Entries for tag "rendering", ordered from most recent. Entry count: 171.
# States and Barriers of Aliasing Render Targets
In my recent post “Initializing DX12 Textures After Allocation and Aliasing” I said that when you use Direct3D 12, you have a render-target or depth-stencil texture “B” aliasing with some other resources “A” in memory and want to use it, you need to initialize it properly every time: 1. issue a barrier of type
D3D12_RESOURCE_BARRIER_TYPE_ALIASING, 2. fully initialize its content using Clear, Discard, or Copy, so that garbage data left from previous usage of the memory as overwritten.
That is actually not the full story. There is a third thing that you need to take care of: a transition barrier for this resource. This is because functions:
ClearDepthStencilView require a texture to be in a correct state –
D3D12_RESOURCE_STATE_DEPTH_WRITE. A question arises here: How does resource state tracking interact with memory aliasing?
If you look at error and warning messages from D3D Debug Layer or PIX, you notice that states are tracked per resource, no matter if they alias in memory. This gives us some clue of how we should handle it correctly to tame the Debug Layer and make things formally correct, but it is still not that obvious where to put the barrier. Let's assume the last usage of our texture B is
D3D12_RESOURCE_STATE_PIXEL_SHADER_RESOURCE – the texture is used for sampling. Where should we put the barrier to transition it to
D3D12_RESOURCE_STATE_RENDER_TARGET, required to do the initializing Discard or Clear after aliasing? I can see 3 solutions here:
1. None at all – skip this barrier, just do the Discard/Clear as if the texture was in the correct RT/DS state. It obviously generates Debug Layer error, but it seems to work fine. My thinking about why this may be fine is: If the Discard/Clear requires the texture to be in the correct RT/DS state and it completely overwrites its content, then it shouldn’t care what initial state the texture is in. Its data are garbage anyway. It will most probably leave the texture initialized properly as for RT/DS state, no matter what.
2. Between the aliasing barrier and Discard/Clear. This would please the Debug Layer, but I have some doubts whether it is correct or necessary. It may not be correct because if the barrier does some transformations of the data, it would transform garbage data left after aliasing, which could be an undefined behavior leading to some persisting corruptions or even to GPU crash. It may not be necessary because we are going to re-initialize the whole content of the texture in a moment anyway with Discard/Clear.
3. After finished working with the texture in the previous frame. The idea is to leave each RT/DS texture that can alias with some other resource in a state that makes it ready for the initializing Discard/Clear next time we want to grab it from the common memory. This will work and will be formally correct, but it also sounds like a well-known and sub-optimal idea of reverting textures to some “base state” after each use. Additional problem appears when your last usage of the texture occurs on the compute queue, because there you cannot transition it to
Conclusion: While this is a tricky question that has no one simple answer, I would recommend to use  if you want to be 100% formally correct or  if you want maximum efficiency.
# CasCmdLine - Few Technical Details
As part of my job, I've written a small console program CasCmdLine with a purpose of testing AMD's FidelityFX Contrast Adaptive Sharpening (CAS) shader on an image from disk, e.g. a screenshot from your game. You can find binary and source code on github.com/GPUOpen-Effects/FidelityFX-CAS/, CasCmdLine subdirectory. See also the blog post and tutorial about it to learn about its features and the syntax of supported command line parameters.
Here I would like to point to three aspects of its implementation that allowed me to make it small and simple. They might interest you if you are a C++/Windows/graphics programmer.
1. To execute a compute shader like CAS, I needed to use a graphics API - Direct3D 11, 12, or Vulkan, as all of them are supported by the effect. I chose D3D11 as the easiest one. What’s interesting is that the API is used without creating a window or swap chain. There are no render frames, no calls to
Present, no depth-stencil texture, no message loop.
D3D11CreateDevice is used to initialize DirectX rather than
D3D11CreateDeviceAndSwapChain. The program just initializes all necessary machinery, does its job, and exits. It is perfectly possible to write a program this way, which may be a good idea for any application that needs to do some GPU-accelerated computations rather than interactive graphics like games do. I suspect this mode of operation would work even on server systems that have no monitor attached, as long as there is a GPU and graphics driver installed. See file “CasCmdLine.cpp” to find out how this is implemented.
2. There is always a question in every graphics app about how to load shaders. Surely, compiling them from HLSL/GLSL source code is the worst option, as it requires the user to have shader compiler installed or attach the compiler to your program. It also takes more time than loading shaders precompiled to the intermediate binary format. But even in this format they need to be loaded from somewhere, whether individual files or some custom compressed archive, like games tend to do. In CasCmdLine I did it differently. I attached precompiled shaders directly to the program binary. To do that, I used command line parameter
/Fh of the "fxc.exe" shader compiler, like this:
fxc.exe /T cs_5_0 /E mainCS /O3 /Fh CompiledShader.h ShaderSource.hlsl
Instead of a binary file, the compiler called with this parameter generates a text file in a format compatible with C/C++ that contains the data of the compiled shader in form of an array, like this:
Shader metadata and assembly is put here, as commented out code...
const BYTE g_mainCS =
68, 88, 66, 67, 8, 233,
11, 94, 141, 165, 83, 251,
50, 166, 219, 219, 84, 109,
128, 23, 1, 0, 0, 0,
Such file can be
#include-d in a C++ code and used to create a D3D shader directly from this data. See files "Shaders/CompiledShader_*.h" to find out how they really look like.
3. The program needs to load and save image files in JPEG, PNG, and preferably other formats. Of course, these formats are very complex, support various pixel formats, involve some compression algorithms etc., so handling them manually would require an enormous amount of work. There are libraries for this, like the official libpng and libjpeg for handling PNG/JPEG formats, respectively, or a multi-format, multi-platform library DevIL.
If the developed program is intended only for Windows, it turns out that no third-party libraries are needed. Native Windows API contains a part called Windows Imaging Component (WIC) that can load and save image files in many formats, including BMP, PNG, JPEG, TIFF, GIF, ICO, WMP, DDS. It can also do some image operations, like rescaling. It is a COM API that involves interfaces like
IWICBitmapFrameDecode, and many more. This is what I used in the program described here. I might write a tutorial about WIC someday... For now, I would just say if you figure out its API, it looks quite powerful. It might be useful for any graphics Windows app that needs to load textures. It is also what Microsoft's DirectXTex library uses under the hood.
# Bezier Curve as Easing Function
Bézier curves are named after Pierre Bézier, and primary used is geometry modeling. They are good at describing various shapes in 2D and 3D. A Bézier curve is a function x(t), y(t) - it gives points in space (x, y) for some parameter t = 0..1. But nowadays they are also used in computer graphics for animation, as easing functions. There, we need to evaluate y(x), because x is the time parameter and y is the evaluated variable.
How does the formula of a Bézier curve look like as y(x)? What constraints do the 4 control points need to meet for this function to be correct - to have only one value of y for each x, with no loops or arcs? Finally, how can this function be approximated to store it in computer memory and evaluate it efficiently in modern game engines? These sound like fundamental questions, but apparently no one researched this topic thoroughly before, so it became the subject of the Ph.D. thesis of my friend Łukasz Izdebski.
A part of his research has just been published as paper "Bézier Curve as a Generalization of the Easing Function in Computer Animation" in Advances in Computer Graphics, 37th Computer Graphics International Conference, CGI 2020, Geneva, Switzerland. We want to share an excerpt of his findings online as an article: Bezier Curve as Easing Function.
# Why Not Use Heterogeneous Multi-GPU?
There was an interesting discussion recently on one Slack channel about using integrated GPU (iGPU) together with discrete GPU (dGPU). Many sound ideas were said there, so I think it's worth writing them down. But because I probably never blogged about multi-GPU before, few words of introduction first:
The idea to use multiple GPUs in one program is not new, but not very widespread either. In old graphics APIs like Direct3D 11 it wasn't easy to implement. Doing it right in a complex game often involved engaging driver engineers from the GPU manufacturer (like AMD, NVIDIA) or using custom vendor extensions (like AMD GPU Services - see for example Explicit Crossfire API).
New generation of graphics APIs – Direct3D 12 and Vulkan – are lower level, give more direct access to the hardware. This includes the possibility to implement multi-GPU support on your own. There are two modes of operation. If the GPUs are identical (e.g. two graphics cards of the same model plugged to the motherboard), you can use them as one device object. In D3D12 you then index them as Node 0, Node 1, ... and specify
NodeMask bit mask when allocating GPU memory, submitting commands and doing all sorts of GPU things. Similarly, in Vulkan you have VK_KHR_device_group extension available that allows you to create one logical device object that will use multiple physical devices.
But this post is about heterogeneous/asymmetric multi-GPU, where there are two different GPUs installed in the system, e.g. one integrated with the CPU and one discrete. A common example is a laptop with "switchable graphics", which may have an Intel CPU with their integrated “HD” graphics plus a NVIDIA GPU. There may even be two different GPUs from the same manufacturer! My new laptop (ASUS TUF Gaming FX505DY) has AMD Radeon Vega 8 + Radeon RX 560X. Another example is a desktop PC with CPU-integrated graphics and a discrete graphics card installed. Such combination may still be used by a single app, but to do that, you must create and use two separate Device objects. But whether you could, doesn't mean you should…
First question is: Are there games that support this technique? Probably few… There is just one example I heard of: Ashes of the Singularity by Oxide Games, and it was many years ago, when DX12 was still fresh. Other than that, there are mostly tech demos, e.g. "WITCH CHAPTER 0 [cry]" by Square Enix as described on DirectX Developer Blog (also 5 years old).
iGPU typically has lower computational power than dGPU. It could accelerate some pieces of computations needed each frame. One idea is to hand over the already rendered 3D scene to the iGPU so it can finish it with screen-space postprocessing effects and present it, which sounds even better if the display is connected to iGPU. Another option is to accelerate some computations, like occlusion culling, particles, or water simulation. There are some excellent learning materials about this technique. The best one I can think of is: Multi-Adapter with Integrated and Discrete GPUs by Allen Hux (Intel), GDC 2020.
However, there are many drawbacks of this technique, which were discussed in the Slack chat I mentioned:
Conclusion: Supporting heterogeneous multi-GPU in a game engine sounds like an interesting technical challenge, but better think twice before doing it in a production code.
BTW If you just want to use just one GPU and worry about the selection of the right one, see my old post: Switchable graphics versus D3D11 adapters.
# Improving the quality of the alpha test (cutout) materials
This is a guest post from my friend Łukasz Izdebski Ph.D.
Today I want to share with you a trick which my collage from previews work mentioned to me a long time ago. It's about alpha tested (also known as cutout) materials. This technique which I want to share with you consists of two neat tricks that can improve the quality of alpha tested (cutout) materials.
Alpha test is an old technique used in computer graphics. The idea behind it is very simple. In a very basic form, a material (shader) of a rendered object can discard processed pixels based on the alpha channel of RGBA texture. When shaded pixel’s final alpha value is less than this threshold value (threshold value is constant for the instance of the material and a typical value is 50%), it is clipped (discarded) and will not land in the shaders output framebuffer. These types of materials are commonly used to render vegetation, fences, impostors/billboards, etc.
Alpha tested materials have some, I will say a little issue. It can be noticed when rendered object (with this material) is far away from the camera. Let the following video below be an example of this issue.
# Secrets of Direct3D 12: Resource Alignment
In the new graphics APIs - Direct3D 12 and Vulkan - creation of resources (textures and buffers) is a multi-step process. You need to allocate some memory and place your resource in it. In D3D12 there is a convenient function
ID3D12Device::CreateCommittedResource that lets you do it in one go, allocating the resource with its own, implicit memory heap, but it's recommended to allocate bigger memory blocks and place multiple resources in them using
When placing a resource in the memory, you need to know and respect its required size and alignment. Size is basically the number of bytes that the resource needs. Alignment is a power-of-two number which the offset of the beginning of the resource must be multiply of (
offset % alignment == 0). I'm thinking about writing a separate article for beginners explaining the concept of memory alignment, but that's a separate topic...
Back to graphics, in Vulkan you first need to create your resource (e.g.
vkCreateBuffer) and then pass it to the function (e.g.
vkGetBufferMemoryRequirements) that will return required size of alignment of this resource (
alignment). In DirectX 12 it looks similar at first glance or even simpler, as it's enough to have a structure
D3D12_RESOURCE_DESC describing the resource you will create to call
ID3D12Device::GetResourceAllocationInfo and get
D3D12_RESOURCE_ALLOCATION_INFO - a similar structure with
Alignment. I've described it briefly in my article Differences in memory management between Direct3D 12 and Vulkan.
But if you dig deeper, there is more to it. While using the mentioned function is enough to make your program working correctly, applying some additional knowledge may let you save some memory, so read on if you want to make your GPU memory allocator better. First interesting information is that alignments in D3D12, unlike in Vulkan, are really fixed constants, independent of a particular GPU or graphics driver that the user may have installed.
So, we have these constants and we also have a function to query for actual alignment. To make things even more complicated, structure
Alignment member, so you have one alignment on the input, another one on the output! Fortunately,
GetResourceAllocationInfo function allows to set
D3D12_RESOURCE_DESC::Alignment to 0, which causes default alignment for the resource to be returned.
Now, let me introduce the concept of "small textures". It turns out that some textures can be aligned 4 KB and some MSAA textures can be aligned to 64 KB. They call this "small" alignment (as opposed to "default" alignment) and there are also constants for it:
|Texture||64 KB||4 KB|
|MSAA texture||4 MB||64 KB|
Using this smaller alignment allows to save some GPU memory that would otherwise be unused as padding between resources. Unfortunately, it's unavailable for buffers and available only for small textures, with a very convoluted definition of "small". The rules are hidden in the description of Alignment member of D3D12_RESOURCE_DESC structure:
GetResourceAllocationInfo calculate all this automatically and just return optimal alignment for a resource, like Vulkan function does? Possibly, but this is not what happens. You have to ask for it explicitly. When you pass
D3D12_RESOURCE_DESC::Alignment = 0 on the input, you always get the default (larger) alignment on the output. Only when you set
D3D12_RESOURCE_DESC::Alignment to the small alignment value, this function returns the same value if the small alignment has been "granted".
There are two ways to use it in practice. First one is to calculate the eligibility of a texture to use small alignment on your own and pass it to the function only when you know the texture fulfills the conditions. Second is to try the small alignment always. When it's not granted,
GetResourceAllocationInfo returns some values other than expected (in my test it's
Alignment = 64 KB and
SizeInBytes = 0xFFFFFFFFFFFFFFFF). Then you should call it again with the default alignment. That's the method that Microsoft shows in their "Small Resources Sample". It looks good, but a problem with it is that calling this function with an alignment that is not accepted generates D3D12 Debug Layer error #721 CREATERESOURCE_INVALIDALIGNMENT. Or at least it used to, because on one of my machines the error no longer occurs. Maybe Microsoft fixed it in some recent update of Windows or Visual Studio / Windows SDK?
Here comes the last quirk of this whole D3D12 resource alignment topic: Alignment is applied to offset used in
CreatePlacedResource, which we understand as relative to the beginning of an
ID3D12Heap, but the heap itself has an alignment too!
D3D12_HEAP_DESC structure has
Alignment member. There is no equivalent of this in Vulkan. Documentation of D3D12_HEAP_DESC structure says it must be 64 KB or 4 MB. Whenever you predict you might create an MSAA textures in a heap, you must choose 4 MB. Otherwise, you can use 64 KB.
Thank you, Microsoft, for making this so complicated! ;) This article wouldn't be complete without the advertisement of open source library: D3D12 Memory Allocator (and similar Vulkan Memory Allocator), which automatically handles all this complexity. It also implements both ways of using small alignment, selectable using a preprocessor macro.
# Links to GDC 2020 Talks and More
March is an important time of year for game developers, as that's when Game Developers Conference (GDC) takes place - the most important conference of the industry. This year's edition has been cancelled because of coronavirus pandemic, just like all other events, or rather postponed to a later date. But many companies prepared their talks anyway. Surely, they had to submit their talks long time ago, plus any preparation, internal technical and legal reviews... The time spent on this shouldn't be wasted. That's why many of them shared their talks online as videos and/or slides. Below, I try to gather links to these materials with a full list of titles, with special focus on programming talks.
They organized multi-day "Virtual Talks" event presented on Twitch, with replays now available to watch and slides accessible on their website.
Monday, March 16
The 'Kine' Postmortem
Storytelling with Verbs: Integrating Gameplay and Narrative
Intrinsically Motivated Teams: The Manager's Toolbox
From 'Assassin's Creed' to 'The Dark Eye': The Importance of Themes
Representing LGBT+ Characters in Games: Two Case Studies
The Sound of Anthem
Is Your Game Cross-Platform Ready?
Forgiveness Mechanics: Reading Minds for Responsive Gameplay
Experimental AI Lightning Talk: Hyper Realistic Artificial Voices for Games
Tuesday, March 17
What to Write So People Buy: Selling Your Game Without Feeling Sleazy
Failure Workshop: FutureGrind: How To Make A 6-Month Game In Only 4.5 Years
Stress-Free Game Development: Powering Up Your Studio With DevOps
Baked in Accessibility: How Features Were Approached in 'Borderlands 3'
Matchmaking for Engagement: Lessons from 'Halo 5'
Forget CPI: Dynamic Mobile Marketing
Integrating Sound Healing Methodologies Into Your Workflow
From 0-1000: A Test Driven Approach to Tools Development
Overcoming Creative Block on 'Super Crush KO'
When Film, Games, and Theatre Collide
Wednesday, March 18
Bringing Replays to 'World of Tanks: Mercenaries'
Developing and Running Neural Audio in Constrained Environments
Mental Health State of the Industry: Past, Present & Future
Empathizing with Steam: How People Shop for Your Game
Scaling to 10 Concurrent Users: Online Infrastructure as an Indie
Crafting A Tiny Open World: 'A Short Hike' Postmortem
Indie Soapbox: UI design is fun!
Don't Ship a Product, Ship Value: Start Your Minimum Viable Product With a Solution
Day of the Devs: GDC Edition Direct
Independent Games Festival & Game Developers Choice Awards
Thursday, March 19
Machine Learning for Optimal Matchmaking
Skill Progression, Visual Attention, and Efficiently Getting Good at Esports
Making Your Game Influencer Ready: A Marketing Wishlist for Developers
How to Run Your Own Career Fair on a Tiny Budget
Making a Healthy Social Impact in Commercial Games
'Forza' Monthly: Live Streaming a Franchise
Aesthetic Driven Development: Choosing Your Art Before Making a Game
Reading the Rules of 'Baba Is You'
Friday, March 20
Beyond Games as a Service with Live Ops
Kill the Hero, Save the (Narrative) World
'Void Bastards' Art Style Origin Story
Writing Tools Faster: Design Decisions to Accelerate Tool Development
Face-to-Parameter Translation via Neural Network Renderer
The Forest Paths Method for Accessible Narrative Design
'Gears 5' Real-Time Character Dynamics
Stop & Think: Teaching Players About Media Manipulation in 'Headliner'
They organized "DirectX Developer Day" where they announced DirectX 12 Ultimate - a fancy name for the updated Direc3D 12_2 with new major features including DXR (Ray Tracing), Variable Rate Shading, and Mesh Shaders.
DXR 1.1 Inline Raytracing
Advanced Mesh Shaders
Reinventing the Geometry Pipeline: Mesh Shaders in DirectX 12
DirectX 12 Sampler Feedback
PIX on Windows
That's actually GPU Technology Conference (GTC) - a separate event. Their biggest announcement this month was probably DLSS 2.0.
RTX-Accelerated Hair Brought to Life with NVIDIA Iray
Material Interoperability Using MaterialX, Standard Surface, and MDL
The Future of GPU Raytracing
Visuals as a Service (VaaS): How Amazon and Others Create and Use Photoreal On-Demand Product Visuals with RTX Real-Time Raytracing and the Cloud
Next-Gen Rendering Technology at Pixar
New Features in OptiX 7
Production-Quality, Final-Frame Rendering on the GPU
Latest Advancements for Production Rendering with V-Ray GPU and Real-Time Raytracing with Project Lavina
Accelerated Light-Transport Simulation using Neural Networks
Bringing the Arnold Renderer to the GPU
Supercharging Adobe Dimension with RTX-Enabled GPU Raytracing
Sharing Physically Based Materials Between Renderers with MDL
Real-Time Ray-Traced Ambient Occlusion of Complex Scenes using Spatial Hashing
I also found some other videos on Google:
DLSS - Image Reconstruction for Real-time Rendering with Deep Learning
NVIDIA Vulkan Features Update – including Vulkan 1.2 and Ray Tracing
3D Deep Learning in Function Space
Unleash Computer Vision at the Edge with Jetson Nano and Always AI
Optimized Image Classification on the Cheap
Cisco and Patriot One Technologies Bring Machine Learning Projects from Imagination to Realization (Presented by Cisco)
AI @ The Network Edge
Animation, Segmentation, and Statistical Modeling of Biological Cells Using Microscopy Imaging and GPU Compute
Improving CNN Performance with Spatial Context
Weakly Supervised Training to Achieve 99% Accuracy for Retail Asset Protection
Combating Problems Like Asteroid Detection, Climate Change, Security, and Disaster Recovery with GPU-Accelerated AI
Condensa: A Programming System for DNN Model Compression
AI/ML with vGPU on Openstack or RHV Using Kubernetes
CTR Inference Optimization on GPU
NVIDIA Tools to Train, Build, and Deploy Intelligent Vision Applications at the Edge
Leveraging NVIDIA’s Technology for the Ultimate Industrial Autonomous Transport Robot
How to Create Generalizable AI?
Isaac Sim 2020 Deep Dive
But somehow I can't find their full list with links to them anywhere on their website. More talks are accessible after free registration on the event website.
Multi-Adapter with Integrated and Discrete GPUs
Optimizing World of Tanks*: from Laptops to High-End PCs
Intel® oneAPI Rendering Toolkit and its Application to Games
Intel® ISPC in Unreal Engine 4: A Peek Behind the Curtain
Variable Rate Shading with Depth of Field
For the Alliance! World of Warcraft and Intel discuss an Optimized Azeroth
Intel® Open Image Denoise in Blender - GDC 2020
Variable Rate Shading Tier 1 with Microsoft DirectX* 12 from Theory to Practice
Does Your Game's Performance Spark Joy? Profiling with Intel® Graphics Performance Analyzers
Boost CPU performance with Intel® VTune Profiler
DeepMotion | Optimize CPU Performance with Intel VTune Profiler
Google for Games Developer Summit 2020 @ YouTube (a collection of playlists)
Google for Games Developer Summit Keynote
What's new in Android game development tools
What's new in Android graphics optimization tools
Android memory tools and best practices
Deliver higher quality games on more devices
Google Play Asset Delivery for games: Product deep dive and case studies
Protect your game's integrity on Google Play
Accelerate your business growth with leading ad strategies
Firebase games SDK news
Cloud Firestore for Game Developers
Google for Games Developer Summit Keynote
Scaling globally with Game Servers and Agones (Google Games Dev Summit)
How to make multiplayer matchmaking easier and scalable with Open Match (Google Games Dev Summit)
Unity Game Simulation: Find the perfect balance with Unity and GCP (Google Games Dev Summit)
How Dragon Quest Walk handled millions of players using Cloud Spanner (Google Games Dev Summit)
Building gaming analytics online services with Google Cloud and Improbable (Google Games Dev Summit)
Google for Games Developer Summit Keynote
Bringing Destiny to Stadia: A postmortem (Google Games Dev Summit)
Stadia Games & Entertainment presents: Creating for content creators (Google Games Dev Summit)
Empowering game developers with Stadia R&D (Google Games Dev Summit)
Stadia Games & Entertainment presents: Keys to a great game pitch (Google Games Dev Summit)
Supercharging discoverability with Stadia (Google Games Dev Summit)
Online Game Technology Summit: Start-And-Discard: A Unified Workflow for Development and Live
Finding Space for Sound: Environmental Acoustics
Game Server Performance
NPC Voice Design
Machine Learning Summit: Ragdoll Motion Matching
Machine Learning, Physics Simulation, Kolmogorov Complexity, and Squishy Bunnies
AMD: No information.
Sony: No information.
Consoles: Last but not least, March 2020 was also the time when the details of the upcoming new generation of consoles have been announced - Xbox Series S and PlayStation 5. You can easily find information about them by searching the Internet, so I won't recommend any links.
If you know about any more GDC 2020 or other important talks related to programming that have been released recently, please contact me or leave a comment below and I will add them!
Maybe there a positive side of this pandemic? With GDC taking place, developers had to pay $1000+ entrance fee for the event. They had to book a flight to California and a hotel in San Francisco, which was prohibitively expensive for many. They had to apply for ESTA or a vista to the US, which not everyone could get. And the talks eventually landed behind a paywall, scoring even more money to the organizers. Now we can educate ourselves for free from the safety and convenience of our offices and homes.
# Initializing DX12 Textures After Allocation and Aliasing
If you are a graphics programmer using Direct3D 12, you may wonder what's the initial content of a newly allocated buffer or texture. Microsoft admitted it was not clearly defined, but in practice such new memory is filled with zeros (unless you use the new flag
D3D12_HEAP_FLAG_CREATE_NOT_ZEROED). See article “Coming to DirectX 12: More control over memory allocation”. This behavior has its pros and cons. Clearing all new memory makes sense, as the operating system surely doesn't want to disclose to us the data left by some other process, possibly containing passwords or other sensitive information. However, writing to a long memory region takes lots of time. Maybe that's one reason GPU memory allocation is so slow. I've seen large allocations taking even hundreds of milliseconds.
There are situations when the memory of your new buffer or texture is not zero, but may contain some random data. First case is when you create a resource using CreatePlacedResource function, inside a memory block that you might have used before for some other, already released resources. That's also what D3D12 Memory Allocator library does by default.
It is important to know that in this case you must initialize the resource in a specific way! The rules are described on page: “ID3D12Device::CreatePlacedResource method” and say: If your resource is a texture that has either
DEPTH_STENCIL flag, you must initialize it after allocation and before any other usage using one of those methods:
Please note that rendering to the texture as a Render Target or writing to it as an Unordered Access View is not on the list! It means that, for example, if you implement a postprocessing effect, you allocated an intermediate 1920x1080 texture, and you want to overwrite all its pixels by rendering a fullscreen quad or triangle (better to use one triangle - see article "GCN Execution Patterns in Full Screen Passes"), then initializing the texture before your draw call seems redundant, but you still need to do it.
What happens if you don't? Why are we asked to perform this initialization? Wouldn't we just see random colorful pixels if we use an uninitialized texture, which may or may not be a problem, depending on our use case? Not really... As I explained in my previous post “Texture Compression: What Can It Mean?”, a texture may be stored in video memory in some vendor-specific, compressed format. If the metadata of such compression are uninitialized, it might have consequences more severe than observing random colors. It's actually an undefined behavior. On one GPU everything may work fine, while on the other you may see graphical corruptions that even rendering to the texture as a Render Target cannot fix (or a total GPU crash maybe?) I've experienced this problem myself recently.
Thinking in terms of internal GPU texture compression also helps to explain why is this initialization required only for render-target and depth-stencil textures. GPUs use more aggressive compression techniques for those. Having the requirements for initialization defined like that implies that you can leave buffers and other textures uninitialized and just experience random data in their content without the danger of anything worse happening.
I feel that a side note on
ID3D12GraphicsCommandList::DiscardResource function is needed, because many of you probably don't know it. Contrary to its name, this function doesn't release a resource or its memory. The meaning of this function is more like the mapping flag
D3D11_MAP_WRITE_DISCARD from the old D3D11. It informs the driver that the current content of the resource might be garbage; we know about it, and we don't care, we don't need it, not going to read it, just going to fill the entire resource with a new content. Sometimes, calling this function may let the driver reach better performance. For example, it may skip downloading previous data from VRAM to the graphics chip. This is especially important and beneficial on tile-based, mobile GPUs. In some other cases, like the initialization of a newly allocated texture described here, it is required. Inside of it, driver might for example clear the metadata of its internal compression format. It is correct to call
DiscardResource and then render to your new texture as a Render Target. It could also be potentially faster than doing
ClearRenderTargetView instead of
DiscardResource. By the way, if you happen to use Vulkan and still read that far, you might find it useful to know that the Vulkan equivalent of
DiscardResource is an image memory barrier with
oldLayout = VK_IMAGE_LAYOUT_UNDEFINED.
There is a second case when a resource may contain some random data. It happens when you use memory aliasing. This technique allows to save GPU memory by creating multiple resources in the same or overlapping region of a
ID3D12Heap. It was not possible in old APIs (Direct3D 11, OpenGL) where each resource got its own implicit memory allocation. In Direct3D you can use
CreatePlacedResource to put your resource in a specific heap, at a specific offset. It's not allowed to use aliasing resources at the same time. Sometimes you need some intermediate buffers or render targets only for a specific, short time during each frame. You can then reuse their memory for different resources needed in later part of the frame. That's the key idea of aliasing.
To do it correctly, you must do two things. First, between the usages you must issue a barrier of special type
D3D12_RESOURCE_BARRIER_TYPE_ALIASING. Second, the resource to be used next (also called "ResourceAfter", as opposed to "ResourceBefore") needs to be initialized. The idea is like what I described before. You can find the rules of this initialization on page “Memory Aliasing and Data Inheritance”. This time however we are told to initialize every texture that has
DEPTH_STENCIL flag with 1. a clear or 2. a copy operation to an entire subresource.
DiscardResource is not allowed. Whether it's an omission or intentional, we have to stick to these rules, even if we feel such clears are redundant and will slow down our rendering. Otherwise we may experience hard to find bugs on some GPUs.
Update 2020-07-14: An engineer from Microsoft told me that the lack of
DiscardResource among valid methods of initializing a texture after aliasing is probably a docs oversight and it is correct to initialize it this way, so the last picture should actually have Discard as well, just like the first one.
Update 2020-12-22: Aliasing barrier and a Clear, Discard, or Copy is not all you need to do to properly initialize a texture after aliasing. You also need to take care of its state by issuing some transition barrier. To read more, see my new post: “States and Barriers of Aliasing Render Targets”.