76 releases (34 breaking)

✓ Uses Rust 2018 edition

0.37.1 Sep 29, 2019
0.36.1 Sep 20, 2019
0.30.1 Sep 10, 2018
0.30.0 Jul 26, 2018
0.6.0 Jul 12, 2016

#8 in Graphics APIs

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What is this?

crates.io License

luminance is an effort to make graphics rendering simple and elegant.

The aims of luminance are:

  • Bringing a safe, type-safe and stateless API to the Rust graphics ecosystem.
  • Providing a simple and easy interface; that is, exposing core concepts without anything extra – just the bare stuff. This is not a 3D or a video game engine. It’s a set of building blocks and graphics primitives you can use to construct more specific abstractions, libraries and applications.
  • To be opinionated enough to allow safety and optimizations but not to force the user into a too strict design: this is not a framework. Some constructs are restricting by design but the overall crate tries to adapt to what the user wants to do, not the over way around.
  • Easy to read with a good documentation and set of tutorials, so that newcomers don’t have to learn a lot of new concepts to get their feet wet. For most primitive concepts, they must be explained and detailed in their corresponding sections / modules.
  • Need-driven: every piece of code added in the project must come from a real use case. If you feel something is missing, feel free to open an issue or even contribute! Issue trackers exist for bug tracking but also for feature requests.
  • The gfx-hal crate is already a good crate, so luminance must stand out by providing an easier crate for people who just want to write some graphics code without having to cope with too low-level details. The crate is low-level but not as much as gfx-hal, though. The goal is to be performant enough and still have an elegant and easy interface.

An important point before starting: some people ask about how to easily render something. luminance is flexible enough to allow people to do rendering as they want and, thus, doesn’t include default shaders or such. So if you are looking for something that has already everything embedded, you’re not looking at the right crate.

luminance is shipped with almost no data — i.e. no default shader, tessellations, etc. You can probably find crates adding those, though. ;)

What’s included?

luminance is a rendering crate, not a 3D engine nor a video game framework. As so, it doesn’t include specific concepts, such as lights, materials, asset management nor scene description. It only provides a rendering library you can plug in whatever you want to.

There are several so-called 3D-engines out there on crates.io. Feel free to have a look around.

However, luminance comes in with several interesting features that might interest you.

Features set

  • Buffers: buffers are ways to communicate with the GPU; they represent regions of memory you can write to and read from. There’re several kinds of buffers you can create, among vertex and index buffers, uniform buffers, and so on and so forth…. They look like regular array but have some differences you might be aware of.
  • Framebuffers: framebuffers are used to hold renders. Each time you want to perform a render, you need to perform it into a framebuffer. Framebuffers can then be combined with each other to produce effects and design render layers.
  • Shaders: luminance supports five kinds of shader stages:
    • Tessellation control shaders.
    • Tessellation evaluation shaders.
    • Vertex shaders.
    • Geometry shaders.
    • Fragment shaders.
  • Vertices, indices, primitives and tessellations: those are used to define a shape you can render into a framebuffer with a shader.
  • Textures: textures represent information packed into arrays on the GPU, and can be used to customize a visual aspect or pass information around in shaders.
  • Blending: blending is the process of taking two colors from two framebuffers and mixing them between each other.
  • Control on the render state: the render state is a set of capabilities you can tweak to draw frames. It includes:
    • The blending equation and factors.
    • Whether we should have a depth test performed.
    • Face culling.
  • And a lot of other cool things like GPU commands, pipelines, uniform interfaces and so on…

How to dig in?

luminance is written to be fairly simple. The documentation is very transparent about what the library does and several articles will appear as the development goes on. Keep tuned! The online documentation is also a good link to have around.

Current implementation

Currently, luminance is powered by OpenGL 3.3: it’s the default. That version of OpenGL is old enough to support a wide range of devices out there. However, it’s possible that your device is older or that you target the Web or Android / iOS. In that case, you should have a look at the set of feature flags, which offers the possibility to compile luminance on several platforms.

Feature flags

  • default = ["std"]
  • std: Compile against the standard library. If you disable that feature, you get a tinier executable but you’re responsible for lots of stuff. Currently, that feature is not well tested and very experimental; use with care and caution and please provide feedback on the issue tracker if you try it out!

Windowing

luminance does not provide a way to create windows because it’s important that it not depend on windowing libraries – so that end-users can use whatever they like. Furthermore, such libraries typically implement windowing and events features, which have nothing to do with our initial purpose.

Nevertheless, an ecosystem effort exists towards luminance: luminance-windowing. That crate provides a windowing API that is implemented by other crates, such as luminance-glfw. You don’t have to use them, though. If you’re interested into how you should setup windowing for luminance to work, this very documentation explains it in the GraphicsContext section.

User-guide and contributor-guide

If you just plan to use luminance, just read the User-guide section.

If you plan to contribute to luminance (by writing a windowing crate or hacking on luminance directly), feel free to read the Contributor-guide section after having read the User-guide section as well.

User-guide

Creating a context

In order to get started, you need to create an object which type implements GraphicsContext. luminance ships with the trait but no implementor. You need to head over crates.io and search for luminance crates to find a windowing backend first.

Such a backend should expose a type which implements GraphicsContext. You can create one per thread. That limitation enables luminance not to perform plenty of runtime branching, minimizing the runtime overhead.

If you really want several contexts, you will need several OS threads.

GraphicsContext is the entry-point of everything luminance provides. Feel free to dig in its documentation for further information on how to use luminance. Most objects you can create will need a mutable reference to such a context object. Even though luminance is stateless in terms of global state, it still requires to have an object representing the GPU somehow.

Understanding the pipeline architecture

luminance has a very particular way of doing graphics. It represents a typical graphics pipeline via a typed AST that is embedded into your code. As you might already know, when you write code, you’re actually creating an AST: expressions, assignments, bindings, conditions, function calls, etc. They all represent a typed tree that represents your program.

luminance uses that property to create a dependency between resources your GPU needs to have in order to perform a render. Typical engines, libraries and frameworks require you to explicitly bind something; instead, luminance requires you to go deeper in the AST by creating a new lower node to mark the dependency.

It might be weird at first but you’ll see how simple and easy it is. If you want to perform a simple draw call of a triangle, you need several resources:

  • A Tess that represents the triangle. It holds three vertices.
  • A shader Program, for shading the triangle with a constant color, for short and simple.
  • A Framebuffer, to accept and hold the actual render.
  • A RenderState, to state how the render should be performed.

There is a dependency graph to represent how the resources must behave regarding each other:

(AST1)

Framebuffer ─> Shader ─> RenderState ─> Tess

The framebuffer must be active, bound, used — or whatever verb you want to picture it with — before the shader can start doing things. The shader must also be in use before we can actually render the tessellation.

That triple dependency relationship is already a small flat AST. Imagine we want to render a second triangle with the same render state and a third triangle with a different render state:

(AST2)

Framebuffer ─> Shader ─> RenderState ─> Tess
                 │            │
                 │            └───────> Tess
                 │
                 └─────> RenderState ─> Tess

That AST looks more complex. Imagine now that we want to shade one other triangle with another shader!

(AST3)

Framebuffer ─> Shader ─> RenderState ─> Tess
     │           │            │
     │           │            └───────> Tess
     │           │
     │           └─────> RenderState ─> Tess
     │
     └───────> Shader ─> RenderState ─> Tess

You can now clearly see the ASTs and the relationships between objects. Those are encoded in luminance within your code directly: lambdas / closures.

If you have followed thoroughly, you might have noticed that you cannot, with such ASTs, shade a triangle with another shader but using the same render state as another node. That was a decision that was needed to be made: how should we allow the AST to be shared? In terms of graphics pipeline, luminance tries to do the best thing to minimize the number of GPU context switches and CPU <=> GPU bandwidth congestion.

The lambda & closure design

A function is a perfect candidate to modelize a dependency. When you look at:

fn tronfibulate(x: Foo) -> Bar;

tronfibulate here is covariant in Bar and contravariant in Foo. What it implies is that for the function itself, if we have a function that does Zoo -> Foo, then we can create a new version of tronfibulate that will have, as input, a Zoo. Contravariance maps backwards while covariance maps forwards (i.e. if you have Bar -> Quux, you can adapt tronfibulate to create a new function that will output Quux value).

All this to say that a dependency (which is contravariant) is pretty interesting in our case since we will be able to adapt and create new functions just by contra-mapping the input. In terms of combinational power, that is gold.

Now, let’s try to represent AST1 with contravariance and, hence, functions, using pseudo-code (this is not real luminance excerpt).

// AST1
use_framebuffer(framebuffer, || {
  // here, we are passing a closure that will get called whenever the framebuffer is ready to
  // receive renders
  use_shader(shader, || {
    // same thing but for shader
    use_render_state(render_state, || {
      // ditto for render state
      triangle.render(); // render the tessellation
    });
  );
);

See how simple it is to represent AST1 with just code and closures? Rust’s lifetimes and existential quantification allows us to ensure that no resource will leave the scope of each closures, hence enforcing memory and coherency safety.

Now let’s try to tackle AST2.

// AST2
use_framebuffer(framebuffer, || {
  use_shader(shader, || {
    use_render_state(render_state, || {
      first_triangle.render();
      second_triangle.render(); // simple and straight-forward
    });

    // we can just branch a new render state here!
    use_render_state(other_render_state, || {
      third.render()
    });
  );
);

And AST3:

// AST3
use_framebuffer(framebuffer, || {
  use_shader(shader, || {
    use_render_state(render_state, || {
      first_triangle.render();
      second_triangle.render(); // simple and straight-forward
    });

    // we can just branch a new render state here!
    use_render_state(other_render_state, || {
      third.render()
    });
  );

  use_shader(other_shader, || {
    use_render_state(yet_another_render_state, || {
      other_triangle.render();
    });
  });
);

The luminance equivalent is a bit more complex because it implies some objects that need to be introduced first.

Pipeline

A Pipeline represents a whole AST as seen as just above. It is created by a GraphicsContext when you ask to create a pipeline and is destroyed as soon as the render has happened. A Pipeline is a special object you can use to bind some specific scarce resources, such as textures and buffers.

Creating a Pipeline requires at least one resource: a Framebuffer to render to.

When you create a pipeline, you’re also handed a ShadingGate. A ShadingGate is an object that allows you to create shader nodes in the AST you’re building. You have no other way to go deeper in the AST. The concept of gates is very important and you should try to familiarize yourself with it.

ShadingGate

As said above, a ShadingGate allows you to create a shader node in the graphics pipeline. That node will typically borrow a shader Program and will move you one level lower in the graph (AST). At that level (i.e. in that closure), you are given two objects:

The ProgramInterface is the only way for you to access your uniform interface. More on this in the dedicated section. It also provides you with the ProgramInterface::query method, that allows you to perform dynamic uniform lookup.

RenderGate

A RenderGate is the second to last gate you will be handling. It allows you to create render state nodes in your AST, creating a new level for you to render tessellations with an obvious, final gate: the TessGate.

The kind of object that node manipulates is RenderState.

TessGate

The TessGate is the final gate you use in an AST. It’s used to create tessellation nodes. Those are used to render actual Tess. You cannot go any deeper in the AST at that stage.

TessGates don’t immediately use Tess as inputs. They use TessSlice. That type is a simple GPU slice into a GPU tessellation (Tess). It can be obtained from a Tess via the TessSliceIndex trait.

Contributor-guide

You want to hack around luminance or provide a windowing crate? Everything you have to know is described in this section.

What it means to be a luminance windowing backend

luminance doesn’t know anything about the context it executes in. That means that it doesn’t know whether it’s used within the SDL, GLFW, glutin, Qt or an embedded specific hardware such as the Nintendo Switch. That is actually powerful, because it allows luminance to be completely agnostic of the execution platform it’s running on: one problem less.

However, the connection between luminance and the execution context must be correctly done. Currently, several points must be enforced:

  • The OpenGL version must be 3.3.
  • The OpenGL profile must be core.
  • OpenGL forward compatibility must be enabled.

Those rules might change and be adapted regarding the feature flags that are enabled. For instance, if you use a feature flag that allows to use OpenGL 2.1, then you should use a 2.1 OpenGL version.

GraphicsContext, GraphicsState and TLS

In order to implement GraphicsContext, you need to know several points:

  • You can get a GraphicsState with the GraphicsState::new function. You have to match the return value. Depending on whether your implementation is the first asking a GraphicsState on the current thread, you might get (or not) an Ok(state). If not, a descriptive error is returned.
  • You’re advised to map and map_err over the GraphicsState::new returned value to implement your own new function for your backend type because of the restriction of having only one context per thread in luminance.

Dependencies