Can a KTX image file be a cubemap arrays? - c++

Question

Is it valid for a KTX image to be a cubemaps arrays, or is that not a thing?
I have some code that I'm currently using for uploading the data from a KTX file to the GPU. Currently, the code works for a regular 2d image, a cubemap, and a texture array. However, it would not support a KTX image that is a cubemap array, if that is a thing.
If it is possible, what is the code below missing to accomplish that?
uint32_t offset = 0;
for (uint32_t layer = 0; layer < layers; layer++) {
for (uint32_t face = 0; face < faces; face++) {
for (uint32_t level = 0; level < mipLevels; level++) {
offset = tex->GetImageOffset(layer, face, level);
vk::BufferImageCopy bufferCopyRegion = {};
bufferCopyRegion.imageSubresource.aspectMask = vk::ImageAspectFlagBits::eColor;
bufferCopyRegion.imageSubresource.mipLevel = level;
bufferCopyRegion.imageSubresource.baseArrayLayer = (faces == 6 ? face : layer); // TexArray or Cubemap, not both.
bufferCopyRegion.imageSubresource.layerCount = 1;
bufferCopyRegion.imageExtent.width = width >> level;;
bufferCopyRegion.imageExtent.height = height >> level;
bufferCopyRegion.imageExtent.depth = 1;
bufferCopyRegion.bufferOffset = offset;
bufferCopyRegions.push_back(bufferCopyRegion);
}
}
}
Vulkan command to transfer the image.
// std::vector<vk::BufferImageCopy> regions;
cmdBuf->copyBufferToImage(srcBufferHandle, destImageHandle,
vk::ImageLayout::eTransferDstOptimal, uint32_t(regions.size()), regions.data());

Answer 1

Yes, KTX also supports cube map arrays (see the KTX specification). Those are stored using layers.
The Vulkan spec states the following on how cube maps are stored in a cube map array:
For cube arrays, each set of six sequential
layers is a single cube, so the number of cube maps in a cube map array view is layerCount / 6, and
image array layer (baseArrayLayer + i) is face index (i mod 6) of cube i / 6.
So you need to change the baseArrayLayer of your buffer copy region accordingly.
Sample code:
// Setup buffer copy regions to get the data from the ktx file to your own image
for (uint32_t layer = 0; layer < ktxTexture->numLayers; layer++) {
for (uint32_t face = 0; face < 6; face++) {
for (uint32_t level = 0; level < ktxTexture->numLevels; level++) {
ktx_size_t offset;
KTX_error_code ret = ktxTexture_GetImageOffset(ktxTexture, level, layer, face, &offset);
VkBufferImageCopy bufferCopyRegion = {};
bufferCopyRegion.imageSubresource.aspectMask = VK_IMAGE_ASPECT_COLOR_BIT;
bufferCopyRegion.imageSubresource.mipLevel = level;
bufferCopyRegion.imageSubresource.baseArrayLayer = layer * 6 + face;
bufferCopyRegion.imageSubresource.layerCount = 1;
bufferCopyRegion.imageExtent.width = ktxTexture->baseWidth >> level;
bufferCopyRegion.imageExtent.height = ktxTexture->baseHeight >> level;
bufferCopyRegion.imageExtent.depth = 1;
bufferCopyRegion.bufferOffset = offset;
bufferCopyRegions.push_back(bufferCopyRegion);
}
}
}
// Create the image view for a cube map array
VkImageViewCreateInfo view = vks::initializers::imageViewCreateInfo();
view.viewType = VK_IMAGE_VIEW_TYPE_CUBE_ARRAY;
view.format = format;
view.components = { VK_COMPONENT_SWIZZLE_R, VK_COMPONENT_SWIZZLE_G, VK_COMPONENT_SWIZZLE_B, VK_COMPONENT_SWIZZLE_A };
view.subresourceRange = { VK_IMAGE_ASPECT_COLOR_BIT, 0, 1, 0, 1 };
view.subresourceRange.layerCount = 6 * cubeMap.layerCount;
view.subresourceRange.levelCount = cubeMap.mipLevels;
view.image = cubeMap.image;
vkCreateImageView(device, &view, nullptr, &cubeMap.view);