Transpose a mat4 in OpenGL ES 2.0 GLSL

Question

I'd like to transpose a matrix in my OpenGL ES 2.0 vertex shader, but apparently my iPad 3 doesn't support GLSL #version 120, which is needed for the built-in function transpose(mat4).

I know there are options to work around that, like transposing the matrix on the CPU before passing it to the graphics chip, but it would make my shader a lot simpler if I could transpose it there.

So, is there an option to transpose a mat4 in a shader on an iOS 6 device?

Another thing: The question

What version of GLSL is used in the iPhone(s)?

says that OpenGL ES 2.0 uses GLSL 1.20. So why doesn't #version 120 work on the iPad 3?

Answer 1

Have you tried just transposing it yourself? Is it a performance problem? If not, I would try it because it's something the optimizer should handle and it will take two minutes. Something like:

highp mat4 transpose(in highp mat4 inMatrix) {
    highp vec4 i0 = inMatrix[0];
    highp vec4 i1 = inMatrix[1];
    highp vec4 i2 = inMatrix[2];
    highp vec4 i3 = inMatrix[3];

    highp mat4 outMatrix = mat4(
                 vec4(i0.x, i1.x, i2.x, i3.x),
                 vec4(i0.y, i1.y, i2.y, i3.y),
                 vec4(i0.z, i1.z, i2.z, i3.z),
                 vec4(i0.w, i1.w, i2.w, i3.w)
                 );

    return outMatrix;
}

Answer 2

As the answer to What version of GLSL is used in the iPhone(s)? correctly states, iOS supports OpenGL ES 2.0 with its companion shading language: ESSL 1.0. ESSL 1.0 is based on, but not identical to GLSL 1.20.

There is no built-in transpose function in ESSL 1.0, so you'll need to implement your own.