A shader is a piece of code that runs in the GPU. Fragment shaders are used to render surfaces in video games and to do post-processing (applying filters, lens-distortion, etc.). Vertex shaders manipulate 3D and 2D objects in order to place them in the screen, before handling the result to fragment shaders.

Shadergif is mostly about Fragment shaders, because it has a vertex shader running in the backend with generates a square which fills the screen. This allows shader programmers to use various 2D and 3D techniques. See some techniques here.

There are many shader languages (GLSL, HLSL, etc.). Shadergif works with GLSL. GLSL is a c-like language with bonus types like `vec2`, `mat2` and others. It also comes with many functions that do not come directly with C, such as math functions like `sin`, `cos`, `atan`, `length` (to measure the length of a vector), `distance` (measure a distance between 2 vector coordinates).

To get started with the GLSL language :

You will enjoy writing shaders if you have had a linear algebra class or if you have at least some vector math background.

• Here is an example of a really quick introduction: (http://blog.wolfire.com/2009/07/linear-algebra-for-game-developers-part-1/)
• Here is a longer one: [http://immersivemath.com/ila/index.html)

When running the Shadergif editor, you will have a canvas on the left which constantly renders the code you type at the right. Don't hesitate to click `examples` in the top bar.

Here is a commented version of the circle example:

``````// Just always put that line to avoid warnings and errors
precision highp float;

// This will give you information on the current pixel you are rendering
varying vec2 UV;
// Screen ration (1 if you are using the default 540x540 size)
uniform float ratio;

// Main code which renders a circle.
// This function is called for every pixel in the screen
// by the GPU.
// You will get different values of UV.x and UV.y.
// The final color will be set in gl_FragColor,
// which the GPU will put on the screen.
//
// To draw a circle, you will have to find the distance
// of the current point (pixel) to the center of the screen.
// If it is smaller than your radius (0.3 here), you can set
// a certain color.
void main(void){
// At first, we set the color to black
vec4 col = vec4(0.0);                                   // vec4(0.0) is equivalent to vec4(0.0, 0.0, 0.0, 0.0)

// Create a variable containing the position of the center of the circle
// (in this case, the middle of the screen)
vec2 center = vec2(0.5,0.5);

// distance is a GLSL function that returns... The distance!
float distance_from_center = distance(pos, center);

// If we are closer to the center than our radius
if(distance_from_center < 0.3){
// Fill the pixel with this color
col.rgba = vec4(0.4, 0.5, 0.6, 1.0);
}

// Set the opacity to 1 so we'll actually see something
col.a = 1.0;

// Pass the pixel color back to OpenGL
gl_FragColor = col;
}
``````