libktx Reference

libktx is a small library of functions for creating and reading KTX (Khronos TeXture) files, version 1 and 2 and instantiating OpenGL® and OpenGL® ES textures and Vulkan images from them. KTX version 2 files can contain images supercompressed with zstd or zlib. They can also contain images in the Basis Universal formats. libktx can deflate and inflate zstd and zlib compressed images, can encode and transcode the Basis Universal formats and can encode ASTC formats.

For information about the KTX format see the formal specification.

Authors

Mark Callow, Edgewise Consulting, formerly at HI Corporation
Mátyás Császár and Daniel Rákos, RasterGrid
Wasim Abbas, Arm
Andreas Atteneder, Independent
Georg Kolling, Imagination Technology
Jacob Ström, Ericsson AB

API Version

v4.0

Date

Fri Mar 14 14:51:46 2025 +0300

Usage Overview

The following ktxTexture examples work for both KTX and KTX2 textures. The texture type is determined from the file contents.

Reading a KTX file for non-GL and non-Vulkan Use

#include <ktx.h>
 
ktxTexture* texture;
KTX_error_code result;
ktx_size_t offset;
ktx_uint8_t* image;
ktx_uint32_t level, layer, faceSlice;
 
result = ktxTexture_CreateFromNamedFile("mytex3d.ktx",
                                        KTX_TEXTURE_CREATE_LOAD_IMAGE_DATA_BIT,
                                        &texture);
 
// Retrieve information about the texture from fields in the ktxTexture
// such as:
ktx_uint32_t numLevels = texture->numLevels;
ktx_uint32_t baseWidth = texture->baseWidth;
ktx_bool_t isArray = texture->isArray;
 
// Retrieve a pointer to the image for a specific mip level, array layer
// & face or depth slice.
level = 1; layer = 0; faceSlice = 3;
result = ktxTexture_GetImageOffset(texture, level, layer, faceSlice, &offset);
image = ktxTexture_GetData(texture) + offset;
// ...
// Do something with the texture image.
// ...
ktxTexture_Destroy(texture);

Creating a GL texture object from a KTX file.

#include <ktx.h>
 
ktxTexture* kTexture;
KTX_error_code result;
ktx_size_t offset;
ktx_uint8_t* image;
ktx_uint32_t level, layer, faceSlice;
GLuint texture = 0;
GLenum target, glerror;
 
result = ktxTexture_CreateFromNamedFile("mytex3d.ktx",
                                        KTX_TEXTURE_CREATE_NO_FLAGS,
                                        &kTexture);
 
// Before the first call to  ktxTexture_GLUpload make libktx load its
// function pointers for the GL functions it uses. The parameter is a
// pointer to the GLGetProcAddress function provided by whatever OpenGL
// framework the application is using. The example shown is for SDL.
//
// Note 1: This is unrelated to any GL function pointers the app may be
//         using.
// Note 2: When this is not called, libktx has fallback mechanisms to
//         find the pointers which work on the vast majority of
//         platforms. The only known failures have occurred on Fedora.
result = ktxLoadOpenGL((PFNGLGETPROCADDRESS)SDL_GL_GetProcAddress);
 
glGenTextures(1, &texture); // Optional. GLUpload can generate a texture.
result = ktxTexture_GLUpload(kTexture, &texture, &target, &glerror);
ktxTexture_Destroy(kTexture);
// ...
// GL rendering using the texture
// ...

Creating a Vulkan image object from a KTX file.

#include <vulkan/vulkan.h>
#include <ktxvulkan.h>
 
ktxTexture* kTexture;
KTX_error_code result;
ktx_size_t offset;
ktx_uint8_t* image;
ktx_uint32_t level, layer, faceSlice;
ktxVulkanDeviceInfo vdi;
ktxVulkanTexture texture;
 
// Set up Vulkan physical device (gpu), logical device (device), queue
// and command pool. Save the handles to these in a struct called vkctx.
// ktx VulkanDeviceInfo is used to pass these with the expectation that
// apps are likely to upload a large number of textures.
ktxVulkanDeviceInfo_Construct(&vdi, vkctx.gpu, vkctx.device,
                              vkctx.queue, vkctx.commandPool, nullptr);
 
ktxresult = ktxTexture_CreateFromNamedFile("mytex3d.ktx",
                                           KTX_TEXTURE_CREATE_NO_FLAGS,
                                           &kTexture);
 
ktxresult = ktxTexture_VkUploadEx(kTexture, &vdi, &texture,
                                  VK_IMAGE_TILING_OPTIMAL,
                                  VK_IMAGE_USAGE_SAMPLED_BIT,
                                  VK_IMAGE_LAYOUT_SHADER_READ_ONLY_OPTIMAL);
 
ktxTexture_Destroy(kTexture);
ktxVulkanDeviceInfo_Destruct(&vdi);
// ...
// Vulkan rendering using the texture
// ...
// When done using the image in Vulkan...
ktxVulkanTexture_Destruct(&texture, vkctx.device, nullptr);

Extracting Metadata

Once a ktxTexture object has been created, metadata can be easily found and extracted. The following can be added to any of the above.

char* pValue;
uint32_t valueLen;
if (KTX_SUCCESS == ktxHashList_FindValue(&kTexture->kvDataHead,
                                          KTX_ORIENTATION_KEY,
                                          &valueLen, (void**)&pValue))
 {
      char s, t;
 
      if (sscanf(pValue, KTX_ORIENTATION2_FMT, &s, &t) == 2) {
         ...
      }
 }

Writing a KTX or KTX2 file

#include <ktx.h>
#include <vkformat_enum.h>
 
ktxTexture2* texture;                   // For KTX2
//ktxTexture1* texture;                 // For KTX
ktxTextureCreateInfo createInfo;
KTX_error_code result;
ktx_uint32_t level, layer, faceSlice;
FILE* src;
ktx_size_t srcSize;
 
createInfo.glInternalformat = GL_RGB8;   // Ignored if creating a ktxTexture2.
createInfo.vkFormat = VK_FORMAT_R8G8B8_UNORM;   // Ignored if creating a ktxTexture1.
createInfo.baseWidth = 2048;
createInfo.baseHeight = 1024;
createInfo.baseDepth = 16;
createInfo.numDimensions = 3.
// Note: it is not necessary to provide a full mipmap pyramid.
createInfo.numLevels = log2(createInfo.baseWidth) + 1
createInfo.numLayers = 1;
createInfo.numFaces = 1;
createInfo.isArray = KTX_FALSE;
createInfo.generateMipmaps = KTX_FALSE;
 
// Call ktxTexture1_Create to create a KTX texture.
result = ktxTexture2_Create(&createInfo,
                            KTX_TEXTURE_CREATE_ALLOC_STORAGE,
                            &texture);
 
src = // Open a stdio FILE* on the baseLevel image, slice 0.
srcSize = // Query size of the file.
level = 0;
layer = 0;
faceSlice = 0;                           
result = ktxTexture_SetImageFromMemory(ktxTexture(texture),
                                       level, layer, faceSlice,
                                       src, srcSize);
// Repeat for the other 15 slices of the base level and all other levels
// up to createInfo.numLevels.
 
ktxTexture_WriteToNamedFile(ktxTexture(texture), "mytex3d.ktx");
ktxTexture_Destroy(ktxTexture(texture));

Modifying a KTX file

#include <ktx.h>
 
ktxTexture* texture;
KTX_error_code result;
ktx_size_t offset;
ktx_uint8_t* image;
ktx_uint32_t level, layer, faceSlice;
 
result = ktxTexture_CreateFromNamedFile("mytex3d.ktx",
                                        KTX_TEXTURE_CREATE_LOAD_IMAGE_DATA_BIT,
                                        &texture);
// The file is closed after all the data has been read.
 
// It is the responsibilty of the application to make sure its
// modifications are valid.
texture->generateMipmaps = KTX_TRUE;
 
ktxTexture_WriteToNamedFile(texture, "mytex3d.ktx");
ktxTexture_Destroy(texture);

Writing a Basis-compressed Universal Texture

Basis compression supports two universal texture formats: BasisLZ/ETC1S and UASTC. The latter gives higher quality at a larger file size. Textures can be compressed to either format using ktxTexture2_CompressBasisEx as shown in this example.

#include <ktx.h>
#include <vkformat_enum.h>
 
ktxTexture2* texture;
ktxTextureCreateInfo createInfo;
KTX_error_code result;
ktx_uint32_t level, layer, faceSlice;
FILE* src;
ktx_size_t srcSize;
ktxBasisParams params = {0};
params.structSize = sizeof(params);
 
createInfo.glInternalformat = 0;  //Ignored as we'll create a KTX2 texture.
createInfo.vkFormat = VK_FORMAT_R8G8B8A8_UNORM;
createInfo.baseWidth = 2048;
createInfo.baseHeight = 1024;
createInfo.baseDepth = 16;
createInfo.numDimensions = 3.
// Note: it is not necessary to provide a full mipmap pyramid.
createInfo.numLevels = log2(createInfo.baseWidth) + 1
createInfo.numLayers = 1;
createInfo.numFaces = 1;
createInfo.isArray = KTX_FALSE;
createInfo.generateMipmaps = KTX_FALSE;
 
result = ktxTexture2_Create(&createInfo,
                            KTX_TEXTURE_CREATE_ALLOC_STORAGE,
                            &texture);
 
src = // Open the file for the baseLevel image, slice 0 and
      // read it into memory.
srcSize = // Query size of the file.
level = 0;
layer = 0;
faceSlice = 0;                           
result = ktxTexture_SetImageFromMemory(ktxTexture(texture),
                                       level, layer, faceSlice,
                                       src, srcSize);
// Repeat for the other 15 slices of the base level and all other levels
// up to createInfo.numLevels.
 
// For BasisLZ/ETC1S
params.compressionLevel = KTX_ETC1S_DEFAULT_COMPRESSION_LEVEL;
// For UASTC
params.uastc = KTX_TRUE;
// Set other BasisLZ/ETC1S or UASTC params to change default quality settings.
result = ktxtexture2_CompressBasisEx(texture, &params);
 
ktxTexture_WriteToNamedFile(ktxTexture(texture), "mytex3d.ktx2");
ktxTexture_Destroy(ktxTexture(texture));

There is a shortcut that can be used when compressing to BasisLZ/ETC1S. Remove the declaration and initialization of params in the previous example and replace ktxtexture2_CompressBasisEx with

// Quality range is 1 - 255. 0 gets the default quality, currently 128.
// The qualityLevel field in ktxBasisParams is set from this.
int quality = 0;
result = ktxTexture2_CompressBasis(texture, quality);

Transcoding a BasisLZ/ETC1S or UASTC-compressed Texture

#include <ktx.h>
 
ktxTexture2* texture;
KTX_error_code result;
 
result = ktxTexture_CreateFromNamedFile("mytex3d_basis.ktx2",
                                        KTX_TEXTURE_CREATE_NO_FLAGS,
                                        (ktxTexture**)&kTexture);
// or
//result = ktxTexture2_CreateFromNamedFile("mytex3d_basis.ktx2",
//                                         KTX_TEXTURE_CREATE_NO_FLAGS,
//                                         &kTexture);
 
if (ktxTexture2_NeedsTranscoding(texture)) {
    ktx_texture_transcode_fmt_e tf;
 
    // Using VkGetPhysicalDeviceFeatures or GL_COMPRESSED_TEXTURE_FORMATS or
    // extension queries, determine what compressed texture formats are
    // supported and pick a format. For example
    vk::PhysicalDeviceFeatures deviceFeatures;
    vkctx.gpu.getFeatures(&deviceFeatures);
    khr_df_model_e colorModel = ktxTexture2_GetColorModel_e(texture);
    if (colorModel == KHR_DF_MODEL_UASTC
        && deviceFeatures.textureCompressionASTC_LDR) {
        tf = KTX_TTF_ASTC_4x4_RGBA;
    } else if (colorModel == KHR_DF_MODEL_ETC1S
               && deviceFeatures.textureCompressionETC2) {
        tf = KTX_TTF_ETC;
    } else if (deviceFeatures.textureCompressionASTC_LDR) {
        tf = KTX_TTF_ASTC_4x4_RGBA;
    } else if (deviceFeatures.textureCompressionETC2)
        tf = KTX_TTF_ETC2_RGBA;
    else if (deviceFeatures.textureCompressionBC)
        tf = KTX_TTF_BC3_RGBA;
    else {
        message << "Vulkan implementation does not support any available transcode target.";
        throw std::runtime_error(message.str());
    }
 
    result = ktxTexture2_TranscodeBasis(texture, tf, 0);
 
    // Then use VkUpload or GLUpload to create a texture object on the GPU.
}

Writing an ASTC-Compressed Texture

#include <ktx.h>
#include <vkformat_enum.h>
 
ktxTexture2* texture;
ktxTextureCreateInfo createInfo;
KTX_error_code result;
ktx_uint32_t level, layer, faceSlice;
FILE* src;
ktx_size_t srcSize;
ktxAstcParams params = {0};
params.structSize = sizeof(params);
 
createInfo.glInternalformat = 0;  //Ignored as we'll create a KTX2 texture.
createInfo.vkFormat = VK_FORMAT_R8G8B8A8_UNORM;
createInfo.baseWidth = 2048;
createInfo.baseHeight = 1024;
createInfo.baseDepth = 16;
createInfo.numDimensions = 3.
// Note: it is not necessary to provide a full mipmap pyramid.
createInfo.numLevels = log2(createInfo.baseWidth) + 1
createInfo.numLayers = 1;
createInfo.numFaces = 1;
createInfo.isArray = KTX_FALSE;
createInfo.generateMipmaps = KTX_FALSE;
 
result = ktxTexture2_Create(&createInfo,
                            KTX_TEXTURE_CREATE_ALLOC_STORAGE,
                            &texture);
 
src = // Open the file for the baseLevel image, slice 0 and
      // read it into memory.
srcSize = // Query size of the file.
level = 0;
layer = 0;
faceSlice = 0;                           
result = ktxTexture_SetImageFromMemory(ktxTexture(texture),
                                       level, layer, faceSlice,
                                       src, srcSize);
// Repeat for the other 15 slices of the base level and all other levels
// up to createInfo.numLevels.
 
params.threadCount = 1;
params.blockDimension = KTX_PACK_ASTC_BLOCK_DIMENSION_6x6;
params.mode = KTX_PACK_ASTC_ENCODER_MODE_LDR;
params.qualityLevel = KTX_PACK_ASTC_QUALITY_LEVEL_MEDIUM;
result = ktxtexture2_CompressAstcEx(texture, &params);
 
ktxTexture_WriteToNamedFile(ktxTexture(texture), "mytex3d.ktx2");
ktxTexture_Destroy(ktxTexture(texture));

There is a shortcut that can be used when the only params field you want to modify is the qualityLevel. Remove the declaration and initialization of params in the previous example and replace ktxtexture2_CompressAstcEx with

// Quality range is 0 - 100. 0 is fastest/lowest. 100 is slowest/highest.
int quality = KTX_PACK_ASTC_QUALITY_LEVEL_MEDIUM;
result = ktxTexture2_CompressAstc(texture, quality);