OpenGL ES Shading Language

Coming from an OpenGL 1.5 + Nvidia Cg environment there are some noticable differences when migrating over to OpenGL ES 2.0. This post is like a cheat sheet for myself and others to use as a quick reference to look up some major keypoints of the Opengl ES Shading Language.

Precision & range of variables

The OpenGL ES Shading Language, used in e.g. IPhone OpenGL ES 2.0, lets you decide upon the range and precision used to represent integer or floating point variables.

Precision qualifiers:

• highp (e.g. highp mat4 myMatrix)
• mediump (e.g. varying mediump vec2 myTexCoord)
• lowp (e.g. lowp float myColor)

Minimum ranges and precision

	Bits	FP Range	FP Precision	Int Range
highp	16	(-262, 262)	Relative 2-16	(-216, 216)
mediump	10	(-214, 214)	Relative 2-10	(-210, 210)
lowp	8	(-2, 2)	Absolute 2-8	(-28, 28)

default vertex shader precisions

• precision highp float
• precision highp int
• precision lowp sampler2D
• precision lowp samplerCube

default FRAGMENT shader precisions

• precision mediump int
• precision lowp sampler2D
• precision lowp samplerCube

Notice the fragment shader is missing a default precision for float. This means you must include precision qualifier for floats.

Built-in Variables

Vertex shader

	Type	Precision
gl_Position	vec4	highp
gl_PointSize	float	mediump

Fragment shader

	Type	Precision
gl_FragCoord	vec4	mediump
gl_FrontFacing	bool
gl_FragColor	vec4	mediump
gl_FragData[gl_MaxDrawBuffers]	vec4	mediump
gl_PointCoord	vec2	mediump