Shaders

Edit

Shaders are small GPU programs that can be used to customize rendering or perform arbitrary computation. This guide explains how shaders work in LÖVR.

GLSL

Shaders are written in GLSL. GLSL is a lower-level language than Lua, so shaders are a bit harder to learn at first. Explaining GLSL is outside the scope of these docs, but some good ways to learn it are:

• The OpenGL wiki is a good reference.
• Copying random shaders from the internet and changing them until they do interesting things is another recommended approach.

LÖVR uses GLSL version 4.60.

Basics

There are 2 types of shaders, given by ShaderType:

• graphics shaders are used for rendering. They compute vertex positions and pixel colors.
• compute shaders run in compute passes. They can perform arbitrary GPU computation.

Shaders have one or more "stages", which are basically functions, given by ShaderStage:

• graphics shaders have 2 stages:
  • The vertex stage computes vertex positions. It loads vertex data from meshes and applies transformations to get the final "on-screen" position for triangles.
  • The fragment stage computes pixel colors. It uses material data to compute lighting and other effects, returning the final color of the pixel.
• compute shaders have a single compute stage. It doesn't have any semantic meaning, instead performing arbitrary computation by writing to Buffer and Texture objects.

Each Pass has an active shader it uses to process draws. Pass:setShader changes the active shader. The active shader will affect all draws until the shader is changed again. When the active shader is nil, LÖVR will use a built-in shader based on the type of draw (unlit for meshes, font for text, etc.).

The set of shaders built in to LÖVR are given by DefaultShader.

Writing Shaders

LÖVR uses the lovrmain function as the GLSL entrypoint. For vertex stages, lovrmain should return the final transformed vertex position. Here's the default vertex shader:

vec4 lovrmain() {
  return Projection * View * Transform * VertexPosition;
}

It can also be written using the DefaultPosition shorthand:

vec4 lovrmain() {
  return DefaultPosition;
}

The vertex position is multiplied by several matrices to get it into "normalized device coordinates", which is the coordinate space the GPU uses internally to process vertices.

For fragment shaders, lovrmain should return the final color of the pixel. Here's the default fragment shader:

vec4 lovrmain() {
  return Color * getPixel(ColorTexture, UV);
}

It can also be written using the DefaultColor shorthand:

vec4 lovrmain() {
  return DefaultColor;
}

The default pixel color is calculated by multiplying the Color from the vertex stage (which includes the vertex color, material color, and pass color) with a pixel sampled from the color texture.

Compute shaders implement the void lovrmain() function, and don't return anything.

Shaders are created with lovr.graphics.newShader, with the code for each stage:

shader = lovr.graphics.newShader([[
  vec4 lovrmain() {
    return DefaultPosition;
  }
]], [[
  vec4 lovrmain() {
    return DefaultColor;
  }
]])

The code can also be loaded from a filename or a Blob. Also, a DefaultShader can be used for the code of one or both of the stages:

shader = lovr.graphics.newShader('vertex.glsl', 'unlit')

Shader Builtins

The following built-in variables and macros are available in vertex and fragment shaders:

Name	Type	Notes
PI	float
TAU	float	(PI * 2)
PI_2	float	(PI / 2)
Resolution	vec2	The size of the render pass texture, in pixels.
Time	float	Current time in seconds (lovr.headset.getTime).
CameraPositionWorld	vec3	The position of the current view (Pass:setViewPose).
Sampler	sampler	The default sampler (Pass:setSampler).

The following built-in variables are available only in vertex shaders:

Name	Type	Notes
VertexPosition	vec4	The local position of the current vertex.
VertexNormal	vec3	The normal vector of the current vertex.
VertexUV	vec2	The texture coordinate of the current vertex.
VertexColor	vec4	The color of the current vertex.
VertexTangent	vec3	The tangent vector of the current vertex.
Projection	mat4	The projection matrix of the current view (Pass:setProjection).
View	mat4	The view matrix of the current view (Pass:setViewPose).
ViewProjection	mat4	The projection matrix multiplied with the view matrix.
InverseProjection	mat4	The inverse of the projection matrix.
Transform	mat4	The model matrix (includes transform stack and draw transform).
NormalMatrix	mat3	Transforms normal vectors from local space to world space.
ClipFromLocal	mat4	Transforms from local space to clip space (Projection * View * Transform).
ClipFromWorld	mat4	Transforms from world space to clip space (Projection * View).
ClipFromView	mat4	Transforms from view space to clip space (Projection).
ViewFromLocal	mat4	Transforms from local space to view space (View * Transform).
ViewFromWorld	mat4	Transforms from world space to view space (View).
ViewFromClip	mat4	Transforms from clip space to view space (InverseProjection).
WorldFromLocal	mat4	Transforms from local space to world space (Transform).
WorldFromView	mat4	Transforms from view space to world space (inverse(View)).
WorldFromClip	mat4	Transforms from clip space to world space (inverse(ViewProjection)).
PassColor	vec4	The color set with Pass:setColor.

The following built-in variables and macros are available only in fragment shaders:

Name	Type	Notes
PositionWorld	vec3	The position of the pixel, in world space.
Normal	vec3	The normal vector of the pixel, in world space.
Color	vec4	The vertex, material, and pass colors multiplied together.
UV	vec2	The texture coordinate of the current pixel.
Tangent	vec3	The tangent vector of the current pixel, in world space.
TangentMatrix	mat3	The tangent matrix, used for normal mapping.

The properties of the active material, set using Pass:setMaterial, can be accessed in vertex and fragment shaders. Textures can be sampled using the getPixel helper function. The Material and lovr.graphics.newMaterial pages have more info on these properties.

Name	Type	Notes
Material.color	vec4	The material color.
Material.glow	vec4	The material glow color (alpha is glow strength).
Material.uvShift	vec2	The material UV shift.
Material.uvScale	vec2	The material UV shift.
Material.metalness	float	The material metalness.
Material.roughness	float	The material roughness.
Material.clearcoat	float	The material clearcoat factor.
Material.clearcoatRoughness	float	The roughness of the clearcoat layer.
Material.occlusionStrength	float	The strength of the occlusion texture.
Material.normalScale	float	The strength of the normal map texture.
Material.alphaCutoff	float	The alpha cutoff.
ColorTexture	texture2D	The color texture.
GlowTexture	texture2D	The glow texture.
OcclusionTexture	texture2D	The ambient occlusion texture.
MetalnessTexture	texture2D	The metalness texture.
RoughnessTexture	texture2D	The roughness texture.
ClearcoatTexture	texture2D	The clearcoat texture.
NormalTexture	texture2D	The normal map.

Shader Inputs

It's also possible to send values or objects from Lua to a Shader. There are a few different ways to do this, each with their own tradeoffs (speed, size, ease of use, etc.).

Constants

Shaders can declare constants, which can be booleans, numbers, vectors, or matrices. There is a very limited amount of space for constants (usually 128 or 256 bytes, depends on the GPU), but they are very easy and inexpensive to update.

Constants are declared in shader code in a Constants block, then individual constants are modified from Lua using Pass:send:

function lovr.load()
  shader = lovr.graphics.newShader('unlit', [[
    Constants {
      vec4 color1;
      vec4 color2;
    };

    // Apply a vertical gradient using the 2 colors from the constants:

    vec4 lovrmain() {
      return mix(color1, color2, dot(Normal, vec3(0, 1, 0)) * .5 + .5);
    }
  ]])
end

function lovr.draw(pass)
  pass:setShader(shader)

  pass:send('color1', { 1, 0, 1, 1 })
  pass:send('color2', { 0, 1, 1, 1 })

  pass:sphere(0, 1.7, -2)
end

The vertex and fragment stages share the constants block, so they should match or one should be a subset of the other.

When the active shader is changed, constants will be preserved.

Vertex Attributes

Vertex attributes are the data for each vertex of a mesh. They should be used for data that varies on a per-vertex basis. Attributes have a name, a type, and a location (an integer ID). LÖVR uses the following default vertex attributes for shapes and meshes:

Name	Type	Location
VertexPosition	vec4	10
VertexNormal	vec3	11
VertexUV	vec2	12
VertexColor	vec4	13
VertexTangent	vec3	14

Custom vertex attributes can be declared for locations 0 through 9:

layout(location = 3) in vec3 customAttribute;

The data in a buffer can then be associated with the attribute, either by name:

vertices = lovr.graphics.newBuffer(vertexCount, {
  { type = 'vec3', name = 'customAttribute' }
})

Or by location:

vertices = lovr.graphics.newBuffer(vertexCount, {
  { type = 'vec3', location = 3 }
})

Buffers

Shaders can access data in Buffer objects. Buffers can store large arrays of data from Lua tables or Blobs. The GPU can also write to buffers using compute shaders.

Data in buffers can be accessed in 2 ways:

• Uniform buffers have a smaller size limit, may be faster, and are read-only in shaders.
• Storage buffers have a large size limit, may be slower, and compute shaders can write to them.

First, the buffer should be declared in the shader. Here's an example declaring a uniform buffer:

layout(set = 2, binding = 0) uniform Colors {
  vec4 colors[100];
};

And an example storage buffer:

layout(set = 2, binding = 0) buffer Colors {
  vec4 colors[100];
};

The first part declares the set and binding of the variable. Right now the set should always be 2 (LÖVR uses set 0 and 1 internally). The binding is a number that can be used to identify the variable. After that, uniform or buffer is used to declare which type of buffer it is, followed by the name of the variable. Finally, there is a block declaring the format of the data in the buffer, which should match the format used to create the Buffer in Lua (structs can be used if the buffer has multiple fields per element).

A Buffer can be sent to one of the above variables like this:

-- palette is a table with 100 colors in it
buffer = lovr.graphics.getBuffer(palette, 'vec4')
pass:send('Colors', buffer)

-- or, using the binding number
pass:send(0, buffer)

The shader can then use the colors array to access the data from the palette table.

It's possible to bind a subset of a buffer to the shader by passing the range as extra arguments to Pass:send.

Textures

Shaders can also access data from Texture objects. Similar to buffers, textures can be accessed in 2 ways:

• Sampled textures are read-only, and can use Sampler objects.
• Storage textures can be written to using compute shaders.

Sampled textures are declared like this:

layout(set = 2, binding = 0) uniform texture2D myTexture;

The texture type can be texture2D, textureCube, texture2DArray, or texture3D (see TextureType).

Storage textures are declared like this:

layout(set = 2, binding = 0) uniform image2D myImage;

A texture can be sent to the shader variable using Pass:send.

The getPixel helper function can be used to sample from a texture:

getPixel(myTexture, UV)

This will sample from the texture using the UV coordinates and the default sampler set using Pass:setSampler. It's the same as writing this for 2D textures:

texture(sampler2D(myTexture, Sampler), UV)

It's also possible to declare a custom sampler variable and use it to sample textures:

layout(set = 2, binding = 0) uniform sampler mySampler;

// texture(sampler2D(myTexture, mySampler), UV)

A Sampler object can be sent to the shader using Pass:send, similar to buffers and textures.