How to write shaders that can be compiled for DirectX, OpenGL, and Vulkan

Question

I recently finished writing the DirectX renderer for my game engine. Now I have an OpenGL, DirectX as well as a not yet finished Vulkan renderer.

Well, the majority of the renderers work perfectly now but I have a problem: I need a shader programming language.

The problem is that OpenGL and Vulkan use GLSL but DirectX uses HLSL (and Apple's Metal API uses MSL). So I searched for a High Level Shader Language and found only C for graphics from NVIDIA. But since this project was deprecated I looked for something else: Without success.

It's a bit annoying to write for 3 shader programming languages at the same time, so I'm looking for a language that can be translated into the native language immediately when the game starts (or is simply compatible with a lot of rendering APIs)

After several weeks of finding nothing, I decided to write my own language for it. But before I invest too much time I want to know if there is another solution to this problem.

Answer 1

This problem is often solved through the use of a transpiler, a program that can translate a shader written in one language into another.

HLSL2GLSL is one such example that was used in Unity up until 2016. Shaders could be authored in a standard HLSL syntax, then transpiled at build time into corresponding GLSL code.

SPIRV-Cross is another transpiler maintained by the Unity team that can serve as a bridge between HLSL, GLSL, MSL, and Vulkan.

Using an existing shader language as your source helps you avoid the overhead of designing a brand new one from scratch, and you can leverage a lot of work shared by other teams via projects like the ones linked above.

There are some extra considerations though, as outlined in the SPIRV-Cross Readme:

Implementation notes

When using SPIR-V and SPIRV-Cross as an intermediate step for cross-compiling between high level languages there are some considerations to take into account, as not all features used by one high-level language are necessarily supported natively by the target shader language. SPIRV-Cross aims to provide the tools needed to handle these scenarios in a clean and robust way, but some manual action is required to maintain compatibility.

The areas they call out include:

• HLSL source to GLSL
  • HLSL entry points
  • Vertex/Fragment interface linking
• HLSL source to legacy GLSL/ESSL
  • Separate image samplers (HLSL/Vulkan) for backends which do not support it (GLSL)
  • Descriptor sets (Vulkan GLSL) for backends which do not support them (HLSL/GLSL/Metal)
  • Linking by name for targets which do not support explicit locations (legacy GLSL/ESSL)
  • Clip-space conventions
  • Reserved identifiers

See the linked document for all the gory details of how to handle these situations.