Vulkan之MultiSample

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字数统计: 1,653 | 阅读时长 ≈ 7

光栅化的步骤会将三角形的线段映射到图像上。但是由于数字图像的不连续性，导致我们没有办法用模拟信号的方式连续的表示直线。实际上是使用小方格，也就是像素来表示直线的颜色，如下面所示。
当线段落在了像素上时，这个像素就染色，没有落在像素上，像素不染色，这就会导致右边的锯齿效果。当图像分辨率较高时，锯齿效果不明显，当分辨率很低时，锯齿效果会比较明显。
multiSampler1
那么有没有什么方法能优化这个显示问题呢？这里我们看到上面的的染色逻辑是我们在每个小方格内采样一个点，这个点在三角形内，整个方格就染色，不在就不染色。之所以会造成锯齿的效果，就在于临近的颜色变化剧烈，会给人眼睛造成冲击，导致人眼能清晰感受到锯齿效果。
那么怎样让其变化不剧烈呢？我们可以增加采样点，每个小方格内采样4次，每次得到一个颜色，然后小方块的颜色是4次的平均。便捷的模糊自然的会导致变化不剧烈，可以欺骗人的眼睛。
multiSampler2
多重采样的原理就是这样的，由于增加了采样次数，必然会带来效率和功耗的提升。

在vulkan中应用多重采样，需要如下的步骤：
查询设备支持的最大的采样数
创建多重采样的colorImage
修改depthImage支持多重采样
将colorImage作为attachment添加到FrameBuffer中
在pipeline中开启多重采样

1）查询设备支持的最大采样数

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VKAPI_ATTR void VKAPI_CALL vkGetPhysicalDeviceProperties(
    VkPhysicalDevice                            physicalDevice,
    VkPhysicalDeviceProperties*                 pProperties);
在pProperties的limit属性中有下面三个attachment的支持的最大采样数目。
VkSampleCountFlags    framebufferColorSampleCounts;
VkSampleCountFlags    framebufferDepthSampleCounts;
VkSampleCountFlags    framebufferStencilSampleCounts;

我们要设置的采样数不能大于这个属性值。

2）创建多重采样的color Image

多重采样是在离屏采样，然后将缓冲区呈现给屏幕。新的缓冲区不同于默认的swapchainImage，这是因为我们要保存每个像素的多个样本。因此我们要创建新的颜色图像资源。

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VkSampleCountFlagBits msaaSamples = VK_SAMPLE_COUNT_1_BIT; // 定义

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VkImage colorImage;
VkDeviceMemory colorImageMemory;
VkImageView colorImageView;

新建一个函数createColorResources，用于创建颜色图像资源。

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void createColorResources() {
    VkFormat colorFormat = swapChainImageFormat;
    createImage(swapChainExtent.width, swapChainExtent.height, 1, msaaSamples, colorFormat, VK_IMAGE_TILING_OPTIMAL, VK_IMAGE_USAGE_TRANSIENT_ATTACHMENT_BIT | VK_IMAGE_USAGE_COLOR_ATTACHMENT_BIT, VK_MEMORY_PROPERTY_DEVICE_LOCAL_BIT, colorImage, colorImageMemory);
    colorImageView = createImageView(colorImage, colorFormat, VK_IMAGE_ASPECT_COLOR_BIT, 1);
    }

这里面的逻辑和纹理图像创建一样的流程，我们需要注意的是他的参数。

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void createImage(uint32_t width, uint32_t height, uint32_t mipLevels, VkSampleCountFlagBits numSamples, VkFormat format, VkImageTiling tiling, VkImageUsageFlags usage, VkMemoryPropertyFlags properties, VkImage& image, VkDeviceMemory& imageMemory);

mipLevels 这里我们设置为1，这是因为我们不是用作纹理图像，所以不需要mipMap的设置，所以这里只要1个细化级别就行，实际上vulkan也要求多重采样的mipLevels要设置为1
numSamples 采样次数，这里需要我们自己设置，这个值不能大于device支持的值。
format跟swapchainImage保持一致即可
tiling 这里我们设置为VK_IMAGE_TILING_OPTIMAL
usage 表示图像用途，这里设置为VK_IMAGE_USAGE_TRANSIENT_ATTACHMENT_BIT | VK_IMAGE_USAGE_COLOR_ATTACHMENT_BIT 表明用作color attachment并且在需要使用时才分配内存空间
properties 为是要用于GPU读取，效率优先，因此设置为VK_MEMORY_PROPERTY_DEVICE_LOCAL_BIT

3）修改DepthImage支持多重采样

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void createDepthResources() {
    VkFormat depthFormat = findDepthFormat();
    createImage(swapChainExtent.width, swapChainExtent.height, 1, msaaSamples, depthFormat, VK_IMAGE_TILING_OPTIMAL, VK_IMAGE_USAGE_DEPTH_STENCIL_ATTACHMENT_BIT, VK_MEMORY_PROPERTY_DEVICE_LOCAL_BIT, depthImage, depthImageMemory);
    depthImageView = createImageView(depthImage, depthFormat, VK_IMAGE_ASPECT_DEPTH_BIT, 1);
    }

4）添加attachment到framebuffer和renderpass中

在创建framebuffer时，将color attachment加入到framebuffer资源中

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std::array<VkImageView, 3> attachments = {
    colorImageView,
    depthImageView,
    swapChainImageViews[i]
};

在renderpass里面添加attachment的引用

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void createRenderPass() {
    VkAttachmentDescription colorAttachment{};
    colorAttachment.format = swapChainImageFormat;
    colorAttachment.samples = msaaSamples;
    colorAttachment.loadOp = VK_ATTACHMENT_LOAD_OP_CLEAR;
    colorAttachment.storeOp = VK_ATTACHMENT_STORE_OP_STORE;
    colorAttachment.stencilLoadOp = VK_ATTACHMENT_LOAD_OP_DONT_CARE;
    colorAttachment.stencilStoreOp = VK_ATTACHMENT_STORE_OP_DONT_CARE;
    colorAttachment.initialLayout = VK_IMAGE_LAYOUT_UNDEFINED;
    colorAttachment.finalLayout = VK_IMAGE_LAYOUT_COLOR_ATTACHMENT_OPTIMAL;

    VkAttachmentDescription depthAttachment{};
    depthAttachment.format = findDepthFormat();
    depthAttachment.samples = msaaSamples;
    depthAttachment.loadOp = VK_ATTACHMENT_LOAD_OP_CLEAR;
    depthAttachment.storeOp = VK_ATTACHMENT_STORE_OP_DONT_CARE;
    depthAttachment.stencilLoadOp = VK_ATTACHMENT_LOAD_OP_DONT_CARE;
    depthAttachment.stencilStoreOp = VK_ATTACHMENT_STORE_OP_DONT_CARE;
    depthAttachment.initialLayout = VK_IMAGE_LAYOUT_UNDEFINED;
    depthAttachment.finalLayout = VK_IMAGE_LAYOUT_DEPTH_STENCIL_ATTACHMENT_OPTIMAL;

    VkAttachmentDescription colorAttachmentResolve{};
    colorAttachmentResolve.format = swapChainImageFormat;
    colorAttachmentResolve.samples = VK_SAMPLE_COUNT_1_BIT;
    colorAttachmentResolve.loadOp = VK_ATTACHMENT_LOAD_OP_DONT_CARE;
    colorAttachmentResolve.storeOp = VK_ATTACHMENT_STORE_OP_STORE;
    colorAttachmentResolve.stencilLoadOp = VK_ATTACHMENT_LOAD_OP_DONT_CARE;
    colorAttachmentResolve.stencilStoreOp = VK_ATTACHMENT_STORE_OP_DONT_CARE;
    colorAttachmentResolve.initialLayout = VK_IMAGE_LAYOUT_UNDEFINED;
    colorAttachmentResolve.finalLayout = VK_IMAGE_LAYOUT_PRESENT_SRC_KHR;

    VkAttachmentReference colorAttachmentRef{};
    colorAttachmentRef.attachment = 0;
    colorAttachmentRef.layout = VK_IMAGE_LAYOUT_COLOR_ATTACHMENT_OPTIMAL;

    VkAttachmentReference depthAttachmentRef{};
    depthAttachmentRef.attachment = 1;
    depthAttachmentRef.layout = VK_IMAGE_LAYOUT_DEPTH_STENCIL_ATTACHMENT_OPTIMAL;

    VkAttachmentReference colorAttachmentResolveRef{};
    colorAttachmentResolveRef.attachment = 2;
    colorAttachmentResolveRef.layout = VK_IMAGE_LAYOUT_COLOR_ATTACHMENT_OPTIMAL;

    VkSubpassDescription subpass{};
    subpass.pipelineBindPoint = VK_PIPELINE_BIND_POINT_GRAPHICS;
    subpass.colorAttachmentCount = 1;
    subpass.pColorAttachments = &colorAttachmentRef;
    subpass.pDepthStencilAttachment = &depthAttachmentRef;
    subpass.pResolveAttachments = &colorAttachmentResolveRef;

    VkSubpassDependency dependency{};
    dependency.srcSubpass = VK_SUBPASS_EXTERNAL;
    dependency.dstSubpass = 0;
    dependency.srcStageMask = VK_PIPELINE_STAGE_COLOR_ATTACHMENT_OUTPUT_BIT;
    dependency.srcAccessMask = 0;
    dependency.dstStageMask = VK_PIPELINE_STAGE_COLOR_ATTACHMENT_OUTPUT_BIT;
    dependency.dstAccessMask = VK_ACCESS_COLOR_ATTACHMENT_WRITE_BIT;

    std::array<VkAttachmentDescription, 3> attachments = { colorAttachment, depthAttachment, colorAttachmentResolve };
    VkRenderPassCreateInfo renderPassInfo{};
    renderPassInfo.sType = VK_STRUCTURE_TYPE_RENDER_PASS_CREATE_INFO;
    renderPassInfo.attachmentCount = static_cast<uint32_t>(attachments.size());
    renderPassInfo.pAttachments = attachments.data();
    renderPassInfo.subpassCount = 1;
    renderPassInfo.pSubpasses = &subpass;
    renderPassInfo.dependencyCount = 1;
    renderPassInfo.pDependencies = &dependency;

    if (vkCreateRenderPass(device, &renderPassInfo, nullptr, &renderPass) != VK_SUCCESS) {
        throw std::runtime_error("failed to create render pass!");
    }
}

下面我们来好好分析一个这个renderpass。

这个跟之前的渲染是有很大的不同的。之前的FrameBuffer里面只需要有两个attachment，如下所示：

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std::array<VkImageView, 3> attachments = {
    swapChainImageViews[i]，
    depthImageView,
};

这里swapChainImageViews[i]作为attachment[0]，直接作为shader的输出写入的attachment。我们也能从他的colorAttachment.finalLayout = VK_IMAGE_LAYOUT_PRESENT_SRC_KHR;看出，他直接就用来输出到屏幕了。

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VkAttachmentDescription colorAttachment{};
colorAttachment.format = swapChainImageFormat;
colorAttachment.samples = VK_SAMPLE_COUNT_1_BIT;
colorAttachment.loadOp = VK_ATTACHMENT_LOAD_OP_CLEAR;
colorAttachment.storeOp = VK_ATTACHMENT_STORE_OP_STORE;
colorAttachment.stencilLoadOp = VK_ATTACHMENT_LOAD_OP_DONT_CARE;
colorAttachment.stencilStoreOp = VK_ATTACHMENT_STORE_OP_DONT_CARE;
colorAttachment.initialLayout = VK_IMAGE_LAYOUT_UNDEFINED;
colorAttachment.finalLayout = VK_IMAGE_LAYOUT_PRESENT_SRC_KHR;

VkAttachmentDescription depthAttachment{};
depthAttachment.format = findDepthFormat();
depthAttachment.samples = VK_SAMPLE_COUNT_1_BIT;
depthAttachment.loadOp = VK_ATTACHMENT_LOAD_OP_CLEAR;
depthAttachment.storeOp = VK_ATTACHMENT_STORE_OP_DONT_CARE;
depthAttachment.stencilLoadOp = VK_ATTACHMENT_LOAD_OP_DONT_CARE;
depthAttachment.stencilStoreOp = VK_ATTACHMENT_STORE_OP_DONT_CARE;
depthAttachment.initialLayout = VK_IMAGE_LAYOUT_UNDEFINED;
depthAttachment.finalLayout = VK_IMAGE_LAYOUT_DEPTH_STENCIL_ATTACHMENT_OPTIMAL;

VkAttachmentReference colorAttachmentRef{};
colorAttachmentRef.attachment = 0;
colorAttachmentRef.layout = VK_IMAGE_LAYOUT_COLOR_ATTACHMENT_OPTIMAL;

VkAttachmentReference depthAttachmentRef{};
depthAttachmentRef.attachment = 1;
depthAttachmentRef.layout = VK_IMAGE_LAYOUT_DEPTH_STENCIL_ATTACHMENT_OPTIMAL;

VkSubpassDescription subpass{};
subpass.pipelineBindPoint = VK_PIPELINE_BIND_POINT_GRAPHICS;
subpass.colorAttachmentCount = 1;
subpass.pColorAttachments = &colorAttachmentRef;
subpass.pDepthStencilAttachment = &depthAttachmentRef;

但是在多重采样这里，我们的设置时不同的，首先我们的swapchainImage是attachment[2]，它确实是用来做present的

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VkAttachmentDescription colorAttachmentResolve{};
colorAttachmentResolve.format = swapChainImageFormat;
colorAttachmentResolve.samples = VK_SAMPLE_COUNT_1_BIT;
colorAttachmentResolve.loadOp = VK_ATTACHMENT_LOAD_OP_DONT_CARE;
colorAttachmentResolve.storeOp = VK_ATTACHMENT_STORE_OP_STORE;
colorAttachmentResolve.stencilLoadOp = VK_ATTACHMENT_LOAD_OP_DONT_CARE;
colorAttachmentResolve.stencilStoreOp = VK_ATTACHMENT_STORE_OP_DONT_CARE;
colorAttachmentResolve.initialLayout = VK_IMAGE_LAYOUT_UNDEFINED;
colorAttachmentResolve.finalLayout = VK_IMAGE_LAYOUT_PRESENT_SRC_KHR;

而fragment shader的输出到哪里了呢？就是我们新创建的colorImage.

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subpass.pColorAttachments = &colorAttachmentRef;

也就是说fragment shader的输出到了新创建的能进行多重采样的color attachment里面，然后vulkan执行多重采样，最终的输出结果位于subpass.pResolveAttachments指定的attachment中，也就是我们的swapchainImage。

这也就意味着多重采样实在fragment shader之后。

5） 修改pipeline支持多重采样

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VkPipelineMultisampleStateCreateInfo multisampling{};
multisampling.sType = VK_STRUCTURE_TYPE_PIPELINE_MULTISAMPLE_STATE_CREATE_INFO;
multisampling.sampleShadingEnable = VK_FALSE;
multisampling.rasterizationSamples = msaaSamples;

6）最后在程序退出的时候，销毁资源

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vkDestroyImageView(device, colorImageView, nullptr);
vkDestroyImage(device, colorImage, nullptr);
vkFreeMemory(device, colorImageMemory, nullptr);
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