How to set image memory to preallocated space without copying?

[Lephar]

Hello,

I have raw image data on a preallocated memory space and I want to set this memory as ktx texture data to avoid extra copies. I scanned the examples and topics on libktx reference and couldn't find what I was looking for. I don't think something like ktxTexture2_SetImageMemory exists (not to be confused with ktxTexture2_SetImageFromMemory).

So far, I can use ktxTexture2_Create with KTX_TEXTURE_CREATE_NO_STORAGE flag but then I couldn't find any way to set memory space for newly created ktxTexture2. ktxTexture2_SetImageFromMemory issues KTX_INVALID_OPERATION error if the texture is created with KTX_TEXTURE_CREATE_NO_STORAGE.

The same thing is also needed when I query the meta data using ktxTexture2_CreateFromNamedFile without KTX_TEXTURE_CREATE_LOAD_IMAGE_DATA_BIT flag and THEN load the image using ktxTexture2_LoadImageData. When created this way, no memory is allocated and we have to provide external buffer to ktxTexture2_LoadImageData via pBuffer and bufSize parameters. Then we can't bind this memory to ktxTexture2 for future operations such as ktxTexture2_TranscodeBasis.

Is simply setting the ktxTexture2->pData to data buffer intended approach and I am just overthinking it?

Thanks in advance.

[Lephar]

A simple example of what I want to do is as following:

// Following function allocates the memory and returns the data
stbi_uc *data = stbi_load(filename, &width, &height, &channels, STBI_rgb_alpha);

ktxTexture2 *texture;
ktxTextureCreateInfo textureCreateInfo = {...};
ktxTexture2_Create(&textureCreateInfo, KTX_TEXTURE_CREATE_NO_STORAGE, &texture);

// This needs allocation and uses copy, I don't want this
// ktxTexture_SetImageFromMemory(texture, ..., data, ...);

ktxTexture2_SetImageMemory((ktx_uint8_t *) data); // <-------- I NEED THIS
// Or simply: texture->pData = (ktx_uint8_t *) data; Is this safe???

// Now I should be able to use following functions in-place
ktxTexture2_CompressBasisEx(texture, ...);
ktxTexture2_TranscodeBasis(texture, ...);

Is this safe? Because I don't think a function exists to do that safely.

[MarkCallow]

ktxTexture2_SetImageMemory((ktx_uint8_t *) data); // <-------- I NEED THIS
// Or simply: texture->pData = (ktx_uint8_t *) data; Is this safe???

Yes, it is safe provided the memory chunk with the data can be freed by a call to free. ktxTexture2_destruct will free a non-zero texture->pData.

But ...

// Now I should be able to use following functions in-place
ktxTexture2_CompressBasisEx(texture, ...);
ktxTexture2_TranscodeBasis(texture, ...);

... you must correctly initialize the DFD (texture->pDfd) for the data you have loaded. That includes the color space information. The correct transferFunction is vital for correct compression and eventually correct transcode results. Also the transferFunction must match that required for the VkFormat you provided in textureCreateInfo.

When I last looked at it STB Image did not have a way to obtain an image's colorspace information from the file it is loading. That is why we do no use it in KTX-Software. Color correctness is important.

[Lephar]

Thanks for your answer, that was more than I wished for!

... you must correctly initialize the DFD (texture->pDfd) for the data you have loaded ...

We are talking about Data Format Descriptors here right? If so, are there any helper functions on libktx for the job or should I manually set them using the KHR_DF_* flags? I don't seem to remember any such functions on libktx reference.

Also, is it necessary to set them for my second use case? Which is:

... ktxTexture2_CreateFromNamedFile without KTX_TEXTURE_CREATE_LOAD_IMAGE_DATA_BIT flag and THEN load the image using ktxTexture2_LoadImageData ...

I think they will be initialized from the original ktx2 file.

Thanks in advance!

[MarkCallow]

The DFD is initially set based on the vkFormat value in your ktxTextureCreateInfo. If you have chosen a format that matches your data then all is good. Otherwise you will have to use the KHR_DF_* flags and macros to change things.

No need for you to set the DFD in your second use case. If the file you are loading is a KTX v2 file, its DFD is loaded. If it is a KTX v1 file there is no DFD and the ktxTexture1 pseudo-class also does not have one. (You can write out a ktxTexture1 as ktxTexture2 with ktxTexture1_WriteKTX2To* functions. In that case the DFD is set to match the Vulkan format that matches the original GL format.)

[MarkCallow]

I have just been informed the stbimage still has no support for acquiring the image's color space info. I am told you will get incorrect results if you load a non-sRGB image with it. Normal maps need to be linear not sRGB. I use lodepng.

[Lephar]

Thanks, that was a great help.

I already know that my normal and metallic roughness textures are linear and base color textures are sRGB, so I can set those DFD values myself. At the end of the day, I offline convert all the textures using ktx create anyway. Even though I only need my first case while prototyping, it takes too long and minor performance gains are welcome.

So that leaves us only my second case and you wholly answered all my questions. Thanks a lot!