Nrthugu

Major edit:

I have one shader setting the values of an SSBO:

#version 430



//Input variables

in vec3 f_pos;

in vec3 f_norm;

in vec3 f_uv;



struct Voxel_Node

{

    vec4 color;

    vec4 normal;

    uint children[8];

};

//Buffer for the rest of the tree

layout(std430, binding = 0) buffer tree_buffer

{

    uint t_index;

    uint pad1;

    uint pad2;

    uint pad3;

    Voxel_Node tree;

};



void main()

{

    tree[0].children[7] = 1;

    tree[1].children[0] = 2;

    tree[2].children[7] = 3;

    tree[3].children[0] = 4;

    tree[4].children[7] = 5;



    tree[0].normal = vec4(0,0,1,1);

    tree[1].normal = vec4(0,1,0,1);

    tree[2].normal = vec4(0,1,1,1);

    tree[3].normal = vec4(1,0,0,1);

    tree[4].normal = vec4(1,0,1,1);

    tree[5].normal = vec4(1,1,0,1);

}

And one shader reading from the SSBO:

#version 430

#pragma optimize (off)



in vec2 f_coord;



out vec4 fragment_color;



struct Voxel_Node

{

    vec4 color;

    vec4 normal;

    uint children[8];

};

//Buffer for the rest of the tree

layout(std430, binding = 0) buffer tree_buffer

{

    uint t_index;

    uint pad1;

    uint pad2;

    uint pad3;

    Voxel_Node tree;

};





void main()

{

    fragment_color = vec4(tree[1].children[0]);

    int pls = int(tree[1].children[0]);

    //set 1

    fragment_color = vec4(pls);         //white

    //fragment_color = vec4(pls-1);     //white

    //fragment_color = vec4(pls-2);     //black

    //fragment_color = vec4(pls)/2.f;   //grey



    //Set 2

    fragment_color = tree[pls].normal;      //blue

    //fragment_color = tree[pls+2].normal;  //cyan

    //fragment_color = tree[2].normal;      //cyan

}

There are 2 sets of tests. first, comment out the tree[value].normal block.

So that the fragment color depends on the value of pls. Only one of the 2 blocks may be uncomented at a time and only one line within each block. The values on the right are the resulting color of the test.

The first block of tests is successful all of those are the expected outputs. For the second block of tests,the first output should be blue and the secddn output should not be cyan.

According to these tests the value of pls is 2 when read by the first set of tests but 0 when read by the second set.

There is no other logic than trying to read the contents of the SSBO, what could cause this kind of behaviour?

Edit:

I think it may be due to this but I am not sure:

https://www.khronos.org/opengl/wiki/Memory_Model#Incoherent_memory_access

Update:

Setting the SSBO to be of constant size "solved" the issue, however I'd rather be able to use varying size ssbo's.

coherent layout(std430, binding = 0) buffer tree_buffer

{

    uint t_index;

    uint pad1;

    uint pad2;

    uint pad3;

    Voxel_Node tree[8];

};

Modifying the code to the above results in teh expected behaviour