Oodle Texture bc7prep data flow

We mostly talk about Oodle Texture encoding to BCN from source art, usually with RDO (rate-distortion optimization).

In the previous post about Oodle Texture we looked at the data flow of texture content in games, for the case of Oodle Texture RDO.

There is a different technology in Oodle Texture called "bc7prep" that can be used differently, or in addition to RDO.

BC7prep is a lossless transform specifically for BC7 (not BC1-6) that rearranges the bits to improve the compression ratio. Unlike Oodle Texture RDO, BC7Prep requires a reverse transform at runtime to unpack it back to BC7. This is a super fast operation, and can also be done with a GPU compute shader so the CPU doesn't have to touch the bits at all.

BC7prep can be used in combination with Oodle Texture RDO encoding, or on BC7 encodings made from any source. It typically improves the compression of BC7 by 5-15%

BC7 is particularly important because it makes up a lot of the size of modern games. It's a commonly used texture format, and without RDO or bc7prep it often doesn't compress much at all.

The similar data flow chart for BC7 textures that use bc7prep is :

RGB uncompressed source art content like BMP or PNG

|     <- this step is where Oodle Texture RDO goes
V

BC7 compressed texture  <- you can also start here for bc7prep

|     <- bc7prep transforms BC7 to "prepped" data
V

BC7Prep transformed texture

|
V

Kraken or zlib compression on the game package containing the BCN textures

|                                                                                   TOOLS ^
V

sent over network, stored on disk

|                                                                                   RUNTIME v
V

decompress Kraken or zlib to load (sometimes with hardware decompressor)

|
V

BC7Prep transformed texture

|     <- bc7prep unpacking, can be done on GPU
V

BC7 compressed texture in memory

|
V

rendered on GPU

Oodle Texture gives you several options for how you use it depending on what best fits your game. You can use only Oodle Texture RDO, in which case no runtime decoding is needed. You can use just bc7prep on existing BC7 encoded data, in which case you don't need to use Oodle Texture's BCN encoding from source art at all. Or you can use both together.

BC7Prep combined with Oodle Texture RDO at "near lossless" levels provides size savings for BC7 with almost no visual quality difference from a non-RDO BC7 encoding.

On varied BC7 textures :

total :
BC7 + Kraken                        : 1.387 to 1
BC7Prep + Kraken                    : 1.530 to 1
NearLossless RDO + BC7Prep + Kraken : 1.650 to 1

The size savings with BC7Prep are not huge and dramatic the way they are with RDO, because the BC7Prep transform is lossless, but they are very large compared to the normal differences between lossless compression options.

For example the compression ratio on that BC7 set with Oodle Leviathan is 1.409 to 1, not much better than Kraken, and a far smaller gain than BC7Prep gives. Oodle Leviathan is a very strong compressor that usually finds bigger gains over Kraken than that, but BC7 data is hard for compressors to parse. BC7Prep and Oodle Texture RDO put the data into a form that increases the benefit of strong compressors like Oodle over weaker ones like zlib. (on data that's incompressible, all compressors are the same).

The runtime BC7Prep untransform is extremely fast. If you're using Oodle Data compression in software, it's best to do the BC7Prep untransform in software right after decompression. If you're on a platform with hardware decompression, you may want to use BC7Prep untransform compute shaders so the texture data never has to be touched by the CPU.

Visit the RAD Game Tools website to read all about Oodle Texture.