.KTX File Format

Introduction

KTX is a container for textures, made by the Khronos Group, the body behind OpenGL and Vulkan. A texture is stored in the form the graphics card uses, often already block-compressed, so it can be loaded without decoding. One file can hold all the mipmap levels, the six faces of a cube map and arrays of layers.

There are two versions. KTX 1 (.ktx) describes the data with OpenGL constants. KTX 2 (.ktx2) uses Vulkan formats, a data format descriptor and optional supercompression with Basis Universal or Zstandard. KTX 2 with Basis Universal is the texture format of the glTF extension KHR_texture_basisu.

Structure

Both versions start with a 12-byte identifier. KTX 1 uses AB 4B 54 58 20 31 31 BB 0D 0A 1A 0A («KTX 11»), and KTX 2 uses AB 4B 54 58 20 32 30 BB 0D 0A 1A 0A («KTX 20»).

In KTX 1 the identifier is followed by an endianness word and twelve more 32-bit fields: the OpenGL type and formats, the width, height and depth, the number of array elements, faces and mipmap levels, and the length of the key and value data. The key and value pairs come next, then each mipmap level with its size in front.

In KTX 2 the identifier is followed by the Vulkan format, the size of the data type, the width, height and depth, the layer, face and level counts and the supercompression scheme. An index gives the offset and length of the data format descriptor, the key and value data and any supercompression data, and then one entry per mipmap level. Levels are stored from the smallest to the largest.

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