Shaders and GLSL

Shaders

What is a Shader

Types of Shaders

Fragment Shaders

Takes a position and outputs a color

graph LR
	position --> shader
	textures --> shader
	shader --> color

More or less

Vertex Shader

??? #inprogress

Responsible for rendering shapes.

GLSL

OpenGLs Shader Language

Single main function that returns void

Returns through some “special global variables” such as gl_FragColor

No automatic casting

Example

#ifdef GL_ES
precision mediump float;
#endif

uniform float u_time;

void main() {
	gl_FragColor = vec4(0.309,0.962,1.000,1.000);
}

Uniforms

https://thebookofshaders.com/03/

What is a uniform? uniforms are the way data is exchanged between the CPU and GPU, this indicates it is UNIFORM across ALL pixels being processed by the GPU, the types supported are:

float, vec2, vec3, vec4, mat2, mat3, mat4, sampler2D and samplerCube

Uniforms are defined with the corresponding type at the top of the shader right after assigning the default floating point precision.

u_ is a common prefix, sometimes they get an i prefix to indicate that they are inputs.

Common Functions

The GPU has hardware accelerated angle, trigonometric and exponential functions. Some of those functions are: sin(), cos(), tan(), asin(), acos(), atan(), pow(), exp(), log(), sqrt(), abs(), sign(), floor(), ceil(), fract(), mod(), min(), max() and clamp().

gl_FragCoord

gl_FragCoord, which holds the screen coordinates of the pixel or screen fragment that the active thread is working on

Normalizing to between 0.0 and 1.0: vec2 st = gl_FragCoord.xy/u_resolution;

(0,0) is bottom left corner

Requirements for Shaders

Cannot know what other threads in the pipeline are doing

Memoryless

Cannot maintain memory past execution

Why?

GPUs execute shaders in parallel, and keep everything busy. This allows for fast parallel operations, has to meet the above requirements