The NVIDIA Texture Tools library (NVTT for short) is a C++ library that allows you to create textures compressed in any of the DX10 texture formats, and apply certain transformations to them such as mipmap generation, and normal map conversion.
By default NVTT is compiled as a shared library. To use it in your application you only have to link it implicitly with your application. With gcc that's achieved using `-lnvtt`, and with Visual Studio you have to add `nvtt.lib` to the Linker -> Input -> Additional Dependencies field of the project settings. Dynamic linking the library explicitly is not recommended.
All the members of the API are in the `nvtt` namespace. Members outside of the `nvtt` namespace should not be considered public and are subject to change. You should not use them if you want your code to always stay compatible with the latest release of NVTT.
As new features are added to NVTT, binary compatibility will always be preserved. That should allow upgrading to new versions without even having to recompile the application code.
It takes the texture defined by `InputOptions`, compresses it according to the `CompressionOptions` and outputs it according to the `OutputOptions`. These classes are documented in detail in the following sections.
The input options class describes the images that compose the texture to be generated, and several image processing operations that could be applied to it.
If the width, height or depth of the provided mipmap does not match the one expected by the layout the function returns false, otherwise it returns true.
Again, since 3D textures are not supported, the depth argument should always be set to 1.
Each of the faces of the surface is composed of a mipmap chain. You can provide each of these images explicitely with the `setMipmapData` method, but you can also disable mipmap generation or let the library compute the mipmaps procedurally. The following method provides some control over this process:
In order to output mipmaps for each of the faces of the surface, the `generateMipmaps` argument should be set to true.
If the mipmap image is not provided explicitly by the application, and generate mipmaps is enabled, then the mipmaps will be generated automatically from the previous level using a downsampling filter.
*TODO*: Partial mipmap chains.
It's possible to specify what filter should be used using the `filter` argument. The available options are:
It's possible to limit the number of mipmap levels that are generated with the `maxLevel` argument. For example, the following example will only generate the first 4 levels of the mipmap chain using a box filter:
Mipmaps are generated using a convolution filter. When evaluating the color of texels that are near the border the convolution filter usually samples outside of the texture. By default, NVTT assumes the texture is mirrored, because that generally looks good, but better results can be achieved by explicitly specifying the wrapping mode. That can be done as follows:
By default, both the `inputGamma` and the `outputGamma` are set to 2.2. Gamma correction is only applied to the RGB channels. It's not applied to the alpha channel and it's never applied to normal maps. You can disable gamma correction by setting these values to 1.0 as follows:
`Quality_Fastest` will select a quick compressor that produces reasonable results in a very short amount of time. Note that this is not a real-time compressor. The code is not highly optimized, but still it's generally one order of magnitude faster than the normal CPU compression mode. If you need real-time compression you can find more resources at: [RealTimeDXTCompression].
`Quality_Production` will generally produce similar results as `Quality_Normal`, but it may double or triple the compression time to obtain minor quality improvements.
`Quality_Highest` is a brute force compressor. In some cases, depending on the size of the search space, this compressor will be extremely slow. Use this only for testing purposes, to determine how much room is left for improvement in the regular compressors.
Selecting a compression quality that's not available won't produce any error, but instead it will fall back to another compression mode. If `Quality_Production` is not available, `Quality_Normal` will be used instead, and if `Quality_Normal` is not available, `Quality_Fastest` will be used. `Quality_Highest` will never be used as a fallback.
Not all compressors are GPU accelerated. Some formats can be compressed relatively fast, but others are much more expensive. Currently GPU acceleration is implemented only for the slowest compression modes:
|| || *GPU Accelerated* ||
|| BC1 || Yes ||
|| BC1a || ||
|| BC2 || Yes ||
|| BC3 || Yes ||
|| BC3n || ||
|| BC4 || ||
|| BC5 || ||
High quality texture compression is a complex problem that would be very hard to solve using traditional GPGPU approaches (that is using the graphics API, vertex and fragment shaders). For this reason GPU compression is implemented in CUDA. Note that currently, CUDA is only available on NVIDIA !GeForce 8 series.
When available, CUDA compression is enabled by default, but can be disabled as follows:
The GPU compressors do not provide exactly the same result as the CPU compressors. However, the difference between the two compressors is always very small and not noticeable to the eye.
In order to minimize memory allocations NVTT will call writeData as soon as compressed data is available. Some compressors output one block at a time, while others output many.
