CS 40: Week2-OpenGL

A couple of minor edits to the qtogl files from last week.

[~]$ cd ~/cs40/examples/w01-intro/qtogl
[qtogl]$ cp ~adanner/public/cs40/examples/w01-intro/qtogl/mypanel* ./
[qtogl]$ cp ~adanner/public/cs40/examples/w01-intro/qtogl/fshader.glsl ./
[qtogl]$ git commit -am "Use QOpenGL classes"
[qtogl]$ git push

[qtogl]$ cd ~/cs40/examples/build/w01-intro/qtogl
[qtogl]$ make -j8
[qtogl]$ ./qtogl

Remember, you could open this in qtcreator too and build the project there. Remember to select qtogl as the target after opening the ~/cs40/examples/CMakeLists.txt project.

QT and OpenGL

The class QGLWidget creates an OpenGL context inside QT. OpenGL functions are called within this context. Additionally, QT has some OpenGL wrapper classes beginning with the prefix QGL.

Creating a QT OpenGL application

Create a new class, e.g., MyPanelOpenGL which inherits from QGLWidget
Add a widget in the UI Designer and a promote it to MyPanelOpenGL
Implement the methods initializeGL(), paintGL() and resizeGL ( int width, int height ) in your MyPanelOpenGL class.
Add additional methods, slots, signals, widgets as needed.

~/cs40/examples/w01-intro/qtogl
MyPanelOpenGL inherits from QGLWidget; provides OpenGL context in QT
clip coordinates 2x2x2 box
geomety -> vertex shader -> clip -> rasterize -> fragment shader -> framebuffer
framebuffer squished into viewport
Copying CPU data to GPU memory using Vertex Buffer Objects (VBOs)

We store the vertices of a triangle in a VBO in GPU memory. The next step is to define, create, load, and compile shaders. The vertex shader runs first and takes vertex data from the VBO and outputs geometry in clip coordinates. This geometry is then clipped, rasterized, and fed to the fragment shader which runs on each fragmenet, or potential output pixel. The output of the fragment shader is written to a framebuffer and displayed in the viewport. Once each shader is compiled, we define, create, and link a shader program, which is the combination of a vertex shader and a fragment shader. Finally, we are ready to draw. The main steps are:

clear the display
bind buffers, programs
connect shader parameters to data
glDraw...
glFlush
repeat as needed using updateGL()

Once the geometric data are copied to GPU memory, almost everything else happens on the GPU. paintGL is just issuing commands to the GPU. The GPU itself will process those commands.

Shaders, GLSL

keywords: in, out, uniform
types: vec3, mat4
variables: gl_Position;

Modifying Shaders

Making fragColor a uniform value
modifying geometry
uniform time variable
QTimer
updateGL -> Animation!

Geometry intro

Before we dive into drawing, manipulating, and lighting shapes, we need to know a little bit more about how to represent geometric objects in OpenGL.

Point, Vector, Line, Ray, Halfplane, Half-Orc
basis and frame

dot product
cross product
Matrices
Matrix/Vector math
w03-geomtry/vector3d.h

CS40 Intro

QT and OpenGL

Creating a QT OpenGL application

Shaders, GLSL

Modifying Shaders