TGA is a common bitmap format used in game development. For more information see the Wikipedia page Truevision TGA.
This format is an intermediary format for use outside a game engine, it is not an in-game format.
Exporting out of Photoshop
TGA images can be exported out of Photoshop in a few different ways.
16 bits per pixel
Exports 3 Channels, Red, Blue and Green. Each Pixel within a channel is broken down into 5 bits per channel. The final 1 bit is unused.
24 bits per pixel
Exports 3 channels, Red, blue and Green. Each pixel within a channel is broken down into 8 bits of information.
32 bits per pixel
Exports 4 channels, Red, blue, Green and Alpha. Each pixel within a channel is broken down into 8 bits of information.
This in conclusion tells us that 16bit/pixel TGA gives us a more compact but lower quality image. 24bit/pixel gives us a larger file size but has a higher quality image. 32bit/pixel gives us the same image quality as 24bits/pixel but with the added bonus of an extra 8bit channel. This can be used form masking transparency, metallic maps or roughness.
When exporting there is also the option for “Compress (RLE)”. RLE stands for Run Length Encoding. It’s a type of Lossless Encoding which means you get to save space without any loss to the quality of your image. It works by grouping values together and reading them a little differently.
Without RLE: 3,3,3,4,5,5,6,6,6
With RLE: (3,3),(1,4),(2,5)(3,6)
If you tally up how many digits of data there you can see that RLE saves us some memory. This technique should always be on due to it’s lossless nature.