[ue4] geometry drawing pipeline

        Geometry drawing routes , Generally speaking , It's notification GPU A complete process of drawing object information ;ue4 It encapsulates a relatively complex framework to maintain the whole process , This article will briefly introduce the whole process .

introduce

        ue4 Most of the core rendering code entries are FDeferredShadingSceneRenderer::Render(FRHICommandListImmediate& RHICmdList) In this function （ And corresponding Mobile edition ）, Here you can see in more detail what has been done in the whole rendering process . among , In the early stage of rendering , An important function is InitViews:

        stay InitViews in , I did the following two important things ：

        (1) Calculate visibility

        (2) Build visible primitive drawing instructions

        All elements in the scene are stored in FScene in , We first need to filter all elements , Determine which primitives we need to draw in the current frame （ Eliminate those that are not in the cone , Eliminate completely occluded , Eliminate small objects at a long distance, etc ）.

        After determining the elements to be drawn , Our ultimate goal is to submit the drawing of these primitives to GPU（drawcall）. To achieve this , We need to construct MeshCommand structure , It contains the rendering information of the object , Such as vertex buffer 、 Index buffer 、 Shader binding, etc .

        In order to build visible primitive drawing instructions ,ue4 Implemented a set of mechanisms , Simply speaking , It is to generate an intermediate data structure for different types of primitives , Different rendering channels load corresponding intermediate data structures , And set the rendering state , structure MeshCommand, Add rendering instructions to DrawList. The overall flow chart is as follows ：

Generate MeshBatch

        As mentioned earlier, we will generate an intermediate temporary data structure , This structure is FMeshBatch, It contains everything the channel needs to determine the final shader binding and render state .

        Proxy

        ue4 There are various types of UPrimitiveComponent, Such as StaticMesh,Sprite,Landscape,Particle,ISM,HISM etc. . In order to do special processing during generation for different types ,ue4 For each UPrimitiveComponent The corresponding FPrimitiveSceneProxy, Different types of Proxy A unified intermediate format will be generated MeshBatch.

        about Proxy for , It is similar to a OEM factory , Just care about how to generate MeshBatch, Don't care about this MeshBatch When, where and how it will be used . As shown in the figure below , from FPrimitiveSceneProxy Derived many types , They have their own corresponding UPrimitiveComponent.

         Static and dynamic generation

        For some objects , Its rendering state will not change in most cases , under these circumstances , Generated MeshBatch Is to determine the , So it can be cached , Regenerate only when necessary . We will generate this kind of MeshBatch The path of is called static path .

        Allied , For another part of the object , Their rendering state may change frequently , Therefore, each frame needs to be generated MeshBatch, We call it dynamic path .

        In order to determine Proxy Generate from static or dynamic paths MeshBatch, You can call GetViewRelevance() function , obtain bDynamicRelevance/bStaticRelevance attribute .

        For static paths , Will call Proxy Medium DrawStaticElements Construct correspondence MeshBatch; For dynamic paths , Will call Proxy Medium GetDynamicMeshElements To construct the corresponding MeshBatch.

        stay InitViews Medium ComputeViewVisibility Function , We can see two calls in succession , Respectively used to collect static MeshBatch And dynamic MeshBatch：

Static path

 

Dynamic path

         Static generation

        For statically generated primitive drawing instructions , When we add primitives to the scene （AddToScene）, Create at the same time MeshBatch, And will MeshBatch Relevant information is bound to the scene . We call Proxy Of DrawStaticElements To construct static objects MeshBatch:

        When necessary, , We generate cached MeshCommand:

 

        For each element , collect MeshBatch It mainly includes ComputeRelevance() and MarkRelevant() Two processes .

        among ,ComputeRelevance Mainly collect all static objects ; and MarkRelevance Mainly responsible for acquiring MeshDrawCommand, And will MeshBatch And the related RenderPass Connect .

        The cache MeshBatch The information is recorded in PrimitiveSceneInfo in ：

       StaticMeshBatch Corresponding MeshDrawCommand There are also caching mechanisms , If you use caching , We find the corresponding Command And collect ; If not used , Directly collect MeshBatch, Wait until later to deal with the relevant logic .

      Cached and uncached MeshBatch The data are recorded in ViewCommands.MeshCommands and ViewCommands.DynamicMeshCommandBuildRequests in .

        Dynamic generation

        Each frame of the dynamic object will regenerate the corresponding MeshBatch, Its overall logic is relatively simple .

        First it will pass Proxy Of GetDynamicMeshElements Function construction corresponds to MeshBatch, after , stay ComputeDynamicMeshRelevance Function , And the related RenderPass Connect .

        Dynamic object MeshBatch adopt Collector To collect , Recorded in the View.DynamicMeshElements in .

Generate MeshBatchCommand 

        ue4 Contains multiple drawings pass, Such as DepthPass, BasePass, ShadowPass etc. , It's through inheritance FMeshPassProcessor Realized , The final drawing takes place here .

        stay ComputeViewVisibility in , When we collect all the static / dynamic MeshBatch after , Then we will submit MeshBatch Convert to the corresponding MeshDrawCommand Request , The whole process takes place in SetupMeshPass in ：

        We have already mentioned , Whether static or dynamic MeshBatch, Will be associated with specific RenderPass relation . there RenderPass contain DepthPass, BasePass, ShadowPass etc. , Different Pass By inheritance FMeshPassProcessor Realized , Every FMeshPassProcessor There will be a AddMeshBatch function .

        And in the SetupMeshPass in , We will traverse all pass, And get the corresponding MeshBatch, call AddMeshBatch, Here's the picture , Two places marked AddMeshBatch Deal with dynamic and static separately （ nothing cache command) Of MeshBatch.

        AddMeshBatch By each pass Separate implementation , It mainly includes the following two parts ： One is based on MeshBatch Get relevant rendering information , Such as bound shaders , Render states （ Mixed mode , Deep reading and writing mode ）; The other is to call general BuildMeshDrawCommands function , from MeshBatch And the rendering state calculated in the previous step MeshDrawCommands.