You can think GLSL as a C language for GPU programming except there are no pointer, union, bitwise operations, and function variables. varying variable is read-only in fragment shader and read/write in vertex shader.

varying modifier indicates that a variable is delivered from a vertex shader to a fragment shader.

attribute modifier indicates that the value of a variable changed for each vertex like position and normal that is in vertex shader only.

uniform modifier indicates that the value of a variable is a constant assigned by application.

Three modifiers are able to be applied on variables: To access an element of matrix you can use m or zero-based row-column position like _m00.

declares a float matrix type with 2x2, 3x3, and 4x4 rows and columns. If, else, do, for, while, break, continue, return, discardįloat, int, bool, true, false, void, lowp, mediump, highp, precision, invariant This instruction below is pay more attention on OpenGL ES 2.0 and plus because of it has been widely used in mobile applications. GLSL is widely used at the effect programming of games on ARM architecture like iOS, Android, as well as Linux platforms. To build a complete shader, GLSL must work along with a host programming language such as C/C++ along with OpenGL library. These high-level languages accelerated the shader development which plays a key role in game development.

OpenGL 1.5 to 4.0 and OpenGL ES started to include GLSL as a standard component. Late, Microsoft's High-Level Shading Languages (HLSL) and OpenGL's OpenGL Shading Language (GLSL) shading languages have been developed for real-time shaders on GPU. Long time ago, Apple's RenderMan was a popular shading language that was used to generate cinematic effects with CPU in render farms. OpenGL Shading Language (GLSL), a programming language for Graphic Processing Unit (GPU) in OpenGL platform, supports shader construction with C-like syntax, types, expressions, statements, and functions. By Chang Li, Draft Version 0.5, March 20, 2013

Glsl attribute opengl es 2.0