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OpenGL learning notes

OpenGL is itself a big statemachine.All significant operations are done to change this machine's state.

Pipe line

Bold items are what programmer can do to refine.

pipe line (or shaders) of graphics workflow.

3D data -> rendered pixels.

  • Vertex shader
  • Shape assembley
  • Geometry shader
  • Tests and blending
  • Fragment shader
  • Rasterization

Inut:Vertex Data

list of 3D coordinates which can be formed to triangiles.

  • so the number of 3D coordinate is times of 3?

Vertex data is a list of vertices

  • vertex

    a collection of data corresponds to a 3D coordinates.

  • vertex attributes

    contaning many useful information such as color in addition to a 3D coordinate.

  • primitives

    a hint of render type to tell OpenGL how to make up of those vertices, such as a line, a triangle or raw points

    so, those vertices may not form a triangle?

Vertex shader

  • first step of the pipeline.
  • takes a sigle vertex.
  • transform one 3D coordinate to another
  • basic processing, such as attributes.

Primitve assemblely

  • take vertices(or vertex if GL_POINTS is specified)
  • form a primitive shape

Geometry shader

  • takes a primitive shape and has the ability to generate new shape.

Raserization

  • mapping the primitive to pixels called fragments.
  • clipping fragments that is outd side of your view.

Fragment Shader

  • a fragment is all the data needed for OpenGL to render a pixel.
  • calculate the final color of one pixel.
  • most advanced effects occured
  • fragment shader contains data about the 3D scene

Tests and blending

  • alpha test
  • blending
  • checks the depth value to decide if one fragment is behind another object.
  • check alpha value and blends the objects.
  • pixel value may entirely different with Fragment shader stage when render multiple objects.

In most cases, we only have to do with Vertex shader and Fragment shader,Geometry shader offen set to default.

In modern OpenGL, wo are required to define at least vertex and fragment shader.

Coordinates

  • Normalized Device Coordinates
  • view port transform(glViewport)
  • Screen space cooordinates

Vertex Buffer Object(VBO) as input

  1. create buffer glgenBuffers(num, store address) **
  2. bind buffer glBindBuffer(type or target, source address)
  3. copy data to buffer glBufferData(type, size, source, data_change_type)

Ceate vertex shader

  1. create vertex shader source code using glsl
  2. create vertex shader glCreateShader(GL_VERTEX_SHADER)
  3. set source code of shader glShaderSource()
  4. compile shader source glCompileShader()
  5. check compile status glgetShaderiv(), glGetShaderInfoLog

Create fragment shader

  1. create vertex shader source code using glsl
  2. create vertex shader glCreateShader(GL_FRAGMENT_SHADER)
  3. set source code of shader glShaderSource()
  4. compile shader source glCompileShader()
  5. check compile status glgetShaderiv(), glGetShaderInfoLog

Shader program

A shader program is the final linked version of multiple shaders, like excutables linked with mutiple libraries.

  1. create shader program glCreateProgram
  2. attach glAttachShader(program, shader), order is important
  3. link glLinkProgram
  4. check link errors glGetProgramiv(), glGetProgramInfoLog()
  5. activate glUseProgram

You can delete shaders after link

Link vertex attributes

  1. how OpenGL should interpret data glVertexAttribPointer
  2. glEnableVertexAttribArray

Vertex Array Object(VAO)

  1. create a vao glGenVertexArrays
  2. bind vao glBindVertexArray
  3. create and configure vbo as we said
  4. 'unbind vao' glBindVertexArray(0)
  5. when use,re-bind vao

Element Buffer Object(EBO)

  1. create ebo glGenBuffers()
  2. bind buffer glBindBuffer
  3. copy indices glBufferData
  4. draw use glDrawElements instead of glDrawArray