CPU Animation. Introduction. CPU skinning. CPUSkin Scalar:

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1 CPU Aniation Introduction The iportance of real-tie character aniation has greatly increased in odern gaes. Aniating eshes ia 'skinning' can be perfored on both a general purpose CPU and a ore specialized GPU. Howeer, to distribute the workload and axiize efficiency on a processor graphics syste, skinning can be perfored on the CPU and lighting and clipping coputations can be perfored on the GPU. One of the adantages of perforing CPU skinning is that the aniated post transfored data are readily aailable for collision detection. Also ulticore and ectorization with Intel Adanced Vector Extensions (Intel AVX) can be used to iproe perforance of the skinning algorith on an AVX capable CPU. The objectie of this saple is to proide best known ethods to perfor CPU skinning on an AVX capable processor graphics syste. The saple copares perforance of linear blend skinning algorith on the CPU and GPU. There are different skinning techniques ipleented in the saple:. CPUSkin Scalar: This technique onloads the CPU and perfors character aniation by skinning the odels on the CPU.. CPUSkin AVX: This technique optiizes the onloaded CPU skinning by using Intel AVX instructions.. GPUSkin: This technique ipleents GPU character aniation by skinning the odels in the ertex shader. There is one dragon odel whose esh is skinned to perfor one of 4 aniations: naely a fly loop, a run loop, a glide loop, and a run fly run loop. The dragon odel has 4,58 ertices and each ertex is influenced by a axiu of 4 bone atrices. CPU skinning This section proides ore details on each of the CPU skinning techniques. CPUSkin Scalar: The aniated position, noral and tangent for a ertex is coputed by transforing the default bindpose position, noral and tangent for that ertex by a set of weighted atrices that influences it. The weights reflect the extent of influence each bone atrix has on the final aniated ertex. For exaple if M, M, M and M are the four bone atrices that influence ertex and W, W, W and W are the weights respectiely then, the new aniated position, noral and tangent is coputed as shown aboe. Pos, Nor and Tan are the bind-pose position, noral, and tangent for ertex.

2 aniatedpos aniatednoral aniatedtangent Pos W M Tan W Nor W Pos M M W M Nor W M Tan W M Pos W M Tan W Pos Nor W M M W M Tan W M Nor W M CPUSkin AVX The CPU skinning algorith was optiized by ipleenting three different ethods using Intel AVX as listed below and ethod which proided the best perforance was retained. This ethod is an extension of the Intel SSE ipleentation explained in (Waeren, 5).. Use Intel AVX to aniate 8 ertices at a tie.. Use Intel AVX to aniate ertices at a tie (ipleent the Intel SSE ersion on ertices at a tie). Use Intel AVX to optiize the atrix transforation and work on only ertex at a tie. The CPUSkin AVX technique uses Intel AVX to transfor the default bind pose position, noral and tangent for each ertex with the weighted su of atrices that influences the ertex. Intel AVX is used to ectorize the atrix transforation while coputing the aniated position as shown below. Siilarly the aniated noral and tangent is coputed. aniatedpos aniatedpos aniatedpos Pos Pos Pos Let WeightedMatrix W W where WeightedMatrix W WeightedMatrix M M M Pos W W W M M M W W Pos M M W W W M M M Pos W M

3 Use AVX to ectorize the atrix ultiply Let Pos Substituti ng in aniatedpos ' ' ' ' x y z and let aniatedpos x y z w ' ' ' ' x y z w x y z tp Pos WeightedMatrix x x x x y y y y z z z z tp x y x y z x y z x y z z Extract the 4 lower FPs and 4 higher ' ' ' ' x y z w x y x y x y x y z FPs and add the together z z z GPU Skinning This technique skins the odels in the ertex shader on the GPU. Each instance of the odel uses a unique set of bone transforation atrices. The set of atrices for all the instances are consolidated and sent to the GPU ia a single bonematrix dynaic buffer. The bonematrix buffer is updated each frae with the atrix inforation for each instance. The ertex buffer contains the default bind pose inforation. Eploying instancing the ertex shader uses the bind pose inforation fro the ertex buffer and the atrix inforation for each instance fro the bonematrix buffer and uniquely skins each odel instance on the GPU. Optiizations for CPU skinning There are seeral optiizations ipleented in the CPUAniation saple. Multi- threading The saple uses the task anager created by ( (Minadakis, ) for ulti-threading the CPU skinning update. The task anager is used as it abstracts and siplifies the task creation and

4 synchronization process. When ulti-threading is enabled in the saple the task anager creates a set of tasks: one task for each dragon instance in the scene. Each task is responsible for copleting the CPU skinning update for a single dragon instance in the scene. Vectorizing with Intel AVX The CPU skinning algorith is optiized by using Intel AVX copiler intrinsics. Please refer to CPU Skinning section for ore details on how the CPU skinning algorith is ectorized. Pipelining /double buffering As the skinning calculations for each frae is independent of the preious frae, the CPU skinning update can be pipelined by using double buffers. Double buffering is used so that skinning update and rendering can be perfored siultaneously. One of the buffers stores the aniation inforation for the current frae and is used by the ain thread for rendering. The other buffer stores aniation inforation for the next frae and is updated by the aniation thread. When pipelining is enabled the user can choose how often the CPU skinning update should be perfored. For exaple the user can choose to perfor CPU skinning once eery fraes. This eans one of the buffers is apped once eery fraes for skinning update and in the eantie the other buffer is used to render the scene. At the end of the fraes the application akes sure that the skinning update is coplete and swaps the two buffers so that the newly updated aniation can be used to render the scene. Saple Usage Figure shows a screenshot of the CPUAniation saple. The saple loads with 5 dragon instances, with each instance perforing one of the aniations. Use the cobo box to choose fro one of the Skinning Techniques. Use the Instances slider to ary the nuber of dragons in the scene. The axiu nuber of dragons in the scene is set to. The Enable Pipeline checkbox enables / disables the ability to use double buffering when perforing the CPU skinning update. Pipelining can be enabled only when ulti-threading is enabled in the saple. Use the Fraes CPU Skin spans slider to choose how often the CPU skinning updates should be perfored. If the Fraes CPU Skin spans slider is set to the CPU skinning update is perfored once eery fraes. When arying the Fraes CPU Skin spans slider ake sure the ertices are skinned on the CPU at least ~ ties per second. The Aniate checkbox enables / disables aniation in the scene. The Multi-Threading checkbox enables /disables Intel TBB optiizations and the Wirefrae checkbox enables / disables wirefrae fill ode in the scene.

5 Figure : Screenshot of the CPUAniation saple The total nuber of ertices skinned on the CPU/GPU, the tie taken to coplete a skinning update on the CPU and the frae tie inus the tie spent on the draw call for CPU skinning are displayed in the top left corner of the screen. Perforance The perforance for the CPUAniation saple is easured on a.ghz Intel icroarchitecture code nae Sandy Bridge-based syste which has 4 cores / 8 threads. The tie is easured in illiseconds. Skinning tie is the tie taken to coplete a single CPU skinning update for all the dragons in the scene and Frae tie is the tie taken to coplete skinning and rendering a single frae. Table shows the perforance for CPU skinning when the application is run in single threaded ode and Table shows the perforance when the application is run in ulti-threaded pipeline enabled ode with CPU skinning perfored once eery frae.

6 Skinning Technique #of dragons # of ertices Skinned CPUSkin Scalar CPU Skin AVX Skinning tie (s) Skinning tie (s) 4, , , ,45, ,79,. 5.7,95, Table : Skinning perforance in single threaded ode Skinning Technique #of dragons # of ertices Skinned CPU Skin Scalar CPU Skin AVX Skinning tie (s) Skinning tie (s) 4, , , ,45, ,79, ,95, Table : Skinning perforance in ultithreaded, pipeline enabled ode with CPU skinning perfored once eery frae Fro Table and Table we can see that for CPU Skin Scalar there is a perforance gain of 8% 5% and for CPU Skin AVX there is perforance gain of 65% % depending upon the nuber of ertices skinned on the CPU in the scene. Figure and Figure copares the perforance for single threaded and ultithreaded pipeline enabled odes for CPU Skin Scalar and CPU Skin AVX techniques respectiely. Fro Table we can see that there is a perforance gain of 9% % fro CPU Skin Scalar to CPU Skin AVX technique depending upon the nuber of ertices skinned on the CPU in the scene. Figure 4 copares the perforance of CPU Skin Scalar and CPU Skin AVX for the ultithreaded pipeline enabled ode. Table shows the frae tie for CPU and GPU skinning techniques. Frae tie for the CPU skinning techniques was easured when the saple was run in a ultithreaded pipeline enabled ode. Irrespectie of the technique, skinning was perfored once eery frae.

7 Tie in illiseconds Tie in illiseconds Skinning tie for CPU Skin Scalar (lower is better) Nuber of ertices skinned on the CPU 56.8 Single threaded Multithreaded pipeline enabled Figure : Copares skinning tie perforance of CPU Skin Scalar technique in single threaded and ultithreaded pipeline enabled ode Skinning tie for CPU Skin AVX (lower is better) Nuber of ertices skinned on the CPU Single threaded Multithreaded pipeline enabled Figure : Copares skinning tie perforance of CPU Skin AVX technique in single threaded and ultithreaded pipeline enabled odes

8 Tie in illiseconds Fro Table we can see that there is a perforance gain of 7% 6% fro GPU Skin technique to CPU Skin AVX technique depending upon the nuber of ertices skinned in the scene. Figure 5 copare the frae tie perforance for the skinning techniques Skinning tie for Scalar s. AVX (lower is better) Nuber of ertices skinned on the CPU CPUSkin Scalar Multithreaded Pipeline Enabled CPUSkin AVX Multithreaded Pipeline enabled Figure 4: Copares skinning tie perforance of CPU Skin Scalar and CPU Skin AVX techniques for ultithreaded pipeline enabled ode Skinning Technique #of dragons # of ertices Skinned CPU Skin Scalar CPU Skin AVX GPU Skin Frae tie (s) Frae tie (s) Frae tie (s) 4, , , ,45, ,79, ,95, Table : Frae tie perforance for ultithreaded pipeline enabled ode with CPU skinning perfored once eery frae

9 Tie in illiseconds Skinning Technique #of dragons # of ertices CPUSkin Scalar CPU Skin AVX Skinned Fraes CPU Frae Tie Fraes CPU Frae Tie Skin spans Skin spans 4, , , ,45, ,79, ,95, Table 4: Frae tie perforance for ultithreaded pipeline enabled ode with CPU skinning perfored once eery N fraes 9 Frae tie perforance (lower is better) Nuber of ertices skinned in the scene CPU Skin Scalar CPU Skin AVX GPU Skin Figure 5: Copares frae tie perforance for the skinning techniques Table 4 shows the tie taken to coplete a single frae in ultithreaded pipeline enabled ode when CPU skinning is perfored once eery N fraes by setting Fraes CPU Skin spans slider to a alue >.

10 Tie in illiseconds There are certain cases in which CPU skinning has to be perfored once eery frae so that there are at least ~ aniation updates per second. For such cases Table 4 has the sae alue as in Table. Fro Table and Table 4 we can see that for CPU Skin Scalar there is a perforance gain of 7% 7% and for CPU Skin AVX there is a perforance gain of % 4% depending upon the nuber of dragons in the scene. Figure 6 and Figure 7 copares the frae tie perforance when CPU skinning is perfored once eery frae and when it s perfored once eery N fraes. Frae tie for CPU Skin Scalar (lower is better) Nuber of ertices skinned on the CPU CPU Skinning perfored once eery frae CPUSkin perfored once eery 'N' fraes Figure 6: Copares perforance of CPU Skin Scalar technique when skinning in perfored once eery frae and when skinning is perfored once eery N fraes Conclusions Fro Table and Figure 6 we can see that CPU Skinning is clearly the winner as copared to GPU skinning on an IVB syste. Future Work There is an opportunity to ipleent ore optiizations for CPU skinning as listed below:

11 Tie in illiseconds Reorder the ertices so that all ertices that use the sae set of atrices for skinning are contiguous in the list. By doing so, we can aoid reloading the atrices ultiple ties. Once the Intel Adanced Vector Extensions (Intel AVX) instruction set is released, we can use fused ultiply adds in the Intel AVX optiization code. Frae tie for CPU Skin AVX (lower is better) Nuber of ertices skinned on the CPU CPU Skinning perfored once eery frae CPUSkin perfored once eery 'N' fraes Figure 7: Copares perforance of CPU Skin Scalar technique when skinning in perfored once eery frae and when skinning is perfored once eery N fraes References (9). Retrieed fro Daies, L. (, February d-gaes/). DXSDK. (, June). Skinning saple. Id Software. (9, August). Fro Texture Virtualization to Massie Parallelization. Intel Threading Building Blocks. (n.d.). Retrieed fro

12 Minadakis, Y. (, March). Using Tasking to Scale Gae Engine Systes. Waeren, J.. (5). Fast Skinning. Id. Software.