Use matrix types as fragment shader output in OpenGL

Question

In OpenGL, a matrix takes up multiple locations in a shader (one for each column). So when using a matrix in a vertex shader as an input it is necessary to call glVertexAttribPointer() multiple times.

I am wondering if it is possible to do something similar for matrix types as outputs in a fragment shader when rendering to multiple framebuffers. That is, I would have 4 different textures in a frame buffer bound to color attachments 0 through 3, and then I could have a layout(location = 0) out matrix4 out_mat that I could write to with the fragment shader.

Am I correct in assuming that this would work?

You can writhe ach column of the matrix to a different target (plane), of the buffer. out0 = m[0]; out1 = m[1]; ... — Rabbid76
– Rabbid76, Commented Apr 24, 2019 at 20:10
@InformationAether: "In OpenGL, a matrix takes up multiple locations in a shader" Matrices only take more than one location when used as vertex shader inputs. In every other case where they can be used (intra-shader in/out variables, uniforms), they only take up one location. — Nicol Bolas
– Nicol Bolas, Commented Apr 24, 2019 at 22:53

Rabbid76 · Accepted Answer · 2019-04-24 21:29:24Z

4

For the output of a fragment shader, a matrix data type is not allowed :

See OpenGL Shading Language 4.60 Specification - 4.3.6. Output Variables, page 54:

Fragment outputs output per-fragment data and are declared using the out storage qualifier. It is a compile-time error to use auxiliary storage qualifiers or interpolation qualifiers in a fragment shader output declaration. It is a compile-time error to declare a fragment shader output with, or that contains, any of the following types:

A boolean type

A double-precision scalar or vector (double, dvec2, dvec3, dvec4)

An opaque type

A matrix type

A structure

But the output of a fragment shader is allowed to be an array:

e.g.

layout (location = 0) out vec4 fragOut[4];

void main()
{
    mat4 m = [...];

    fragOut[0] = m[0];
    fragOut[1] = m[1];
    [...]
}

The matrix can be assigned to the array by an array constructor (see 4.1.13. Arrays, page 36):

layout (location = 0) out vec4 fragOut[4];

void main()
{
    mat4 m = [...];

    fragOut = vec4[4](m[0], m[1], m[2], m[3]);
}

edited Apr 24, 2019 at 21:29

answered Apr 24, 2019 at 20:53

Rabbid76

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2 Comments

Yakov Galka Over a year ago

Do you know if arrays of vectors are allowed though? Couldn't find it in the spec.

Rabbid76 Over a year ago

@ybungalobill It is not expressly forbidden. In general an output variable can be an array. I just tried it and it works.

Community · Accepted Answer · 2020-06-20 09:12:55Z

2

GLSL spec says (Version 4.5, 4.3.6 Output Variables):

It is a compile-time error to declare a fragment shader output that contains any of the following:

...

Any matrix type

Therefore

layout(location = 0) out mat4 out_mat;

is illegal.

Instead you should output the elements of the matrix into four separate outputs:

 layout(location = 0) out vec4 out_mat0;
 layout(location = 1) out vec4 out_mat1;
 layout(location = 2) out vec4 out_mat2;
 layout(location = 3) out vec4 out_mat3;

 void main() {
     mat4 M = ...;
     out_mat0 = M[0]; // column 0
     out_mat1 = M[1]; // column 1
     out_mat2 = M[2]; // column 2
     out_mat3 = M[3]; // column 3
 }

edited Jun 20, 2020 at 9:12

CommunityBot

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answered Apr 24, 2019 at 20:53

Yakov Galka

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