simple-glarea-example

Simple example of using GtkGLArea to draw a triangle.

I find existing glarea examples too complex so i decided to write my own. It's just a single main.c file that depends only on GTK+ and GTK+ dependencies. You has to ensure that you have gtk-3-dev installed.
Then you can compile code with:
gcc main.c `pkg-config --cflags gtk+-3.0 --libs epoxy` and run with ./a.out.
It have to work, if it don't, open issue and provide as more info as possible.
I also wrote an short explanation so, if you are interested it is below

!!! NOT A PRODUCTION CODE !!!
In this example I do stuff which is unacceptable in real code. I don't check errors after openGL, I write whole code in one file and I store shaders source in same file as C source. It not good, but I did it for keeping example easy to understand. So read this code, understand it, and try to find some more realistic examples.
They will be more complex but i hope you'll be able to understand them.

How it works

Here im going to explain what this code mean. I'm not going to explain GTK usage and i'm not going deep into openGL workflow you can read it here:

• GTK
• openGL
  note that you already have openGL context, provided by GtkGLArea

Functions

main()

All that main() do in this code is creating GtkApplication, and binding activate function to it

activate()

Here we create a GtkGLArea instance and bind two signals to it

1. realize - it will run once? when window appears to the user
2. render - every time GTK updates the window this function will be runned
   it is default behavior it possible to disable auto-render and call update function manualy

buildShaderProgram()

Here we just compile shaders which source is stored as global variable. It's bad, I know, but this is just an example. After the comilation we link shaders to program and delete them.

on_realize()

here we do some magic. First thing first we call buildShaderProgram() and then we put our vertices in buffers, so openGL can read them. Vertices is just cordinates of points in normalized space. We take all our area put (0;0;0) in center and normalize all points to [-1;1]. So top left corner is (-1;1;Z) top right (1;1;Z) and so on...

render()

This function set openGL context to current GLarea and call draw_triangle() function. You can write function like draw_triangle() (ex. draw_rectangle() :) and call it here. This function exist just for code organisation

draw_triangle()

The actual triangle! First we clear glarea with normalized to [0;1] RGBA color. Then we tell openGL to use our shader program and vertex buffer. And asking openGL to draw binded vertecies with glDrawArrays() and colorize it with binded shader program. After that we can bind nulls to clear context and flush changes to screen

shaders

Aim of shaders is tell us color of pixel based on it position, to keep thing simple we just always return light orange color RGBA(1.0;0.5;0.2;1.0)

Contribution

If you don't understand something, open an issue i will try answer your question and update readme. It would be awesome contrubution because it help make this example easier to understand. If you found typo, i have already open an issue so tell me about it there

Give thanks

If you like this example. Or it helps you fell free to star this repo or contact me in LinkedIn