Blackmagic Design ATEM Mini Manuel D'installation Et D'utilisation page 52

You will now see an alpha channel appear underneath the RGB color channels.
The alpha channel contains a greyscale version of the combined color channels
in your graphic. Don't forget to click on the alpha channel's 'eye' icon to make
sure it is selected so it will be included when the TGA file is saved.
5 Your selection has now been used to create the greyscale matte in the alpha channel.
If you want to, you can now go to the 'menu' bar and click 'select/deselect' to remove
the selection marquee.
6 Now it's time to save your TGA file.
Go to the file menu and click on 'save as'. Type the file name and select the location for
your file. In the format box, select 'targa', which is the full name for a TGA file and make
sure the 'alpha channels' checkbox is selected.
7 Click 'save'. A targa options box will appear asking which resolution you want to save.
Select '32 bits/pixel'. This provides enough data for four 8 bit channels which includes
the red, green and blue color channels, plus the alpha channel. Click 'OK'.
ATEM Software Control 52