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luminance designs #

luminance-0.7.0 was released a few days ago and I decided it was time to explain exactly what luminance is and what were the design choices I made. After a very interesting talk with nical about other rust graphics frameworks (e.g. gfx, glium, vulkano, etc.), I thought it was time to give people some more information about luminance and how to compare it to other frameworks.

Origin#

luminance started as a Haskell package, extracted from a “3D engine” I had been working on for a while called quaazar. I came to the realization that I wasn’t using the Haskell garbage collector at all and that I could benefit from using a language without GC. Rust is a very famous language and well appreciated in the Haskell community, so I decided to jump in and learn Rust. I migrated luminance in a month or two. The mapping is described in this blog entry.

What is luminance for?#

I’ve been writing 3D applications for a while and I always was frustrated by how OpenGL is badly designed. Let’s sum up the lack of design of OpenGL:

weakly typed: OpenGL has types, but… it actually does not. GLint, GLuint or GLbitfield are all defined as aliases to primary and integral types (i.e. something like typedef float GLfloat). Try it with grep -Rn "typedef [a-zA-Z]* GLfloat" /usr/include/GL. This leads to the fact that framebuffers, textures, shader stages, shader program or even uniforms, etc. have the same type (GLuint, i.e. unsigned int). Thus, a function like glCompileShader expects a GLuint as argument, though you can pass a framebuffer, because it’s also represented as a GLuint – very bad for us. It’s better to consider that those are just untyped – :( – handles.
runtime overhead: Because of the point above, functions cannot assume you’re passing a value of a the expected type – e.g. the example just above with glCompileShader and a framebuffer. That means OpenGL implementations have to check against all the values you’re passing as arguments to be sure they match the type. That’s basically several tests for each call of an OpenGL function. If the type doesn’t match, you’re screwed and see the next point.
error handling: This is catastrophic. Because of the runtime overhead, almost all functions might set the error flag. You have to check the error flag with the glGetError function, adding a side-effect, preventing parallelism, etc.
global state: OpenGL works on the concept of global mutation. You have a state, wrapped in a context, and each time you want to do something with the GPU, you have to change something in the context. Such a context is important; however, some mutations shouldn’t be required. For instance, when you want to change the value of an object or use a texture, OpenGL requires you to bind the object. If you forget to bind for the next object, the mutation will occurs on the first object. Side effects, side effects…

The goal of luminance is to fix most of those issues by providing a safe, stateless and elegant graphics framework. It should be as low-level as possible, but shouldn’t sacrifice runtime performances – CPU charge as well as memory bandwidth. That is why if you know how to program with OpenGL, you won’t feel lost when getting your feet wet with luminance.

Because of the many OpenGL versions and other technologies (among them, vulkan), luminance has an extra aim: abstract over the trending graphics API.

Types in luminance#

In luminance, all graphics resources – and even concepts – have their own respective type. For instance, instead of GLuint for both shader programs and textures, luminance has Program and Texture. That ensures you don’t pass values with the wrong types.

Because of static warranties provided by compile-time, with such a scheme of strong-typing, the runtime shouldn’t have to check for type safety. Unfortunately, because luminance wraps over OpenGL in the luminance-gl backend, we can only add static warranties; we cannot remove the runtime overhead.

Error handling#

luminance follows the Rust conventions and uses the famous Option and Result types to specify errors. You will never have to check against a global error flag, because this is just all wrong. Keep in mind, you have the try! macro in your Rust prelude; use it as often as possible!

Even though Rust needs to provide an exception handler – i.e. panics – there’s no such thing as exceptions in Rust. The try! macro is just syntactic sugar to:

match result {
  Ok(x) => x,
  Err(e) => return e
}

Stateless#

luminance is stateless. That means you don’t have to bind an object to be able to use it. luminance takes care of that for you in a very simple way. To achieve this and keep performances running, it’s required to add a bit of high-level to the OpenGL API by leveraging how binds should happen.

Whatever the task you’re trying to reach, whatever computation or problem, it’s always better to gather / group the computation by batches. A good example of that is how magnetic hard drive disks work or your RAM. If you spread your data across the disk region (fragmented data) or across several non-contiguous addresses in your RAM, it will end up by unnecessary moves. The hard drive’s head will have to go all over the disk to gather the information, and it’s very likely you’ll destroy the RAM performance (and your CPU caches) if you don’t put the data in a contiguous area.

If you don’t group your OpenGL resources – for instances, you render 400 objects with shader A, 10 objects with shader B, then 20 objects with shader A, 32 objects with shader C, 349 objects with shader A and finally 439 objects with shader B, you’ll add more OpenGL calls to the equation – hence more global state mutations, and those are costly.

Instead of this:

400 objects with shader A
10 objects with shader B
20 objects with shader A
32 objects with shader C
348 objects with shader A
439 objects with shader B

luminance forces you to group your resources like this:

400 + 20 + 348 objects with shader A
10 + 439 objects with shader B
32 objects with shader C

This is done via types called Pipeline, ShadingCommand and RenderCommand.

Pipelines#

A Pipeline gathers shading commands under a Framebuffer. That means that all ShadingCommand embedded in the Pipeline will output to the embedded Framebuffer. Simple, yet powerful, because we can bind the framebuffer when executing the pipeline and don’t have to worry about framebuffer until the next execution of another Pipeline.

ShadingCommand#

A ShadingCommand gathers render commands under a shader Program along with an update function. The update function is used to customize the Program by providing uniforms – i.e. Uniform. If you want to change a Programs Uniform once a frame – and only if the Program is only called once in the frame – it’s the right place to do it.

All RenderCommand embedded in the ShadingCommand will be rendered using the embedded shader Program. Like with the Pipeline, we don’t have to worry about binding: we just have to use the embedded shader program when executing the ShadingCommand, and we’ll bind another program the next time a ShadingCommand is ran!

RenderCommand#

A RenderCommand gathers all the information required to render a Tessellation, that is:

the blending equation, source and destination blending factors
whether the depth test should be performed
an update function to update the Program being in use – so that each object can have different properties used in the shader program
a reference to the Tessellation to render
the number of instances of the Tessellation to render
the size of the rasterized points (if the Tessellation contains any)

What about shaders?#

Shaders are written in… the backend’s expected format. For OpenGL, you’ll have to write GLSL. The backends automatically inserts the version pragma (#version 330 core for OpenGL 3.3 for instance). In the first place, I wanted to migrate cheddar, my Haskell shader EDSL. But… the sad part of the story is that Rust is – yet – unable to handle that kind of stuff correctly. I started to implement an EDSL for luminance with macros. Even though it was usable, the error handling is seriously terrible – macros shouldn’t be used for such an important purpose. Then some rustaceans pointed out I could implement a (rustc) compiler plugin. That enables the use of new constructs directly into Rust by extending its syntax. This is great.

However, with the hindsight, I will not do that. For a very simple reason. luminance is, currently, simple, stateless and most of all: it works! I released a PC demo in Köln, Germany using luminance and a demoscene graphics framework I’m working on:

pouët.net link

youtube capture

ion demoscene framework

While developping Céleri Rémoulade, I decided to bake the shaders directly into Rust – to get used to what I had wanted to build, i.e., a shader EDSL. So there’re a bunch of constant &'static str everywhere. Each time I wanted to make a fix to a shader, I had to leave the application, make the change, recompile, rerun… I’m not sure it’s a good thing. Interactive programming is a very good thing we can enjoy – yes, even in strongly typed languages ;).

I saw that gfx doesn’t have its own shader EDSL either and requires you to provide several shader implementations (one per backend). I don’t know; I think it’s not that bad if you only target a single backend (i.e. OpenGL 3.3 or Vulkan). Transpiling shaders is a thing, I’ve been told…

sneaking out…

Feel free to dig in the code of Céleri Rémoulade here. It’s demoscene code, so it had been rushed on before the release – read: it’s not as clean as I wanted it to be.

I’ll provide you with more information in the next weeks, but I prefer spending my spare time writing code than explaining what I’m gonna do – and missing the time to actually do it. ;)

Keep the vibe!

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