0, in DSS0_OVR_ATTRIBUTES2_0 register
- For layer 1, Z-order 1, in DSS0_OVR_ATTRIBUTES2_1 register
- For layer 2, Z-order 2, in DSS0_OVR_ATTRIBUTES2_2 register
- For layer 3, Z-order 3, in DSS0_OVR_ATTRIBUTES2_3 register
- For layer 4, Z-order 4, in DSS0_OVR_ATTRIBUTES2_4 register
Note: The output of an input pipeline can be mapped to more than one overlay manager simultaneously, if and only if the following conditions are true:
- All video port timing generators controlling the overlay managers are identically programmed (same frame width/height with same blanking parameters running at same frame rate) and clocked by the same pixel clock source, AND ...
- The pipeline output is positioned on each display output at the same frame position, OR ...
- One of the overlay managers is connected to the write-back path only.
The software is responsible for managing the pipeline assignment and setting up the video port timing generators properly in order to avoid any resource conflicts that may cause the DISPC to lock up (sync loss).