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

IPhone Development Reference Post

	3G	3GS	4	IPad
Resolution	480x320	480x320	960x640	1024x768
Ratio (*)	3:2 (1.5)	3:2 (1.5)	3:2 (1.5)	4:3 (1.33)
OpenGL ES	1.1	2.0	2.0	2.0

(*) The bigger the ratio the more “widescreen” the display is. Eg: 16:9 = 1.78, 16:10 = 1.6

Doom Classic Iphone Source Code

The Doom source code for IPhone can be downloaded from this site http://www.idsoftware.com/doom-classic/.

At the bottom of that page there is a broken link to the actual zip file: ftp://www.idsoftware.com/idstuff/doom/doomclassic_iphone_v1.0_source.zip

I could not find a mirror site which hosted this file, but after a closer look at the link I was able to find the source at the correct link here:
ftp://ftp.idsoftware.com/idstuff/doom/doomclassic_iphone_v1.0_source.zip

From Visual Studio to XCode – Lessons Learned #1

In Visual Studio I use HOME/END to jump back and forth on rows, extensively. Using those keys in XCode results in the cursor jumping to the beginning/end of the document instead of the current row. There is a simple way to fix this:

Open:

XCode->Preferences->Key Bindings->Text Key Bindings

1. First, duplicate “Xcode Default” Key Binding Set, by clicking on Duplicate…
2. Name your new Key Binding Set
3. Assign “Home” to “Move to Beginning of Line”
4. Assign “End” to “Move to End of Line”
5. Remove the previous bindings for Home/End from “Scroll to Beginning of Document/Scroll to End of Document” (if necessary. Xcode might remove these automatically).