Using HLSL shaders in Vulkan with Vulkan-Hpp

The source for this sample can be found in the Khronos Vulkan samples github repository.

This is a transcoded version of the API sample that illustrates the usage of the C++ bindings of vulkan provided by vulkan.hpp.

This tutorial, along with the accompanying example code, shows how to use shaders written in the High Level Shading Language (HLSL) in Vulkan at runtime, using Vulkan-Hpp.

Vulkan does not directly consume shaders in a human-readable text format, but instead uses SPIR-V as an intermediate representation. This opens the option to use shader languages other than e.g. GLSL, as long as they can target the Vulkan SPIR-V environment. One such language is Microsoft’s HLSL, which is the shading language for DirectX.

Details on how HLSL fits into the Vulkan ecosystem can be found in this Vulkan guide chapter.

HLSL Syntax

HLSL is a bit more object-oriented than GLSL, but the general structure of a shader is similar Vulkan-specific functions are marked with the implicit vk namespace:

struct VSInput
{
[[vk::location(0)]] float3 Pos : POSITION0;
[[vk::location(1)]] float2 UV : TEXCOORD0;
[[vk::location(2)]] float3 Normal : NORMAL0;
};

struct UBO
{
	float4x4 projection;
	float4x4 model;
	float4 viewPos;
};

cbuffer ubo : register(b0, space0) { UBO ubo; }

struct VSOutput
{
	float4 Pos : SV_POSITION;
[[vk::location(0)]] float2 UV : TEXCOORD0;
};

VSOutput main(VSInput input)
{
	VSOutput output = (VSOutput)0;
	output.UV = input.UV;
	output.Pos = mul(ubo.projection, mul(ubo.model, float4(input.Pos.xyz, 1.0)));
	return output;
}

Glslang

The Vulkan samples use Glslang for converting shaders to SPIR-V at runtime. Glslang is the reference GLSL validator and translator, but also supports HLSL as an input language.

HLSL support in Glslang limited though and for a more feature complete HLSL to SPIR-V compiler, you can also use the DirectX shader compiler.

For the basic shader in this tutorial, we can go with Glslang though, as it supports all features we require.

Converting HLSL to SPIR-V

Loading HLSL with Glslang is similar to loading GLSL, but requires different parameters. Here are the relevant parts that differ from loading HLSL from the HlslShaders::load_hlsl_shader function of the sample:

std::vector<uint32_t> spirvCode;

// Use HLSL parsing rules and semantics (EShMsgReadHlsl)
EShMessages messages = static_cast<EShMessages>(EShMsgReadHlsl | ...);
...

// Language needs to be selected based on the shader stage
EShLanguage language = EShLangVertex;
glslang::TShader shader(language);
...

// Set the source language to HLSL
shader.setEnvInput(glslang::EShSourceHlsl, language, glslang::EShClientVulkan, 1);
...

// Parse the HLSL input
if (!shader.parse(&glslang::DefaultTBuiltInResource, 100, false, messages))
{
	...
}

// Add shader to new program object.
glslang::TProgram program;
program.addShader(&shader);

// Link program.
if (!program.link(messages))
{
	...
}
...

// Translate to SPIRV
glslang::TIntermediate *intermediate = program.getIntermediate(language);
...
glslang::GlslangToSpv(*intermediate, spirvCode, &logger);
...

Creating the shader module

The call to glslang::GlslangToSpv will generate the SPIR-V bytecode that we can use to create the Vulkan shader module from:

vk::ShaderModuleCreateInfo module_create_info({}, spirvCode);
vk::ShaderModule           shader_module = get_device()->get_handle().createShaderModule(module_create_info);