iOS Metal开发指南：GPU加速与3D图形渲染

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"IOS Metal开发指南翻译版，详细介绍了Metal框架的核心概念、命令提交模型、内存管理模型以及独立编译代码（如图形着色器函数或数据并行计算函数）的使用。" Metal是Apple推出的一个高性能图形渲染和数据并行计算框架，专为iOS、macOS等平台的硬件加速而设计。Metal提供了一个现代且高效的API，允许开发者以细粒度、低级别的控制来组织、处理和提交图形与计算命令，以及管理这些命令相关的数据和资源，目标是降低执行GPU工作负载所需的CPU开销。 Metal的主要特点包括： 1. **GPU驱动的高级3D图形渲染**：Metal支持创建复杂的3D场景，允许开发者直接控制渲染管线，提高图形性能和效率。这包括顶点处理、纹理映射、光照计算等一系列3D图形处理任务。 2. **低级控制**：通过Metal API，开发者可以更直接地与GPU进行交互，定制化图形和计算任务，从而实现特定需求的优化。 3. **命令提交模型**：在Metal中，命令缓冲区用于存储和组织GPU命令。开发者可以创建多个命令缓冲区，每个缓冲区可以包含一系列相关操作，然后提交到GPU执行。这种模型有利于并行处理和多线程编程。 4. **内存管理**：Metal提供了高效的内存管理机制，包括纹理、缓冲区和其他资源的分配和释放。它考虑了GPU和CPU之间的内存同步，以确保高效的数据传输。 5. **独立编译的代码**：Metal支持图形着色器和数据并行计算函数，这些函数在应用程序启动前预先编译，提高了运行时的性能。开发者可以编写自定义的着色语言（例如，Metal shading language, MSL）代码，实现对像素、顶点或其他计算任务的处理。 6. **状态管理**：在Metal中，设置硬件状态是必要的，比如纹理单位、深度测试等。开发者需要明确指定GPU应如何处理不同的渲染任务。 7. **性能优化**：通过减少CPU和GPU之间的通信开销，Metal能够最大化设备的GPU性能，实现更快的计算速度和更流畅的图形渲染。这份翻译版的《Metal Programming Guide》详细解释了如何使用Metal API来编写应用，涵盖了设置硬件状态、提交GPU执行的命令、管理内存分配等多个方面，是iOS Metal入门学习的重要参考材料。对于想要深入理解GPU编程和提升应用性能的开发者来说，这是一个不可或缺的资源。

command buffercommand buffer

command buffercommand buffer

MTLCommandBufferenqueuecommand queue

command buffercommand queue

command buffer

Only one CPU thread can access a command buffer at time. Multithreaded apps can use one thread per command buffer

to create multiple command buffers in parallel.

Figure 2-2 (page 17) shows an example with three threads. Each thread has its own command buffer. For each thread,

one command encoder at a time has access to its associated command buffer. Figure 2-2 (page 17) also shows each

command buffer receiving commands from different command encoders. When you are done encoding, call the

endEncoding method of the command encoder, and a new command encoder object can then begin encoding

commands to the command buffer.

A MTLParallelRenderCommandEncoder object allows a single rendering pass to be broken up across multiple

command encoders and assigned to separate threads. For more information about

MTLParallelRenderCommandEncoder, see Encoding a Single Rendering Pass Using Multiple Threads (page 55).

GPUcommand buffercommand buffer

command buffer

2-23command bufferEncoder

command bufferEncoderendEncodingEncoder

command buffer

MTLParallelRenderCommandEncoderpassEncoder



Figure 2-2 Metal Command Buffers with Multiple Threads

Resource Objects: Buffers and Textures



This chapter describes Metal resource objects (MTLResource) for storing unformatted memory and formatted image

data. There are two types of MTLResource objects:

MTLBuffer represents an allocation of unformatted memory that can contain any type of data. Buffers are often used for

vertex, shader, and compute state data.

MTLTexture represents an allocation of formatted image data with a speciﬁed texture type and pixel format. Texture

objects are used as source textures for vertex, fragment, or compute functions, as well as to store graphics rendering

output (i.e., an attachment).

MTLSamplerState objects are also discussed in this chapter. Although samplers are not resources themselves, they are

used when performing lookup calculations with a texture object.

Metal(MTLResource)  

MTLResource

MTLBuffer



MTLTexture



MTLSamplerState



Buffers are Typeless Allocations of Memory



A MTLBuffer object represents an allocation of memory that can contain any type of data.

MTLBuffer

Creating a Buffer Object



The following MTLDevice methods create and return a MTLBuffer object:

The newBufferWithLength:options: method creates a MTLBuffer object with a new storage allocation.

The newBufferWithBytes:length:options: method creates a MTLBuffer object by copying data from existing

storage (located at the CPU address pointer) into a new storage allocation.

The newBufferWithBytesNoCopy:length:options:deallocator: method creates a MTLBuffer object with an

existing storage allocation and does not allocate any new storage for this object.

All buffer creation methods have the input value length to indicate the size of the storage allocation, in bytes. All the

methods also accept a MTLResourceOptions object for options that can modify the behavior of the created buffer. If

the value for options is 0, the default values are used for resource options.

MTLDeviceMTLBuffer

newBufferWithLength:options:MTLBuffer

newBufferWithBytes:length:options:MTLBuffer

CPU

newBufferWithBytesNoCopy:length:options:deallocator:MTLBuffer



lengthbyteoptions

MTLResourceOptionsoptions0

Buffer Methods



The MTLBuffer protocol has the following methods:

The contents method returns the CPU address of the buffer’s storage allocation.!

The newTextureWithDescriptor:offset:bytesPerRow: method creates a special kind of texture

object that references the buffer's data. This method is detailed in Creating a Texture Object (page 19).



contentsCPU

newTextureWithDescriptor:offset:bytesPerRow:

Textures are Formatted Image Data



A MTLTexture object represents an allocation of formatted image data that can be used as a resource for a vertex

shader, fragment shader, or compute function, or as an attachment to be used as a rendering destination. A MTLTexture

object can have one of the following structures:

a 1D, 2D, or 3D image, arrays of 1D or 2D images, or a cube of six 2D images.

MTLPixelFormat speciﬁes the organization of individual pixels in a MTLTexture object. Pixel formats are discussed

further in Pixel Formats for Textures (page 23).

MTLTexture



123

12

62

MTLPixelFormatMTLTexture

Creating a Texture Object



The following methods create and return a MTLTexture object:

The newTextureWithDescriptor: method of MTLDevice creates a MTLTexture object with a new storage

allocation for the texture image data, using a MTLTextureDescriptor object to describe the texture’s properties.

The newTextureViewWithPixelFormat: method of MTLTexture creates a MTLTexture object that shares the same

storage allocation as the calling MTLTexture object. Since they share the same storage, any changes to the pixels of the

new texture object are reﬂected in the calling texture object, and vice versa. For the newly created texture, the

newTextureViewWithPixelFormat: method reinterprets the existing texture image data of the storage allocation of

the calling MTLTexture object as if the data was stored in the speciﬁed pixel format. The MTLPixelFormat of the new

texture object must be compatible with the MTLPixelFormat of the original texture object. (See Pixel Formats for

Textures (page 23) for details about the ordinary, packed, and compressed pixel formats.)

The newTextureWithDescriptor:offset:bytesPerRow: method of MTLBuffer creates a MTLTexture object that

shares the storage allocation of the calling MTLBuffer object as its texture image data. As they share the same storage,

any changes to the pixels of the new texture object are reﬂected in the calling texture object, and vice versa. Sharing

storage between a texture and a buffer may prevent applying certain texturing optimizations (e.g., pixel swizzling or

tiling).

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