第十五章 · 多重采样 (MSAA)

15.1 MSAA 概述

MSAA (Multi-Sampling Anti-Aliasing):
────────────────────────────────────

单采样 (No AA):
┌───┬───┬───┬───┐
│ ■ │ ■ │ ■ │ ■ │  ← 锯齿边缘
├───┼───┼───┼───┤
│ ■ │ ■ │ ■ │ ■ │
└───┴───┴───┴───┘

4x MSAA:
┌───┬───┬───┬───┐
│ 2 │ 2 │ 2 │ 2 │  ← 每个像素 4 个子采样点
│ 2 │ 2 │ 2 │ 2 │
├───┼───┼───┼───┤
│ 2 │ 2 │ 2 │ 2 │
│ 2 │ 2 │ 2 │ 2 │
└───┴───┴───┴───┘
         2 = 子采样点

原理:
- 对每个像素的多边形边缘进行多次采样
- 取子采样点的平均值作为最终颜色
- 性能开销: 4x MSAA ≈ 1.5x 渲染时间

15.2 创建多重采样 Image

import vkbottle

# --- 创建多重采样颜色/深度 Image ---
msaa_samples = vkbottle.SampleCountFlagBits._4  # 4x MSAA

# 颜色附件（多重采样）
msaa_color_image = device.create_image(
    vkbottle.ImageCreateInfo(
        imageType=vkbottle.ImageType._2D,
        format=swapchain_image_views[0].format,
        extent=vkbottleExtent(swapchain_image_extent.width, swapchain_image_extent.height, 1),
        mipLevels=1,
        arrayLayers=1,
        samples=msaa_samples,  # 多重采样
        tiling=vkbottle.ImageTiling.OPTIMAL,
        usage=vkbottle.ImageUsageFlag.COLOR_ATTACHMENT | vkbottle.ImageUsageFlag.TRANSFER_SRC,
        initialLayout=vkbottle.ImageLayout.UNDEFINED,
    ),
)

# 深度附件（多重采样）
msaa_depth_image = device.create_image(
    vkbottle.ImageCreateInfo(
        imageType=vkbottle.ImageType._2D,
        format=vkbottle.Format.D32_SFLOAT,
        extent=vkbottleExtent(swapchain_image_extent.width, swapchain_image_extent.height, 1),
        mipLevels=1,
        arrayLayers=1,
        samples=msaa_samples,
        tiling=vkbottle.ImageTiling.OPTIMAL,
        usage=vkbottle.ImageUsageFlag.DEPTH_STENCIL_ATTACHMENT,
        initialLayout=vkbottle.ImageLayout.UNDEFINED,
    ),
)

15.3 Render Pass 修改

15.3.1 Attachment 定义

# --- 颜色附件 (多重采样) ---
color_attachment = vkbottle.AttachmentDescription(
    format=swapchain_image_views[0].format,
    samples=msaa_samples,  # 关键：多重采样
    loadOp=vkbottle.AttachmentLoadOp.CLEAR,
    storeOp=vkbottle.AttachmentStoreOp.DONT_CARE,  # 不存储（Resolve 后存储到单采样）
    stencilLoadOp=vkbottle.AttachmentLoadOp.DONT_CARE,
    stencilStoreOp=vkbottle.AttachmentStoreOp.DONT_CARE,
    initialLayout=vkbottle.ImageLayout.UNDEFINED,
    finalLayout=vkbottle.ImageLayout.COLOR_ATTACHMENT_OPTIMAL,
)

# --- Resolve 附件 (单采样，用于呈现) ---
resolve_attachment = vkbottle.AttachmentDescription(
    format=swapchain_image_views[0].format,
    samples=vkbottle.SampleCountFlagBits._1,  # 单采样
    loadOp=vkbottle.AttachmentLoadOp.DONT_CARE,
    storeOp=vkbottle.AttachmentStoreOp.STORE,  # 存储到 swapchain
    stencilLoadOp=vkbottle.AttachmentLoadOp.DONT_CARE,
    stencilStoreOp=vkbottle.AttachmentStoreOp.DONT_CARE,
    initialLayout=vkbottle.ImageLayout.UNDEFINED,
    finalLayout=vkbottle.ImageLayout.PRESENT_SRC_KHR,
)

# --- 深度附件 (多重采样) ---
depth_attachment = vkbottle.AttachmentDescription(
    format=vkbottle.Format.D32_SFLOAT,
    samples=msaa_samples,
    loadOp=vkbottle.AttachmentLoadOp.CLEAR,
    storeOp=vkbottle.AttachmentStoreOp.DONT_CARE,
    stencilLoadOp=vkbottle.AttachmentLoadOp.DONT_CARE,
    stencilStoreOp=vkbottle.AttachmentStoreOp.DONT_CARE,
    initialLayout=vkbottle.ImageLayout.UNDEFINED,
    finalLayout=vkbottle.ImageLayout.DEPTH_STENCIL_READ_ONLY_OPTIMAL,
)

15.3.2 Subpass 定义

# --- Subpass 添加 Resolve Attachment ---
subpass = vkbottle.SubpassDescription(
    pipelineBindPoint=vkbottle.PipelineBindPoint.GRAPHICS,
    colorAttachments=[
        vkbottle.AttachmentReference(
            attachment=0,  # Color (multisampled)
            layout=vkbottle.ImageLayout.COLOR_ATTACHMENT_OPTIMAL,
        ),
    ],
    resolveAttachments=[  # Resolve 附件
        vkbottle.AttachmentReference(
            attachment=1,  # Resolve (single sampled)
            layout=vkbottle.ImageLayout.COLOR_ATTACHMENT_OPTIMAL,
        ),
    ],
    depthStencilAttachment=vkbottle.AttachmentReference(
        attachment=2,  # Depth
        layout=vkbottle.ImageLayout.DEPTH_STENCIL_ATTACHMENT_OPTIMAL,
    ),
)

15.4 Framebuffer 修改

# --- Framebuffer 使用 Resolve Image 作为输出 ---
attachments = [
    msaa_color_image_view,  # 多重采样颜色
    resolve_image_view,      # Resolve 附件（单采样）
    msaa_depth_image_view,  # 多重采样深度
]
framebuffer = device.create_framebuffer(
    vkbottle.FramebufferCreateInfo(
        renderPass=render_pass,
        attachments=attachments,
        width=swapchain_image_extent.width,
        height=swapchain_image_extent.height,
        layers=1,
    ),
)

15.5 Pipeline 修改

# --- Pipeline 中启用 MSAA ---
multisample_info = vkbottle.MultisampleStateCreateInfo(
    rasterizationSamples=msaa_samples,  # 关键：指定采样数
    sampleShadingEnable=vkbottle.Bool(0),
    minSampleShading=1.0,
    alphaToCoverageEnable=vkbottle.Bool(0),
    alphaToOneEnable=vkbottle.Bool(0),
)

15.6 可用采样数查询

# 查询 GPU 支持的最大采样数
props = device.get_physical_device().get_properties()
sample_counts = props.sampleRates
# 常见支持: _1, _2, _4, _8, _16, _32, _64

# 检查图像格式是否支持多重采样
image_props = device.get_physical_device().get_image_format_properties(
    format=vkbottle.Format.B8G8R8A8_SRGB,
    imageType=vkbottle.ImageType._2D,
    tiling=vkbottle.ImageTiling.OPTIMAL,
    usage=vkbottle.ImageUsageFlag.COLOR_ATTACHMENT,
    flags=vkbottle.ImageCreateFlag(0),
)
# 检查 sampleCounts 是否包含 _4

15.7 MSAA 配置速查

参数	值	说明
采样数	_1 / _2 / _4 / _8	1x/2x/4x/8x
性能开销	4x ≈ 1.5x	8x ≈ 2x
推荐	_4	平衡点 ✅
移动端	_1 / _2	性能优先
桌面端	_4 / _8	质量优先

15.8 MSAA 注意事项

1. 多重采样 Image 必须使用 OPTIMAL tiling
2. 多重采样 Image 不能直接用于纹理采样
3. 需要 Resolve 到单采样 Image 才能呈现
4. Depth Buffer 也需要多重采样（深度测试在子采样级进行）
5. Sample count 必须在整个 Render Pass 中一致
6. Image 创建时必须指定 samples > 1
7. Resolve 操作在 Render Pass 结束时自动执行

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