When setting rendertargets in Unity LWRP or URP templates, it will triggered the resolve operation of the old rendertarget if MSAA is used automatically. On some platforms, resolve operation is costly(0.6ms on Iphone XR for a full-screen msaa's buffer's resolve operation). This demo gives more control over rendertargets by setting resolve operations manually.
This image shows the redundant resolve operations in LWRP or UWP. A MSAA rendertarget is used as a shader resource for two consecutive passes, two resolve operation is triggered although the second is not needed.
In this demo, the frame analysis is this:
The above image shows in this demo, only one resolve operation is triggered in the same scenario.
The magic here is to use RenderTexture instead of RenderTargetIdentifier. Rendertexture gives a way to control the auto-resolve, and a method to trigger the resolve operation on command. The related changes are in submit resolve rendertarget on command.
Xcode capture frame also visualizes it：
The previous way has two resolve operations. Each one costs 0.6ms
Now two resolve operations merge into a single one. Saves 0.6ms.
带有MSAA的RT常被用于增加画面表现细节，又由于MSAA操作在某些手机硬件上绘制是free的，所以在手机上一种常有的操作是开MSAA同时降分辨率来增加效率。但是在unity引擎上有个掣肘，每次切换rendertarget都会出现切换出去的rendertarget发生aa的resolve操作。在某些情况下rendertarget的resolve操作是冗余的,而且还[影响效率] (https://forum.unity.com/threads/every-graphics-blit-causes-rendertexture-resolveaa-if-msaa-enabled-which-is-killing-framerate.457653/)。
此图显示了使用LWRP,UWP中的冗余AA resolve操作，一张Texture2DMS连续被用作shader resource,每一次使用都触发了resolve操作，但是实际上第二次resolve是不需要的。
使用本文介绍的修改通过1.取消自动resolve配置和2.手动设置resolve操作来取消操作的冗余。
LWRP,URP模板使用RenderTargetIdentifier进行RT的切换，这个类很难设置resolve的频率，但是RT也可以由RenderTexture设置，这个类的创建时候的bindMS参数来控制是否自动进行RT的resolve和ResolveAntiAliasedSurface手动控制resolve的操作。 具体修改可见resolve rendertarget on command修改。存储上，本修改在使用msaa的rendertarget通过开启bindms设置增加一个带msaa的rendertexure(m_color_handle)和一个不带msaa的rendertexture(m_resolve_handle)。看上去增加一个rt，但是在unity本身中如果关闭bindms,只用一个rt内部也会有两个rendertexutre handle,一个带msaa的texture2dMS，一个不带msaatexture2d。所以概念上是等同的。
修改后再rendertarget的创建函数可以指定rt由RenderTargetIdentifier还是rendertexture创建。如果是使用msaa的rendertarget，需要用GetShaderResource()返回Texture2D，原来的Identifier()则返回rendertarget handle 或者Texture2DMS.
在render中则需要显示增加一个resolvePass添加resolveAA操作。
显示只用一个resolve操作就能实现之前的操作，省下一个resolve操作。
Xcode 截帧也显示出了这一点：
原来resolve操作有两个（图中的蓝色节点），在iphone XR耗时0.6ms。经过优化后
可以看到只有一个resolve操作，最后resolve的texture2d被两个pass所用，iphone XR上节省了一个600us的全屏resolve操作。