In computer graphics, a shader is a computer program operation which is applied to data as it moves through the rendering pipeline.
Shaders can execute a wide variety of operations and can run on different types of hardware. In modern real-time computer graphics, shaders are run on graphics processing units (GPUs) — dedicated hardware which provides highly parallel execution of programs. As rendering an image is embarrassingly parallel, fragment and pixel shaders scale well on SIMD hardware. Historically, the drive for faster rendering has produced highly-parallel processors which can in turn be used for other SIMD amenable algorithms.
History
The first known use of the term "shader" was introduced to the public by
Pixar with version 3.0 of their RenderMan Interface Specification, originally published in May 1988.
As graphics processing units evolved, major graphics software libraries such as OpenGL and Direct3D began to support shaders. The first shader-capable GPUs only supported pixel shading, but vertex shaders were quickly introduced once developers realized the power of shaders. The first video card with a programmable pixel shader was the Nvidia GeForce 3 (NV20), released in 2001.
Graphics shaders
The traditional use of shaders is to operate on data in the graphics pipeline to control the rendering of an image. Graphics shaders can be classified depending on their position in the pipeline, the data being manipulated, and the graphics API being used.
Fragment shaders
Fragment shaders, also known as
pixel shaders, compute
color and other attributes of each "fragment": a unit of rendering work affecting at most a single output pixel. The simplest kinds of pixel shaders output one screen pixel as a color value; more complex shaders with multiple inputs/outputs are also possible.
Pixel shaders range from simply always outputting the same color, to applying a
lighting value, to doing
bump mapping,
shadows, specular highlights,
translucency and other phenomena. They can alter the depth of the fragment (for
Z-buffering), or output more than one color if multiple
are active. In 3D graphics, a pixel shader alone cannot produce some kinds of complex effects because it operates only on a single fragment, without knowledge of a scene's geometry (i.e. vertex data). However, pixel shaders do have knowledge of the screen coordinate being drawn, and can sample the screen and nearby pixels if the contents of the entire screen are passed as a texture to the shader. This technique can enable a wide variety of two-dimensional postprocessing effects such as
Gaussian blur, or
edge detection/enhancement for
Cel shader. Pixel shaders may also be applied in
intermediate stages to any two-dimensional images—sprites or textures—in the pipeline, whereas
vertex shaders always require a 3D scene. For instance, a pixel shader is the only kind of shader that can act as a postprocessor or
video filter for a
video stream after it has been
rasterization.
Vertex shaders
Vertex shaders are run once for each 3D vertex given to the graphics processor. The purpose is to transform each vertex's 3D position in virtual space to the 2D coordinate at which it appears on the screen (as well as a depth value for the Z-buffer).
Vertex shaders can manipulate properties such as position, color and texture coordinates, but cannot create new vertices. The output of the vertex shader goes to the next stage in the pipeline, which is either a geometry shader if present, or the
rasterizer. Vertex shaders can enable powerful control over the details of position, movement, lighting, and color in any scene involving 3D models.
Geometry shaders
Geometry shaders were introduced in Direct3D 10 and OpenGL 3.2; formerly available in OpenGL 2.0+ with the use of extensions.
[ Geometry Shader - OpenGL. Retrieved on December 21, 2011.] This type of shader can generate new graphics primitives, such as points, lines, and triangles, from those primitives that were sent to the beginning of the graphics pipeline.
Geometry shader programs are executed after vertex shaders. They take as input a whole primitive, possibly with adjacency information. For example, when operating on triangles, the three vertices are the geometry shader's input. The shader can then emit zero or more primitives, which are rasterized and their fragments ultimately passed to a pixel shader.
Typical uses of a geometry shader include point sprite generation, geometry tessellation, shadow volume extrusion, and single pass rendering to a cube map. A typical real-world example of the benefits of geometry shaders would be automatic mesh complexity modification. A series of line strips representing control points for a curve are passed to the geometry shader and depending on the complexity required the shader can automatically generate extra lines each of which provides a better approximation of a curve.
Tessellation shaders
As of OpenGL 4.0 and Direct3D 11, a new shader class called a tessellation shader has been added. It adds two new shader stages to the traditional model: tessellation control shaders (also known as hull shaders) and tessellation evaluation shaders (also known as Domain Shaders), which together allow for simpler meshes to be subdivided into finer meshes at run-time according to a mathematical function. The function can be related to a variety of variables, most notably the distance from the viewing camera to allow active level-of-detail scaling. This allows objects close to the camera to have fine detail, while further away ones can have more coarse meshes, yet seem comparable in quality. It also can drastically reduce required mesh bandwidth by allowing meshes to be refined once inside the shader units instead of downsampling very complex ones from memory. Some algorithms can upsample any arbitrary mesh, while others allow for "hinting" in meshes to dictate the most characteristic vertices and edges.
Primitive and Mesh shaders
Circa 2017, the
AMD Vega microarchitecture added support for a new shader stage—primitive shaders—somewhat akin to compute shaders with access to the data necessary to process geometry.
Nvidia introduced mesh and task shaders with its Turing microarchitecture in 2018 which are also modelled after compute shaders.
In 2020, AMD and Nvidia released RDNA 2 and Ampere microarchitectures which both support mesh shading through DirectX 12 Ultimate.
Ray tracing shaders
Ray tracing shaders are supported by
Microsoft via DirectX Raytracing, by
Khronos Group via Vulkan,
GLSL, and
SPIR-V,
by Apple via Metal.
NVIDIA and
AMD called "ray tracing shaders" as "ray tracing cores". Unlike unified shader, one ray tracing shader can contain multiple ALUs.
Compute shaders
are not limited to graphics applications, but use the same execution resources for
GPGPU. They may be used in graphics pipelines e.g. for additional stages in animation or lighting algorithms (e.g. tiled forward rendering). Some rendering APIs allow compute shaders to easily share data resources with the graphics pipeline.
Tensor shaders
Tensor shaders may be integrated in
AI accelerator or
. Tensor shaders are supported by
Microsoft via
DirectML, by
Khronos Group via
OpenVX, by Apple via
Core ML, by Google via
TensorFlow, by
Linux Foundation via
ONNX.
[ ] NVIDIA and
AMD called "tensor shaders" as "tensor cores". Unlike unified shader, one tensor shader can contains multiple ALUs.
Programming
The language in which shaders are programmed depends on the target environment. The official OpenGL and
OpenGL ES shading language is OpenGL Shading Language, also known as GLSL, and the official Direct3D shading language is
HLSL, also known as HLSL. Cg, a third-party shading language which outputs both OpenGL and Direct3D shaders, was developed by
Nvidia; however since 2012 it has been deprecated. Apple released its own shading language called Metal Shading Language as part of the Metal framework.
GUI shader editors
Modern
video game development platforms such as Unity,
Unreal Engine and Godot increasingly include node-based editors that can create shaders without the need for actual code; the user is instead presented with a
directed graph of connected nodes that allow users to direct various textures, maps, and mathematical functions into output values like the diffuse color, the specular color and intensity, roughness/metalness, height, normal, and so on. Automatic compilation then turns the graph into an actual, compiled shader.
See also
Further reading
External links
