TL;DR: I was doing a tutorial on how to use procedural graphics on the GPU and was irritated by the lack of higher-order functions, parametric polymorphism, and an effect system in GLSL. 🥲
Signed distance functions (SDFs) are a powerful approach for rendering procedural graphics in parallel on the GPU, without the use of mesh geometry. For more details see the resources listed below. This is an embedded DSL for composing SDFs in OCaml and compiling them to shader programs.
(** A scene to render, assuming UV coordinates in (-0.5, 0.5) *)
let scene : (vec3f repr) Env.t =
(* Some shapes defined using signed distance functions *)
let* s1 = circle !!0.3 |> move (L.vec2 !!0.0 !!0.0) in
let* s2 = square !!0.2 |> move (L.vec2 !!0.2 !!0.0) in
(* Combine the two shapes, meeting at a rounded edge *)
let shape = union_round s1 s2 !!0.05 in
(* Colours to use in the background and foreground *)
let background_color = L.vec3 !!0.35 !!0.45 !!0.50 in
let shape_color = L.vec3 !!1.0 !!1.0 !!1.0 in
(* The final output colour to render at the current UV coordinate. *)
Env.pure (background_color |> overlay ~shape:shape ~color:shape_color)The resulting DSL is clunkier than I’d like. This is due to OCaml’s odd approach to custom operators (which don’t allow for custom precedences), and lack of implicit overloading. Still, I think it’s an interesting proof-of-concept!
A full example can be found in examples/Basic.ml, which renders the following image using the sequential, CPU-based back-end:
Internally an approach called tagless-final style is used to implement the DSL. This helps make supporting either compilation to GLSL shaders or rendering directly on the CPU reasonably straightforward. In the future alternate back-ends for targets like HLSL, MSL and SPIR-V could also be implemented.
The compiled GLSL code can be seen at test/basic.glsl. For now the compiler assigns the result of each intermediate computation to a new local variable, attempting to avoid introducing duplicate computations. I’m not sure if this is the most efficient approach, or whether attempting to compile to higher-level code would work better (this would probably depend on the how GLSL code is compiled or interpreted by the GPU driver).
- Conditionals
- 3D SDFs
- Examples of raymarching
- Typed coordinate systems
- PPX to improve ergonomics of embedded DSL
- Replace monads with algebraic effects
- Optimise compiled shaders (eg. constant folding)
- SPIR-V Backend
- Browser-based shader previews
- Extenal DSL with improved ergonomics
- Browser-based shader playground