OpenTK suited tutorial, OpenTK allows you to use OpenGL in C#

good explanation videos

good explanation slides

"If you can’t explain it simply, you don’t understand it well enough." - Albert Einstein

"Laziness can be the downfall of the best of us." - Always Google

What is OpenGL ?

"Standartized API for rendering 2D and 3D vector graphic." https://en.wikipedia.org/wiki/OpenGL
API = a set of functions you use to render vector graphic
Standartized = same API and behaves the same on any platform in any programing language
CPU tells what to render -> GPU renders it
vendor = GPU manufacturer = NVidia, ATI, ...
different vendor = different GPU architecture = different way of inner workings,
each vendor implements the same set of functions (OpenGL API), inside the implmenetation each does something different but end result is the same (rendered vector graphics)
the OpenGL API implementation is inside vendor drivers, most systems come preinstalled with (outdated?) drivers

What is Shading Language

introduced in OpenGL 1.5 (2003)
OpenGL Shading Language (glsl) is programing language used to describe how to adjust parts of rendering pipeline
before OpenGL 1.5 there was only fixed pipeline, you had little control over the final image
is similar to C, has many inbuilt functions and types
shader = our own custom part of rendering pipeline written in shading language
rendering pipeline has several stages (points) which you can replace with your own shaders
each stage has some inbuilt variables

How does one render triangle on screen ?

Simplified rendering pipeline

vertex shader that adds 5 units on X axis to every vertex

vec3 vertex_main(vec3 in)
{
  return in + vec3(5,0,0);
}

fragment shader that outputs color based on screen position

vec3 fragment_main()
{
 return vec3(gl_FragCoord.x, gl_FragCoord.y, 0);
}