如何在 Vulkan 中从 gpu 读取 R32G32_SFLOAT 图像

Question

我可以从 R32G32B32A32 图像中转储内容以进行屏幕截图。我也想从 R32G32_SFLOAT 图像中读出一个像素。但是结果看起来很奇怪。

下面是我的工作图像转储代码（无验证错误）

void DumpImageToFile(VkTool::VulkanDevice &device, VkQueue graphics_queue, VkTool::Wrapper::CommandBuffers &command_buffer, VkImage image, uint32_t width, uint32_t height, const char *filename)
{
    auto image_create_info = VkTool::Initializer::GenerateImageCreateInfo(VK_IMAGE_TYPE_2D, VK_FORMAT_R8G8B8A8_UNORM, {width, height, 1},
        VK_IMAGE_USAGE_TRANSFER_SRC_BIT | VK_IMAGE_USAGE_TRANSFER_DST_BIT, VK_SAMPLE_COUNT_1_BIT);
    VkTool::Wrapper::Image staging_image(device, image_create_info, VK_MEMORY_HEAP_DEVICE_LOCAL_BIT);
    auto buffer_create_info = VkTool::Initializer::GenerateBufferCreateInfo(width * height * 4, VK_BUFFER_USAGE_TRANSFER_DST_BIT);
    VkTool::Wrapper::Buffer staging_buffer(device, buffer_create_info, VK_MEMORY_PROPERTY_HOST_VISIBLE_BIT | VK_MEMORY_PROPERTY_HOST_COHERENT_BIT);

    // Copy texture to buffer

    command_buffer.Begin();

    auto image_memory_barrier = VkTool::Initializer::GenerateImageMemoryBarrier(VK_IMAGE_LAYOUT_UNDEFINED, VK_IMAGE_LAYOUT_TRANSFER_DST_OPTIMAL,
    { VK_IMAGE_ASPECT_COLOR_BIT, 0, 1, 0, 1 }, staging_image.Get());
    device.vkCmdPipelineBarrier(command_buffer.Get(), VK_PIPELINE_STAGE_TOP_OF_PIPE_BIT, VK_PIPELINE_STAGE_TRANSFER_BIT, 0
        , 0, nullptr, 0, nullptr, 1, &image_memory_barrier);


    image_memory_barrier = VkTool::Initializer::GenerateImageMemoryBarrier(VK_IMAGE_LAYOUT_UNDEFINED, VK_IMAGE_LAYOUT_TRANSFER_SRC_OPTIMAL,
    { VK_IMAGE_ASPECT_COLOR_BIT, 0, 1, 0, 1 }, image);
    device.vkCmdPipelineBarrier(command_buffer.Get(), VK_PIPELINE_STAGE_TOP_OF_PIPE_BIT, VK_PIPELINE_STAGE_TRANSFER_BIT, 0
        , 0, nullptr, 0, nullptr, 1, &image_memory_barrier);


    // Copy!!
    VkImageBlit region = {};
    region.srcSubresource = { VK_IMAGE_ASPECT_COLOR_BIT, 0, 0, 1 };
    region.srcOffsets[0] = { 0, 0, 0 };
    region.srcOffsets[1] = { static_cast<int32_t>(width), static_cast<int32_t>(height), 1};
    region.dstSubresource = { VK_IMAGE_ASPECT_COLOR_BIT, 0, 0, 1 };
    region.dstOffsets[0] = { 0, 0, 0 };
    region.dstOffsets[1] = { static_cast<int32_t>(width), static_cast<int32_t>(height), 1 };
    device.vkCmdBlitImage(command_buffer.Get(), image, VK_IMAGE_LAYOUT_TRANSFER_SRC_OPTIMAL, staging_image.Get(), VK_IMAGE_LAYOUT_TRANSFER_DST_OPTIMAL, 1, &region, VK_FILTER_LINEAR);


    image_memory_barrier = VkTool::Initializer::GenerateImageMemoryBarrier(VK_IMAGE_LAYOUT_TRANSFER_SRC_OPTIMAL, VK_IMAGE_LAYOUT_PRESENT_SRC_KHR,
    { VK_IMAGE_ASPECT_COLOR_BIT, 0, 1, 0, 1 }, image);
    device.vkCmdPipelineBarrier(command_buffer.Get(), VK_PIPELINE_STAGE_TRANSFER_BIT, VK_PIPELINE_STAGE_COLOR_ATTACHMENT_OUTPUT_BIT, 0
        , 0, nullptr, 0, nullptr, 1, &image_memory_barrier);

    image_memory_barrier = VkTool::Initializer::GenerateImageMemoryBarrier(VK_IMAGE_LAYOUT_TRANSFER_DST_OPTIMAL, VK_IMAGE_LAYOUT_TRANSFER_SRC_OPTIMAL,
    { VK_IMAGE_ASPECT_COLOR_BIT, 0, 1, 0, 1 }, staging_image.Get());
    device.vkCmdPipelineBarrier(command_buffer.Get(), VK_PIPELINE_STAGE_TRANSFER_BIT, VK_PIPELINE_STAGE_TRANSFER_BIT, 0
        , 0, nullptr, 0, nullptr, 1, &image_memory_barrier);

    auto buffer_image_copy = VkTool::Initializer::GenerateBufferImageCopy({ VK_IMAGE_ASPECT_COLOR_BIT , 0, 0, 1 }, { width, height, 1 });
    device.vkCmdCopyImageToBuffer(command_buffer.Get(), staging_image.Get(), VK_IMAGE_LAYOUT_TRANSFER_SRC_OPTIMAL, staging_buffer.Get(), 1, &buffer_image_copy);

    command_buffer.End();

    std::vector<VkCommandBuffer> raw_command_buffers = command_buffer.GetAll();
    auto submit_info = VkTool::Initializer::GenerateSubmitInfo(raw_command_buffers);
    VkTool::Wrapper::Fence fence(device);
    device.vkQueueSubmit(graphics_queue, 1, &submit_info, fence.Get());
    fence.Wait();
    fence.Destroy();

    const uint8_t *mapped_address = reinterpret_cast<const uint8_t *>(staging_buffer.MapMemory());
    lodepng::encode(filename, mapped_address, width, height);
    staging_buffer.UnmapMemory();

    staging_image.Destroy();
    staging_buffer.Destroy();
}

对不起，丑陋的自制包装纸，没有官方包装纸。基本上，它创建一个暂存图像和缓冲区。首先使用 vkCmdBlitImage 从源图像复制到暂存图像。然后使用 vkCmdCopyImageToBuffer 并将缓冲区映射到主机内存。此方法适用于多个 gpu，无需担心填充。（我猜，如果我错了请纠正我）。

但是，我没有运气用这种方法阅读R32G32_SFLOAT。一开始我以为是字节序问题，直到我把整个图片都转出来了。

上图是我直接把R32G32_SFLOAT转成R8G8B8A8_UNORM，我知道没有意义。但是在不改变格式的情况下，图像中仍然有很多 "hole" 并且值是致命的错误。

Answer 1

我不太确定这是否是问题所在，但如果我理解你的代码，你想将 image 放入 filename。所以你想从这个图像中读取。但是，您说此图像（不是暂存图像）的旧布局是未定义布局。该实现可以自由假设您不关心存储在其中的数据。使用真正的布局（我认为它是 COLOR_ATTACHMENT 或类似的东西）。

此外，您正在使用一个暂存映像和一个暂存缓冲区。我真的不明白你为什么要这样做？为什么不简单地将 vkCmdCopyImageToBuffer 函数与 image 一起使用到 staging_buffer？

顺便说一句，对于 Vulkan，并不是因为一个代码在某些 GPU 上工作，所以这段代码是正确的。

此外，我认为您必须在传输到暗示 HOST_STAGE 和 HOST_READ 的缓冲区后使用内存屏障。在规范中，它是这样写的：

Signaling a fence and waiting on the host does not guarantee that the results of memory accesses will be visible to the host, as the access scope of a memory dependency defined by a fence only includes device access. A memory barrier or other memory dependency must be used to guarantee this. See the description of host access types for more information.

Answer 2

你的这部分代码看起来很奇怪：

image_memory_barrier = VkTool::Initializer::GenerateImageMemoryBarrier(VK_IMAGE_LAYOUT_UNDEFINED, VK_IMAGE_LAYOUT_TRANSFER_SRC_OPTIMAL, { VK_IMAGE_ASPECT_COLOR_BIT, 0, 1, 0, 1 }, image);
device.vkCmdPipelineBarrier(command_buffer.Get(), VK_PIPELINE_STAGE_TOP_OF_PIPE_BIT, VK_PIPELINE_STAGE_TRANSFER_BIT, 0, 0, nullptr, 0, nullptr, 1, &image_memory_barrier);

这基本上意味着在屏障之后您的源图像可能没有任何数据。用作源布局的 UNDEFINED 值不能保证保留图像的内容。

如何在 Vulkan 中从 gpu 读取 R32G32_SFLOAT 图像

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