Shader

This article is about the kind of computer program. For other uses, see Shader (album) and Tattoo machine.

A simplified illustration of the graphics pipeline in the Vulkan API. The yellow sections are programmable shader stages which can modify the different data being processed by the pipeline.

In computer graphics, a shader is a programmable operation which is applied to data as it moves through the rendering pipeline. Shaders can act on data such as vertices and primitives — to generate or morph geometry — and fragments — to calculate the values in a rendered image.

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. Such shaders executing in a compute pipeline are commonly called compute shaders.

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. Geometry shaders were introduced with Direct3D 10 and OpenGL 3.2. Eventually, graphics hardware evolved toward a unified shader model.

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 render targets 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 blur, or edge detection/enhancement for cartoon/cel shaders. 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 filter for a video stream after it has been rasterized.

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. 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. Nvidia Turing is the world's first GPU microarchitecture that supports mesh shading through DirectX 12 Ultimate API, several months before Ampere RTX 30 series was released.

In 2020, AMD and Nvidia released RDNA 2 and Ampere microarchitectures which both support mesh shading through DirectX 12 Ultimate. These mesh shaders allow the GPU to handle more complex algorithms, offloading more work from the CPU to the GPU, and in algorithm intense rendering, increasing the frame rate of or number of triangles in a scene by an order of magnitude. Intel announced that Intel Arc Alchemist GPUs shipping in Q1 2022 will support mesh shaders.

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

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 NPUs or GPUs. 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 High Level Shader Language, 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

• GLSL
• SPIR-V
• HLSL
• Compute kernel
• Shading language
• GPGPU
• List of common shading algorithms
• Vector processor

Further reading

• Upstill, Steve (1990). The RenderMan Companion: A Programmer's Guide to Realistic Computer Graphics. Addison-Wesley. ISBN 0-201-50868-0.
• Ebert, David S; Musgrave, F. Kenton; Peachey, Darwyn; Perlin, Ken; Worley, Steven (1994). Texturing and modeling: a procedural approach. AP Professional. ISBN 0-12-228730-4.
• Fernando, Randima; Kilgard, Mark (2003). The Cg Tutorial: The Definitive Guide to Programmable Real-Time Graphics. Addison-Wesley Professional. ISBN 0-321-19496-9.
• Rost, Randi J (2004). OpenGL Shading Language. Addison-Wesley Professional. ISBN 0-321-19789-5.

External links

• OpenGL geometry shader extension
• Riemer's DirectX & HLSL Tutorial: HLSL Tutorial using DirectX with much sample code
• Pipeline Stages (Direct3D 10)