Graphics Shaders

Graphics is rendered using a rendering pipeline split into stages. There are multiple APIs to control graphics rendering. Qt supports OpenGL, Metal, Vulcan, and Direct3D. Looking at a simplified OpenGL pipeline, we can spot a vertex and fragment shader. These concepts exists for all other rendering pipelines too.

In the pipeline, the vertex shader receives vertex data, i.e. the location of the corners of each element that makes up the scene, and calculates a gl_Position. This means that the vertex shader can move graphical elements. In the next stage, the vertexes are clipped, transformed and rasterized for pixel output. Then the pixels, also known as fragments,are passed through the fragment shader, which calculates the color of each pixel. The resulting color returned through the gl_FragColor variable.

To summarize: the vertex shader is called for each corner point of your polygon (vertex = point in 3D) and is responsible for any 3D manipulation of these points. The fragment (fragment = pixel) shader is called for each pixel and determines the color of that pixel.

As Qt is independent of the underlying rendering API, Qt relies on a standard language for writing shaders. The Qt Shader Tools rely on a Vulcan-compatible GLSL. We will look more at this in the examples in this chapter.