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www.imgtec.comApril 2013
It’s all about triangles!
Understanding the GPU in your pocket to
write better code

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Introductions
 Who?
 Guillem Vinals Gangolells (guillem.vinalsgangolells@imgtec.com)
 Developer Technology Engineer, PowerVR Graphics
 What?
 It’s all about triangles! Understanding the GPU in your pocket to write better code

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Company overview
 Leading silicon, software & cloud IP supplier
 Multimedia: graphics; GPU compute; video; vision
 Communications: demodulation; connectivity; sensors
 Processors: applications CPUs; embedded MCUs
 Cloud: device and user management; services
 Targeting high volume, high growth markets
 Top semis and OEMs for mobile, connected home consumer
automotive and more
 Pure: our strategic product division
 Digital radio, internet connected audio, home automation
 Established technology powerhouse
 Founded 1985; London FTSE 250 (IMG.L); ~1,500 employees
 UK HQ; global operations
Comprehensive IP
portfolio for SoCs
& cloud connectivity
IP business pathfinder
Market maker/driver

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A Crash Course in Graphics Architectures

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Immediate Mode Renderer (IMR)
 Buffers kept in system memory
 High bandwidth use, power consumption & latency
 Each triangle is processed to completion in submission order
 Wastes processing time and thus power due to “overdraw”
 ‘Early-Z’ techniques help but are only as good as your geometry sorting

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Concept: Tiling
 Frame buffer sub-divided into Tiles
 32x32 pixels per tile, for example
 Varies by device
 Geometry is sorted into affected tiles
 Allows each tile to be processed independently
 Small number of fragments per tile
 Allows on-chip memory to be used

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Tile Based Renderer (TBR)
 Rasterizing performed per-tile
 Allows the use of fast, on-chip, buffers
 Each triangle is processed to completion in submission order
 Wastes processing time and thus power due to “overdraw”
 ‘Early-Z’ techniques help but are only as good as your geometry sorting

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Concept: Deferred Rendering
 Fragments - Two stage process
 Hidden Surface Removal (HSR)
 Shading
 HSR is pixel perfect
 Only visible fragments pass, no ‘overdraw’
 Only requires position data
 Less bandwidth & processing, saves power
 HSR is submission order independent
 No need for applications to submit geometry front to back

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Tile Based Deferred Renderer (TBDR) = PowerVR
 Rasterizing performed per-tile
 Allows the use of fast, on-chip, buffers
 Hidden Surface Removal (HSR) reduces overdraw
 Pixel perfect, and submission order independent, no geometry sorting needed
 Optimised to only retrieve information required (*), saving even more bandwidth
 Saves power and bandwidth

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PowerVR Hardware Overview

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Pipeline Summary
Geometry Processing

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Pipeline Summary
Fragment Processing

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Bandwidth Saving
 Bandwidth usage is the biggest contributor to GPU power consumption
 Saving bandwidth means staying ‘on chip’ as much as possible
 It also means throwing away work you don’t need to do
 PowerVR is designed from the ground up to do all of these

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Unified Architecture

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Pixel Back End (PBE)
 Combines sub-samples for on-chip MSAA
 MSAA Performed per-tile
 Done using sub-sampling
 Negligible impact on bandwidth
 Each sub-sample benefits from HSR
 Series5/5XT: 4x MSAA
 Series6: 8x MSAA
 Performs final format conversions
 Up scaling, down scaling etc. (Internal True
Colour)

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Further Considerations

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Micro Kernel
 Specialised software running on the USSE (Series5) or its own core (Series6)
 Allows the GPU and CPU to operate with minimal synchronisation
 Improves performance by handling interrupts on the GPU
 Competing solutions handle interrupts on CPU (in the driver)

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Multicore
 Near linear performance scaling
 Small fixed overhead known at design time
 Geometry processing load-balanced
 Cores share the processing effort
 Tiling enables parallel fragment processing
 Any core can work on any tile when available
 Each tile is self-contained
 Multi-core logic is handled by the hardware
 Completely transparent to the developer

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Alpha Blending
 Tiling GPUs don’t need to reach in to system memory to perform an alpha blend
 The colour buffer is on-chip
 This means that alpha blending doesn’t cost you any additional bandwidth
 It also means that alpha blending is fast…very fast
 HSR will also save you some work by throwing away occluded blending work
 Remember: Opaque, Alpha Test, Alpha Blend

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Golden Rules

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Common Bottlenecks
Based on past observation
Most Likely
CPU Usage
Bandwidth Usage
CPU/GPU Synchronisation
Fragment Shader Instructions
Geometry Upload
Texture Upload
Vertex Shader Instructions
Geometry Complexity
Least Likely

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Warning!
Some of these rules may seem obvious to you…
…we still see them broken everyday…
…if you know them, please bear with us

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Understand Your Target Device
 No two devices are identical
 Even when they look the same
 Different SoCs will have different bottlenecks
 Make sure you test against different chips
 Make sure you understand the hardware
 You don’t want your optimisation to make things worse
 Clearly, you’re already doing this….your here 
Golden Rule 1

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Don’t Waste GPU Time
 The Principle of “Good Enough”
 Don't waste polygons on un-needed detail
 Textures should never be much larger than their size on screen
 Why waste time loading a 1Kx1K texture if it’s never going to appear bigger than 128x128?
 If the user won't notice it, don’t waste time processing it
Golden Rule 2

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Promote Calculations up The Chain
 Don’t do a calculation you don’t need to do
 If you can do it once per scene, do it once per scene
 If you can’t, try and do it per vertex
 There are generally fewer vertices in a scene than fragments.
 If you can, pre-bake
 E.g. lighting
 Remember, ‘Good Enough’
Golden Rule 3

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Don’t Access an Active Render Target
 Accessing a render target from the CPU is very bad for performance
 If it’s not done properly it will synchronise the GPU and CPU….This is Bad™
Golden Rule 4

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Accessing Render Targets Safely
 Use EGL_KHR_fence_sync
 Use CPU side handles to GPU mapped memory to avoid blocking calls
 E.g. GraphicsBuffer (or gralloc) on Android
Golden Rule 4 Cont.

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Avoid Updating Active Assets
 Assets may need to stay the same for multiple frames
 We refer to this as an asset’s ‘Lifespan’
Golden Rule 5
 Changing a texture during its lifespan may cause ‘Ghosting’
 Changing a buffer during its lifespan is blocking
 This can be managed using circular buffers, similarly to render targets

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Use VBOs and Indexed Geometry
 VBOs benefit from driver level optimisations
 Vertex Array Objects (VAOs) may be even better
 Index your geometry
 It makes your data smaller
 It also benefits from driver level optimisations
 Use static VBOs ideally, and consider the assets lifespan
 Don’t use a VBO for dynamic data
Golden Rule 6

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Batch Your Draw Calls
 Group static objects, and draw once
 Static objects are objects that are static relative to each other
 Sort objects by render state
 Emphasis on texture and program state changes
 Try using texture atlases
 Remember Golden Rule 5 if your going to update the contents
Golden Rule 7

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Compress Your Textures
 The lower the bitrate the less bandwidth consumed
 Use PVRTC & PVRTC2, at 2 & 4bpp RGB/RGBA
 Don’t confuse this with PNG or JPG which are
decompressed in memory
 Usually to 24bpp or 32bpp
 PVRTC is read directly from the compressed form
 It stays in memory at 2bpp or 4bpp
 Use MIP-Mapping and remember ‘Good Enough’
Golden Rule 8

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Alpha Test/Discard & Alpha Blend
 Alpha Test removes advantages of ‘Early-Z’ techniques and HSR
 Fragment visibility isn’t known until fragment shader is run
 Prefer Alpha Blending, and render in the order Opaque, Alpha Test, Alpha Blend
 Makes best use of HSR
Golden Rule 9

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Use ‘Clear’ and ‘DiscardFrameBuffer’
 Calling ‘Clear’ ensures the previous render isn’t uploaded to the GPU
 By default, the depth/stencil buffers are written to memory at the end of a render
 Calling DiscardFrameBufferExt(…) ensures these buffers aren’t written to system memory
 Look for the ‘GL_EXT_discard_framebuffer’ extension
Do both if you can!
Golden Rule 10

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Questions ?
Or drop us an email: devtech@imgtec.com
Download our PowerVR SDK: bit.ly/PVR_SDK
Also, you can download examples, tools and
shell as an Android SDK add-on:
http://install.powervrinsider.com/androidsdk.xml

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www.imgtec.com
April 2013