# Shader

Shader is a program running on the [graphics processing unit](gpu.md) (GPU), typically in many parallel instances as to utilize the GPU's highly parallel nature. As such they are simple to mid complexity programs.

The word *shader* is also used more loosely to stand for any specific effect, material or look in 3D graphics (e.g. [games](games.md)), as shaders are usually the means of achieving such effects.

Shaders are normally written in a special **shading language** such as [GLSL](glsl.md) in the [OpenGL](opengl.md) [API](api.md), [HLSL](hlsl.md) ([proprietary](proprietary.md)) in [Direct3D](d3d.md) API or the Metal shading language ([proprietary](proprietary.md)) in [Metal](metal.md) API. These languages are often similar to [C](c.md) with some additions (e.g. vector and matrix data types) and simplifications (e.g. no function [recursion](resursion.md)). High level [frameworks](framework.md) like [Blender](blender.md) many times offer [visual programming](visual_programming.md) (point-n-click) of shaders with graph/node editors.

Initially (basically early 2000s) shaders were used only for graphics, i.e. to transform 3D vertices, draw triangles and compute pixel colors. Later on as GPUs became more [general purpose](gpgpu.md), flexibility was added to shaders that allowed to solve more problems with the GPU and eventually general *compute* shaders appeared (OpenGL added them in version  3.3 in 2010).

To put shaders in the context, the flow of data is this: a [CPU](cpu.md) uploads some data (3D models, textures, ...) to the GPU and then issues a draw command -- this makes the GPU start its **pipeline** consisting of different **stages**, e.g. the vertices of 3D models are transformed to screens space (the vertex stage), then triangles are generated and rasterized (the shading stage) and the data is output (on screen, to a buffer etc.). Some of these stages are programmable and so they have their own type of a shader. The details of the pipeline differ from API to API, but in general, depending on the type of data the shader processes (the stage), we talk about:

- **vertex shaders**: Perform per-vertex computations on 3D models, typically their transformation from their world position to the position in the camera and screen space.
- **fragment/pixel shaders**: Compute the final color of each pixel (sometimes called more generally *fragments*), i.e. work per-pixel. A typical use is to perform [texturing](texture.md) and amount of reflected light (lighting model).
- **geometry shaders**: Advanced stage, serves to modify the geometry of 3D models (these shaders can, unlike vertex shaders, generate of discard vertices).
- **tesselation shaders**: Advanced stage, serves for subdividing primitives to create smoother geometry.
- **compute shaders**: Perform general computations, used e.g. for computing physics simulations, [AI](ai.md) etc.
- **[ray tracing](ray_tracing.md) shaders** and other specific types