Lumberyard’s AssetBuilderSDK can be used to process a custom asset type for use at runtime. “Creating a Custom Asset Builder” in the User Guide shows how to do this in C++. This is another extension point using shared libraries which could potentially be written in Rust.

For a concrete example, look at MaterialBuilder found in Code\Tools\AssetProcessor\Builders\MaterialBuilder\ because it’s one of the simplest. At the bottom of MaterialBuilder\Source\MaterialBuilderApplication.cpp you’ll find the important sounding REGISTER_ASSETBUILDER macro.

It’s defined in Code\Tools\AssetProcessor\AssetBuilderSDK\AssetBuilderSDK\AssetBuilderSDK.h:

//! This macro should be used by every AssetBuilder to register itself,
//! AssetProcessor uses these exported function to identify whether a dll is an Asset Builder or not
//! If you want something highly custom you can do these entry points yourself instead of using the macro.
#define REGISTER_ASSETBUILDER                                                      \
    extern void BuilderOnInit();                                                   \
    extern void BuilderDestroy();                                                  \
    extern void BuilderRegisterDescriptors();                                      \
    extern void BuilderAddComponents(AZ::Entity * entity);                         \
    extern "C"                                                                     \
    {                                                                              \
    AZ_DLL_EXPORT int IsAssetBuilder()                                             \
    {                                                                              \
        return 0;                                                                  \
    }                                                                              \
                                                                                   \
    AZ_DLL_EXPORT void InitializeModule(AZ::EnvironmentInstance sharedEnvironment) \
    {                                                                              \
        AZ::Environment::Attach(sharedEnvironment);                                \
        BuilderOnInit();                                                           \
    }                                                                              \
                                                                                   \
    AZ_DLL_EXPORT void UninitializeModule()                                        \
    {                                                                              \
        BuilderDestroy();                                                          \
        AZ::Environment::Detach();                                                 \
    }                                                                              \
                                                                                   \
    AZ_DLL_EXPORT void ModuleRegisterDescriptors()                                 \
    {                                                                              \
        BuilderRegisterDescriptors();                                              \
    }                                                                              \
                                                                                   \
    AZ_DLL_EXPORT void ModuleAddComponents(AZ::Entity * entity)                    \
    {                                                                              \
        BuilderAddComponents(entity);                                              \
    }                                                                              \
    }
// confusion-reducing note: above end-brace is part of the macro, not a namespace

Asset builders need to export particular functions. We can use the same approach as with Editor plugins to register a builder written in Rust:

// Ignore warnings like:
// warning: function `IsAssetBuilder` should have a snake case name
#![allow(non_snake_case)]

use lmbr_sys::root::AZ;

#[no_mangle]
pub fn IsAssetBuilder() -> i32 {
    0
}

#[no_mangle]
pub fn InitializeModule(shared_environment: AZ::EnvironmentInstance) {
    unsafe {
        AZ::Environment::Attach(shared_environment, false);
    }
    // Implement custom init
}

#[no_mangle]
pub fn UninitializeModule() {
    // Implement custom uninit

    unsafe {
        AZ::Environment::Detach();
    }
}

#[no_mangle]
pub fn ModuleRegisterDescriptors() {
    // Implement
}

#[no_mangle]
pub fn ModuleAddComponents(entity: *mut AZ::Entity) {
    // Implement
}

Notice how in Rust Environment::Attach() takes two arguments. Bindgen generates the following for AZ::Environment::Attach():

pub mod Environment {
    //...
    extern "C" {
        #[link_name = "\u{1}?Attach@Environment@AZ@@YAXPEAVEnvironmentInterface@Internal@2@_N@Z"]
        pub fn Attach(
            sourceEnvironment: root::AZ::EnvironmentInstance,
            useAsGetFallback: bool,
        );
    }
}

The extra bool is from the default argument in Code\Framework\AzCore\AzCore\Module\Environment.h:

/**
    * Attaches the current module environment from sourceEnvironment.
    * note: this is not a copy it will actually reference the source environment, so any variables
    * you add remove will be visible to all shared modules
    * \param useAsFallback if set to true a new environment will be created and only failures to GetVariable
    * which check the shared environment. This way you can change the environment.
    */
void Attach(EnvironmentInstance sourceEnvironment, bool useAsGetFallback = false);

Cargo.toml:

[package]
name = "asset_builder_plugin"
version = "0.1.0"
edition = "2018"

[lib]
crate-type = ["cdylib"] # Produce shared library (.dll)

[build-dependencies]
lmbr_build = {version = "0.1", path = "../lmbr_build"}

[dependencies]
lmbr_logger = { version = "0.1", path = "../lmbr_logger" }
lmbr_sys = { version = "0.1", path = "../lmbr_sys" }
log = "0.4"

cargo build and check the binary with dumpbin /exports <path>\target\debug\asset_builder_plugin.dll (in Visual Studio shell):

Microsoft (R) COFF/PE Dumper Version 14.23.28106.4
Copyright (C) Microsoft Corporation.  All rights reserved.


Dump of file D:\projects\lmbr\target\debug\asset_builder_plugin.dll

File Type: DLL

  Section contains the following exports for asset_builder_plugin.dll

    00000000 characteristics
    FFFFFFFF time date stamp
        0.00 version
           1 ordinal base
           6 number of functions
           6 number of names

    ordinal hint RVA      name

          1    0 00001010 InitializeModule = InitializeModule
          2    1 00001000 IsAssetBuilder = IsAssetBuilder
          3    2 00001050 ModuleAddComponents = ModuleAddComponents
          4    3 00001040 ModuleRegisterDescriptors = ModuleRegisterDescriptors
          5    4 00001030 UninitializeModule = UninitializeModule
          6    5 00001200 rust_eh_personality = rust_eh_personality

  Summary

       5A000 .data
        A000 .pdata
       33000 .rdata
        2000 .reloc
       6C000 .text
        3000 _RDATA

Copy target/debug/asset_builder_plugin.dll to Lumberyard’s Bin64vc141/Builders/ folder. Launch AssetProcessor (Bin64vc141/AssetProcessor.exe), and in Logs > Messages:

12/12/2019 4:09 PM - AssetProcessor - Initializing and registering builder D:/projects/lumberyard/dev/Bin64vc141/Builders/asset_builder_plugin.dll

Looks good so far.

Component

The implementation for ModuleRegisterDescriptors() and ModuleAddComponents() are deferred to BuilderRegisterDescriptors() and BuilderAddComponents(), respectively:

void BuilderRegisterDescriptors()
{
    EBUS_EVENT(AssetBuilderSDK::AssetBuilderBus, RegisterComponentDescriptor, MaterialBuilder::BuilderPluginComponent::CreateDescriptor());
}

void BuilderAddComponents(AZ::Entity* entity)
{
    entity->CreateComponentIfReady<MaterialBuilder::BuilderPluginComponent>();
}

The first is working with ebus like we investigated previously and easy enough to do from Rust. First, wrap the C++:

void AssetBuilderBus_Broadcast_RegisterComponentDescriptor(AZ::ComponentDescriptor* descriptor)
{
    AssetBuilderSDK::AssetBuilderBus::Broadcast(&AssetBuilderSDK::AssetBuilderBus::Events::RegisterComponentDescriptor, descriptor);
}

Then, call the wrapped method from Rust:

// In lmbr_sys crate
extern "C" {
    pub fn AssetBuilderBus_Broadcast_RegisterComponentDescriptor(descriptor: *mut AZ::ComponentDescriptor);
}

// Elsewhere
let descriptor: *mut AZ::ComponentDescriptor = /* ... */;
unsafe {
    lmbr_sys::AssetBuilderBus_Broadcast_RegisterComponentDescriptor(descriptor);
}

MaterialBuilder calls CreateComponentIfReady(), but if we look at the definition in Entity.h:

/// @cond EXCLUDE_DOCS 
/**
 * @deprecated In tools, use AzToolsFramework::EntityCompositionRequestBus 
 * to ensure component requirements are met. 
 */
template<class ComponentType>
ComponentType* CreateComponentIfReady()
{
    return static_cast<ComponentType*>(CreateComponentIfReady(AzTypeInfo<ComponentType>::Uuid()));
}
/// @endcond

It’s not immediately clear to me how to replace it with AzToolsFramework::EntityCompositionRequestBus, but it should be ok for our purposes.

Debugging

Note that if you want to debug the AssetProcessor

  • GemRegistry; only build with certain specs
  • AssetProcessor/Builders; only built with “all” spec

Logging

Message logging

lmbr_logger::init().unwrap();
log::warn!("Hello AssetBuilder");

Output ends up in Bin64vc141\logs\JobLogs\Editor\Icons\Components\Flipbook.png-525422DE-05B3-4095-966F-90CD7657A7E1_createJobs.log:

~~1576157159161~~1~~0000000000001D80~~AssetBuilder~~: Hello AssetBuilder

unsafe {
    let uuid = "{166A7962-A3E4-4451-AC1A-AAD32E29C52C}";
    let uuid = AZ::Uuid_CreateString(uuid.as_ptr() as *const _, uuid.len());
    AssetBuilderSDK::BuilderLog(uuid, std::ffi::CString::new("Hello AssetBuilder").unwrap().as_ptr())
}

Code\Tools\AssetProcessor\Builders\LuaBuilder\Source\LuaBuilderApplication.cpp:

AssetBuilderSDK::AssetBuilderDesc builderDescriptor;
//...
builderDescriptor.m_busId = azrtti_typeid<LuaBuilderWorker>(); // 166A7962-A3E4-4451-AC1A-AAD32E29C52C
//...
AssetBuilderSDK::AssetBuilderBus::Broadcast(&AssetBuilderSDK::AssetBuilderBusTraits::RegisterBuilderInformation, builderDescriptor);

Code\Tools\AssetProcessor\Builders\LuaBuilder\Source\LuaBuilderWorker.h

class LuaBuilderWorker
        : public AssetBuilderSDK::AssetBuilderCommandBus::Handler
    {
    public:
        AZ_TYPE_INFO(LuaBuilderWorker, "{166A7962-A3E4-4451-AC1A-AAD32E29C52C}");
    //...

Bad Strings

AZStd::string is defined in string.h. An abridged version:

typedef basic_string<char >     string;

// In allocator.h
class allocator
{
public:

    AZ_TYPE_INFO(allocator, "{E9F5A3BE-2B3D-4C62-9E6B-4E00A13AB452}");

    typedef void*               pointer_type;
    typedef AZStd::size_t       size_type;
    typedef AZStd::ptrdiff_t    difference_type;

    //...
}

// In string.h
template<class Element, class Traits = char_traits<Element>, class Allocator = AZStd::allocator >
class basic_string
{
public:
    typedef Element*                                pointer;
    typedef const Element*                          const_pointer;

    typedef Element&                                reference;
    typedef const Element&                          const_reference;
    typedef typename Allocator::difference_type     difference_type;
    typedef typename Allocator::size_type           size_type;

    //...

protected:
    enum
    {   // length of internal buffer, [1, 16]
        SSO_BUF_SIZE = 16 / sizeof (Element) < 1 ? 1 : 16 / sizeof(Element)
    };
    enum
    {   // roundup mask for allocated buffers, [0, 15]
        _ALLOC_MASK = sizeof (Element) <= 1 ? 15 : sizeof (Element) <= 2 ? 7 : sizeof (Element) <= 4 ? 3 : sizeof (Element) <= 8 ? 1 : 0
    };

    //...

    union //Storage
    {
        Element m_buffer[SSO_BUF_SIZE];     //< small buffer used for small string optimization
        pointer m_data;                     //< dynamically allocated data
    };

    size_type m_size;           // current length of string
    size_type m_capacity;       // current storage reserved for string
    allocator_type m_allocator;
}
  • For char, SSO_BUF_SIZE is 16 so m_buffer provides 16 bytes of inline storage for short strings. Longer strings use m_data and a heap allocation.
  • size_type and difference_type are size_t and ptrdiff_t, respectively.
  • When targetting 64-bit and ignoring alignment/padding, sizeof(string) == 40 (bytes).

Bindgen outputs:

pub struct basic_string<Element, Allocator> {
    pub __bindgen_anon_1: root::AZStd::basic_string__bindgen_ty_3<Element>,
    pub m_size: root::AZStd::basic_string_size_type,
    pub m_capacity: root::AZStd::basic_string_size_type,
    pub m_allocator: root::AZStd::basic_string_allocator_type<Allocator>,
    pub _phantom_0: ::std::marker::PhantomData<::std::cell::UnsafeCell<Element>>,
    pub _phantom_1: ::std::marker::PhantomData<::std::cell::UnsafeCell<Allocator>>,
}
//...
pub type basic_string_difference_type = [u8; 0usize];
pub type basic_string_size_type = [u8; 0usize];
//...
pub const basic_string_SSO_BUF_SIZE: root::AZStd::basic_string__bindgen_ty_1 =
    basic_string__bindgen_ty_1::SSO_BUF_SIZE;
#[repr(i32)]
#[derive(Debug, Copy, Clone, PartialEq, Eq, Hash)]
pub enum basic_string__bindgen_ty_1 {
    SSO_BUF_SIZE = 0,
}
pub const basic_string__ALLOC_MASK: root::AZStd::basic_string__bindgen_ty_2 =
    basic_string__bindgen_ty_2::_ALLOC_MASK;
#[repr(i32)]
#[derive(Debug, Copy, Clone, PartialEq, Eq, Hash)]
pub enum basic_string__bindgen_ty_2 {
    _ALLOC_MASK = 0,
}
#[repr(C)]
pub union basic_string__bindgen_ty_3<Element> {
    pub m_buffer: *mut Element,
    pub m_data: root::AZStd::basic_string_pointer<Element>,
    _bindgen_union_align: u64,
    pub _phantom_0: ::std::marker::PhantomData<::std::cell::UnsafeCell<Element>>,
}
  • SSO_BUF_SIZE is 0 and m_buffer inline storage is just a pointer *mut Element
  • basic_string::size_type and basic_string::different_type are [u8; 0usize]- array of 0 bytes
  • Ignoring alignment/padding, sizeof(basic_string) == 16 (std::mem::size_of::<[u8; 0]>() == 0 and PhantomData is also zero-sized)

An AZStd::string returned from C++ was ok, but as soon as you did anything with it in Rust you’d get invalid memory access exceptions. This wasn’t Rust’s fault, we lied about the size and contents of the type.

We can manually fix it by changing the following lines:

pub type basic_string_difference_type = u64;
pub type basic_string_size_type = u64;
//...
pub const basic_string_SSO_BUF_SIZE: root::AZStd::basic_string__bindgen_ty_1 =
    basic_string__bindgen_ty_1::SSO_BUF_SIZE;
#[repr(i32)]
#[derive(Debug, Copy, Clone, PartialEq, Eq, Hash)]
pub enum basic_string__bindgen_ty_1 {
    SSO_BUF_SIZE = 16,
}

#[repr(C)]
pub union basic_string__bindgen_ty_3<Element> {
    pub m_buffer: [u8; basic_string_SSO_BUF_SIZE as usize],
    //...
}

Now string is usable from Rust.

Closing

The remainder of fleshing out our asset builder scaffolding involves calling bindgen generated bindings from lmbr_sys and creating wrappers for stickier C++ functions.

Wrappers are getting common enough I need to split them out from lmbr_sys so it only contains bindings and another crate contains the various “helpers” and utility functions. Perhaps another day.