We have detected your current browser version is not the latest one. Xilinx.com uses the latest web technologies to bring you the best online experience possible. Please upgrade to a Xilinx.com supported browser:Chrome, Firefox, Internet Explorer 11, Safari. Thank you!

# AR# 57278

## Description

What is the valid range for the Global Alpha Value for each layer?

## Solution

For all versions of the Video On Screen Display (OSD), the full range of the Global Alpha Value is:
• 0- 256 (0x100) - 8-bit - a maximum value of 256 is 0% transparent (100% opaque)
• 1-1024 (0x400) - 10-bit - a maximum value of 1024 is 0% transparent (100% opaque)
• 1-4096 (0x1000) - 12-bit - a maximum value of 4096 is 0% transparent (100% opaque)

Video On Screen Display (OSD) v5.01.a:
Due to the size of the registers, the working range is limited to (2^(DATA_WIDTH) - 1) in both the GUI and via the AXI-4 Lite interface.
Because the full global alpha range is (2^DATA_WIDTH) the core layers cannot be full opacity. The maximum they can be set to is (2^DATA_WIDTH)/(2^(DATA_WIDTH-1)), which though it is not completely opaque it as close to 0% transparent (100% opaque) as is possible in the OSD v5.01.a.

For example, if the input bit width is 8, then the maximum internal global alpha value will actually be 256 (2^DATA_WIDTH)), but the maximum value that could be set is 255 (2^(DATA_WIDTH) - 1).
The maximum global alpha value for each data width is:
• 8-bit - (2^(DATA_WIDTH) - 1) = 256 for a maximum opacity of 255/256
• 10-bit - (2^(DATA_WIDTH) - 1) = 4095 for a maximum opacity of 1023/1024
• 12-bit - (2^(DATA_WIDTH) - 1) = 4095 for a maximum opacity of 4095/4096

Video On Screen Display (OSD) v6.0 and v6.0 (Rev. 1):
In the Video On Screen Display (OSD) v6.0 and v6.0 (Rev. 1), the Global Alpha Value core registers were updated to support the full range of (2^DATA_WIDTH), but the GUI still limits this setting to (2^(DATA_WIDTH) - 1).

A user can work around the GUI limitation by updating the registers via the AXI4-Lite interface, using the MAX values listed at the beginning of the solution (2^DATA_WIDTH). The range for the Global Alpha Value, can be found in the Video On-Screen Display v6.0 Product Guide (PG010).

Video On Screen Display (ODS) v6.0 (Rev. 2) and later:
The Video On-Screen Display (OSD) v6.0 (Rev. 2) and later GUI has been updated to support the full available range for the Global Alpha Value.

For a detailed list of LogiCORE IP Video On Screen Display Release Notes and Known Issues, see (Xilinx Answer 33257).
For a detailed list of LogiCORE IP Video On Screen Display Release Notes and Known Issues for Vivado 2013.1 and newer tool versions, see (Xilinx Answer 54539).