Graphics Card Explained: How the Vertex Shader, Pixel Pipeline, Stream Processor, And ROP Unit Work

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You may be familiar with the terms surrounding the physical aspects of a graphics card. However, the same might not be said for the terms used to describe the graphics card’s actual performance indicators.

A general understanding of the way in which your graphics card renders images can help you get the most out of it, whether your primary purpose is for video editing or gaming.

Knowing what each of the properties means will allow you to make a more informed choice about the power and performance of the card you are considering, and this article will feature on the terms used to describe the aspects under a graphics card’s ‘hood’ which determine the card’s rendering performance, with the most commonly encountered terms being:

Vertex Shader

Vertices are points on a 3D map that are used to create the outlines of the images that you see within 3D games. Images are typically made up of many vertices, and are used to determine every object’s position within the scene to be rendered. Once each object’s location has been established on the map, the map is passed to the vertex shader.

The Vertex Shader is responsible for adding special effects to objects in a 3D environment. It does this by performing mathematical calculations on the objects' vertex data using an array of variables, such as the object’s co-ordinates, colour and position and space. The vertex shader is responsible for calculating the 3D aspects of a scene, such as colouring, lighting etc and converting the data into a 2D map which it is passed to the pixel shader for further processing and rendering.

Pixel Pipeline

Every image you see on a screen is made up of thousands of pixels. A pixel is a single point within an image, and is normally capable of displaying either three colours (red, green, blue) or four colours (cyan, yellow, magenta, black). Pixels are associated with the screen resolution of your display, so if you were to play a game at a common resolution such as 1280x1024, your display would show 1280 pixels across the screen, and 1024 pixels from top to bottom.

The pixel pipeline processes the pixel, texture and geometric data received from the Vertex Shaders. Different GPUs (Graphics Processing Unit) have different numbers of pixel pipelines, but as a rule of thumb, the more pipelines a graphics card has, the faster the card can process the data for rendering the images on-screen.

Each pixel is made up of a series of fragments, which are processed by the pixel shader according to calculations made by the vertex shader. Once each fragment is processed it is held in a buffer where it is built into a complete pixel by the Raster Operator unit.

Pixel shading is usually the most intensive part of the graphics rendering process on a modern GPU and so usually takes the most time.

ROP Unit

The Raster Operator handle the final transition from the pixel pipeline to the display by building the pixel fragments generated from the pixel pipeline into complete pixels. Most modern graphics cards have multiple ROP units. The ROP unit also optimizes the display image to save memory bandwidth, such as when dealing with depth compression and colour comparison.

Stream Processer

Stream processors are a relatively new technology to be introduced to graphics cards. Essentially, stream processors can be allocated different processes to perform depending on what graphical environment is to be generated. For example, in indoor scenes the stream processors can be set as shaders, while in outdoor scenes the stream processors can be used to map vertices.

Stream processors are commonly used in the newer generations of graphics cards, replacing dedicated vertex shaders and pixel pipelines.

The above are the main elements which make up the graphics processing power of your graphics card. There are, however, other areas of the card which can be set and controlled using software such as antialiasing and anisotropic filtering. Depending on the game you are playing, and the more powerful the graphics card you have, the higher these additional factors can be pushed.