Ios – Transpose a mat4 in OpenGL ES 2.0 GLSL

glsliosipadopengl-esopengl-es-2.0

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?

Best Answer

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;
}

Related Solutions

Provide an integer for an array index in OpenGL ES 2.0

Specify the attribute as a float. You can use floats as indices into arrays.

Javascript – WebGL/GLSL – How does a ShaderToy work

ShaderToy is a tool for writing pixel shaders.

What are pixel shaders?

If you render a full screen quad, meaning that each of four points is placed in one of the four corners of the viewport, then the fragment shader for that quad is called pixel shader, because you could say that now each fragment corresponds to exactly one pixel of the screen. So a pixel shader is a fragment shader for a fullscreen quad.

So attributes are always the same and so is a vertex shader:

positions = [ [-1,1], [1,1], [-1,-1], [1,-1] ]
uv = [ [0.0, 0.0], [1.0, 0.0], [0.0, 1.0], [1.0, 1.0] ]

And that quad is rendered as TRIANGLE_STRIP. Also, instead of setting UVs explicitly, some prefer to use fragment shader's built-in variable gl_FragCoord, which is then divided with, for example, a uniform vec2 uScreenResolution.

Vertex shader:

attribute vec2 aPos;
attribute vec2 aUV;
varying vec2 vUV;

void main() {
    gl_Position = vec4(aPos, 0.0, 1.0);
    vUV = aUV;
}

And fragment shader would then look something like this:

uniform vec2 uScreenResolution;
varying vec2 vUV;

void main() {
    // vUV is equal to gl_FragCoord/uScreenResolution
    // do some pixel shader related work
    gl_FragColor = vec3(someColor);
}

ShaderToy can supply you with a few uniforms on default, iResolution (aka uScreenResolution), iGlobalTime, iMouse,... which you can use in your pixel shader.

For coding geometry directly into the fragment shader (aka pixel shader), developer use something called ray-tracing. That is quite complex area of programming but in short: You present your geometry through some mathematical formulas, and later in pixel shader, when you wish to check if some pixel is a part of your geometry you use that formula to retrieve that information. Google-ing a bit should give you plenty of resources to read from what and how ray tracers are built exactly, and this might help: How to do ray tracing in modern OpenGL?

Hope this helps.

Best Answer

Related Solutions

Provide an integer for an array index in OpenGL ES 2.0

Javascript – WebGL/GLSL – How does a ShaderToy work

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