GLSL uniform block with an array of vec3

Question

I'm trying to send a model-view matrix to the vertex shader in the form of an array. I have tried three methods of doing this; an array of floats, an array of vec4-s and an array of vec3-s. The shader is shown below (the model-view matrix is named "locate").

layout (shared) uniform uView
{
    float locate[12];   // <-- METHOD 1
    //vec4 locate[4];   // <-- METHOD 2
    //vec3 locate[4];   // <-- METHOD 3
    mat4 project;
};

in vec3 aPosition;
in vec4 aColour;

out vec4 fColour;

void main()
{
    gl_Position = project*vec4(     // <-- METHOD 1
        locate[ 0]*aPosition.x + locate[ 3]*aPosition.y + locate[ 6]*aPosition.z + locate[ 9],
        locate[ 1]*aPosition.x + locate[ 4]*aPosition.y + locate[ 7]*aPosition.z + locate[10],
        locate[ 2]*aPosition.x + locate[ 5]*aPosition.y + locate[ 8]*aPosition.z + locate[11],
        1.0);
    //gl_Position = project*vec4(   // <-- METHODS 2 & 3
    //    locate[0].xyz*aPosition[0] +
    //    locate[1].xyz*aPosition[1] +
    //    locate[2].xyz*aPosition[2] +
    //    locate[3].xyz,
    //    1.0);

    fColour = aColour;
}

Methods 1 and 3 only send the top three rows of the matrix; 12 floats, or 4 vec3-s. The row which generates the homogeneous coordinate just has a one on the diagonal, so it can be omitted. Method 2 sends the last row, but doesn't use it.

Methods 1 and 2 work. Method 3 doesn't. I can't figure out why. I don't get any shader compile errors. It just doesn't draw anything. I am using PyOpenGL. Is there some padding issue with arrays of vec3-s that I am not aware of?

Side note... I appreciate this is a bit of a silly example when I could just send the model-view matrix as a mat4. This is a minimal 3-dimensional example of an N-dimensional shader I am trying to implement. In N-dimensions, the aPosition variable is an array of N floats, and the model-view matrix can be a (N+1)x3 matrix. I therefore want to send the model-view matrix as an array of N+1 vec3-s.

Nicol Bolas Nicol Bolas · Accepted Answer · 2016-02-20T15:32:31

vec3 is a weird thing in GLSL, particular in buffer-backed block layouts. It always takes up just as much room as a vec4. Always. Whether it's just a vec3 someName or a vec3 someArrray[30], each vec3 may as well be a vec4 as far as layout is concerned.

You will save no memory by using a vec3 here. So you may as well make the padding explicit and use a vec4.

GLSL uniform block with an array of vec3

1 Answers