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 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. If you want your code to always stay compatible with the latest release of NVTT, you should not include other headers, nor use members outside of the `nvtt` namespace.
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 `InputOptions` class describes the images that compose the compressed texture, and several image processing operations that can be applied to them.
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.
By default the entire mipmap chain is generated, but it's possible to limit the number of mimap levels generated with the `maxLevel` argument. When `maxLevel` is set to -1, this argument has no effect.
`MipmapFilter_Box` is a [http://developer.nvidia.com/object/np2_mipmapping.html polyphase box filter]. It's the default option and good choice for most cases. It's also much faster than the other filters.
`MipmapFilter_Kaiser` is a Kaiser-windowed sinc filter. That's generally considered the best choice for downsampling filters, but in order to obtain best results it may be required to tweak some of the parameters of the filter, otherwise the resulting images could suffer from ringing artifacts or the result may not be as sharp as desired. The default values of the parameters are generally safe, but it's possible to tweak them using the following method:
Larger kernel widths are supposed to approximate better a perfect low pass filter, but sometimes result in ringing artifacts. Values above 5 are not recommended. You should not change the alpha and stretch values unless you know what you are doing.
You can compare the result of each of these filters at [ResizeFilters]. For more information about mipmap generation algorithms see [MipmapGeneration].
When evaluating the color of texels that are near the border, most the filters usually sample outside of the texture. By default, NVTT assumes the texture wrapping mode is to mirror, because that generally looks good, but better results can be achieved by explicitly specifying the desired wrapping mode. That can be done with the following method:
Note that the `wrapMode` is also used for other image processing operations, like normal map generation. So, it's a good idea to set it even if you are not generating mipmaps. The default `wrapMode` is `WrapMode_Mirror`.
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:
Note, that using different values for `inputGamma` and `outputGamma` could result in quantization artifacts when the input components only have 8 bits (that is always the case in the current version).
If the input image is a normal map, it may require some special treatment. For example, gamma correction won't be applied, and some operations will only be active for normal maps. To indicate that the input image is a normal map the following method is used:
Normal map mipmaps are generated by down filtering the previous normal map level, just like with regular images. However, after down filtering the resulting normals are generally not unit length. By default, these normals are re-normalized, but you can chose to left them unnormalized with the following method:
The height factors do not necessarily sum 1. So, you can also use them to change the steepness of the height map, and the sharpness of the resulting normal map.
Mipmap generation behaves as with regular normal maps. The input images are first converted to normal map. Then the mipmaps are generated from the normal maps as usual, unless the user provides the mipmaps explicitly, in which case normal map conversion is applied again.
*TODO*: Normal map conversion is only done to the input images. Mipmaps are generated from the normal map as usual.
It's possible to constrain the size of the textures to a certain limit. This is accomplished with the following method:
{{{
void InputOptions::setMaxExtents(int d);
}}}
NVTT is able to handle non power of two textures correctly, but not all hardware supports them. It's possible to round the size of the input images to the next, nearest, or previous power of two. The rounding mode is specified with the following method.
`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 the 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, as shown in the following table:
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.
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.
By default the compressor calls the `writeData` method before indicating the image that the data belongs to. That means that the data does not belong to any of the surface images, but is part of the header file. If you don't want to output the header file, you can ignore these calls, or disable this behavior with:
