Core Profile and Compatibility Profile

The Key Versions to Revolute The OpenGL

OpenGL 3.0 introduced deprecation model to gradually remove legacy functions.
OpenGL 3.1 removed the deprecated functions and the remaining part forms the Core Profile.
OpenGL 3.2 introduced Compatibility Profile to maintain backwards compatibility.

Core Profile OpenGL

Have access to later OpenGL functionalities
More suitable for modern GPU
Many familiar functionalities removed
All rendering must use shaders
- A shader is a program loaded and executed in a stage of the rendering pipeline
- The shader is a user program which means that the users have to/can define their own shaders to realize the rendering

OpenGL Pipeline

A simplified view of OpenGL Graphics Pipeline is shown as follows:

Vertex Shading

Excuted per vertex
Produces vertex NDC (normalized device coordinates) coordinates

Tesssllation

Tessellation Control Shader (TCS)
- Executed per patch vertex
- Receives patch vertices (control points) from vertex shader
- Generates tessellation output patch vertices to pass to the Tessellation Evaluation Shader (TES)
- Specifies tessellation level factors
- Optional
Tessellation Primitive Generator (TPG)
- Receives tessellation level factors from TCS
- Performs tessellation and generates tessellation coordinates for the resulting primitives and pass them to the Tessellation Evaluation Shader (TES)
- not programmable
Tessellation Evaluation Shader (TES)
- Executed per tessellation coordinate
- Receives patch vertices (control points) from TCS
- Computes vertex position from tessellation coordinate
- Optional

Geometry Shading

Executed per primitive
Receives complete primitive (as arrays of vertices/attributes) from earlier stage
Outputs zero primitive (culling) or more primitives (geometry amplification)
Can output primitive type different from input
Optional

Primitive Assembly, etc.

This part is fixed, cannot be programmed

Primitive assembly
- Vertex data is collected into complete primitives
- Necessary for clipping and back-face culling
Clipping
Perspective division
- To normalized device coordinate (NDC) space
Viewport transformation
- To windows space
- Include depth range scaling
Back-face culling

Rasterization

If geometric primitive is not clipped out, the appropriate pixels in the frame buffer must be assigned colors
Rasterizer produces a set of framgements for each primitive
- Fragments are “potential pixels”, they have pixel/fragment locations (in window space) and have color and depth attributes (and others)

Fragment Shading

Executed per fragment
Each generated fragment is processed to determine the color of the corresponding pixel in the framebuffer

Per-Fragment Operations

After fragment shading, each resulting fragment is submitted to a set of simple operations before reaching the framebuffer

Framebuffer

A framebuffer (frame buffer or framestore) is a portion of random-access memory (RAM) containing a bitmap that drives a video display. It is a memory buffer containing a complete frame of data.