How to write const array in GLSL ES

Question

I am trying to write a simple vertex shader for an OpenGL ES app on the iPhone, but my array constructor is causing me trouble.

attribute vec4 normal;
attribute vec4 position;

void main(void){

    const vec4 vertices[3] = vec4[](vec4(0.25, -0.25, 0.5, 1.0), 
                             vec4(-0.25, -0.25, 0.5, 1.0),
                             vec4(0.25, 0.25, 0.5, 1.0));
    gl_Position = vertices[gl_VertexID];

}

When using this code the shader is unable to compile, and gives me the eror message:

ERROR: 0:13: '(' : syntax error: Array size must appear after variable name

Answer 1

The GLSL version used with ES 2.0 does not support constant arrays. From section "4.3.2 Constant Qualifier" on page 30 of the spec:

Arrays and structures containing arrays may not be declared constant since they cannot be initialized.

This restriction is lifted in ES 3.0, where it says in the corresponding section:

The const qualifier can be used with any of the non-void transparent basic data types as well as structures and arrays of these.

As an alternative, you should be able to use a non-constant array, where you assign the values one by one (untested):

vec4 vertices[3];
vertices[0] = vec4(0.25, -0.25, 0.5, 1.0);
vertices[1] = vec4(-0.25, -0.25, 0.5, 1.0);
vertices[2] = vec4(0.25, 0.25, 0.5, 1.0);

Or you could use a series of if-statements.