序（为什么要抄unreal源码）

unreal所有源码很长，几百万行，一辈子也抄不完。然而，只需要关注渲染部分，和工作中用到的插件，还是可以的。
统计了一下，rhi（含注释47949行，纯代码35462行）,rhicore（含注释3985行，纯代码2838行）,renderer（含注释264440行，纯代码205227行）和RendererCore（含注释66807行，纯代码49492行）。总共381881行（含注释），纯代码是203019行，相当于一个中型引擎，是可以接受的。每天300行（含注释）就行了,三四年就可以抄完了。即使版本再更新，增量学习就可以了，那时候也没多少了。

从重要性上看，ue使用了最先进的技术，比如从枚举上就可以看到，很多过时的技术都不再用了。有种时不我待之感。
namespace ERHIFeatureLevel
{
enum Type
{
/** Feature level defined by the core capabilities of OpenGL ES2. Deprecated */
ES2_REMOVED,

	/** Feature level defined by the core capabilities of OpenGL ES3.1 & Metal/Vulkan. */
	ES3_1,

	/**
	 * Feature level defined by the capabilities of DX10 Shader Model 4.
	 * SUPPORT FOR THIS FEATURE LEVEL HAS BEEN ENTIRELY REMOVED.
	 */
	SM4_REMOVED,

	/**
	 * Feature level defined by the capabilities of DX11 Shader Model 5.
	 *   Compute shaders with shared memory, group sync, UAV writes, integer atomics
	 *   Indirect drawing
	 *   Pixel shaders with UAV writes
	 *   Cubemap arrays
	 *   Read-only depth or stencil views (eg read depth buffer as SRV while depth test and stencil write)
	 * Tessellation is not considered part of Feature Level SM5 and has a separate capability flag.
	 */
	SM5,

	/**
	 * Feature level defined by the capabilities of DirectX 12 hardware feature level 12_2 with Shader Model 6.5
	 *   Raytracing Tier 1.1
	 *   Mesh and Amplification shaders
	 *   Variable rate shading
	 *   Sampler feedback
	 *   Resource binding tier 3
	 */
	SM6,

	Num
};

};