OpenGL pass integer array with GL_ARRAY_BUFFER - opengl

Question

I am trying to pass some integer values to the Vertex Shader along with the vertex data.
I generate a buffer while vertex array is bound and then try to attach it to a location but it seems like in vertex shader the value is always 0.
here is part of the code that generates the buffer and it`s usage in the shader.
glm::vec3 materialStuff = glm::vec3(31, 32, 33);
glGenBuffers(1, &materialBufferIndex);
glBindBuffer(GL_ARRAY_BUFFER, materialBufferIndex);
glBufferData(GL_ARRAY_BUFFER, sizeof(glm::vec3), &materialStuff, GL_STATIC_DRAW);
glEnableVertexAttribArray(9);
glVertexAttribIPointer(9, 3, GL_INT, sizeof(glm::vec3), (void*)0);
And here is part of the shader that suppose to receive the integer values
// Some other locations
layout (location = 0) in vec3 vertex_position;
layout (location = 1) in vec2 vertex_texcoord;
layout (location = 2) in vec3 vertex_normal;
layout (location = 3) in vec3 vertex_tangent;
layout (location = 4) in vec3 vertex_bitangent;
layout (location = 5) in mat4 vertex_modelMatrix;
// layout (location = 6) in_use...
// layout (location = 7) in_use...
// layout (location = 8) in_use...
// The location I am attaching my integer buffer to
layout (location = 9) in ivec3 vertex_material;
// I also tried with these variations
//layout (location = 9) in int vertex_material[3];
//layout (location = 9) in int[3] vertex_material;
// and then in vertex shader I try to retrieve the int value by doing something like this
diffuseTextureInd = vertex_material[0];
That diffuseTextureInd should go to fragment shader through
out flat int diffuseTextureInd;
And I am planning to use this to index into an array of bindless textures that I already have set up and working. The issue is that it seems like vertex_material just contains 0s since my fragment shader always displays the 0th texture in the array.
Note: I know that my fragment shader is fine since if I do
diffuseTextureInd = 31;
in the vertex shader, the fragment shader correctly receives the correct index and displays the correct texture. But when I try to use the value from the layout location 9, it seems like I always get a 0. Any idea what I am doing wrong here?

Answer 1

The following definitions:
glm::vec3 materialStuff = glm::vec3(31, 32, 33);
glVertexAttribIPointer(9, 3, GL_INT, sizeof(glm::vec3), (void*)0);
...
layout (location = 9) in ivec3 vertex_material;
practically mean that:
glm::vec3 means that you declare vector of 3 floats rather than integers. glm::ivec3 should be used for vector of integer.
ivec3 vertex attribute means a vector of 3 integer values is expected for each vertex. At the same moment, materialStuff defines values only for a single vertex (makes no sense for a triangle, which would require at least 3 glm::ivec3).
What is supposed to be declared for passing a single integer vertex attribute:
layout (location = 9) in int vertex_material;
(without any array qualifier)
GLint materialStuff[3] = { 31, 32, 33 };
glVertexAttribIPointer(9, 1, GL_INT, sizeof(GLint)*3, (void*)0);
It should be noticed though, that passing different per-vertex integer to fragment shader makes no sense, which I suppose you solved by flat keyword. Existing pipeline defines only per-vertex inputs, not per-triangle or something like this. There are glVertexAttribDivisor() defining the vertex attribute rate, but it is applicable only to rendering instances via glDrawArraysInstanced()/glDrawElementsInstanced() (specific vertex attribute might be increment per instance), not triangles.
There are ways to handle per-triangle inputs - this could be done by defining Uniform Buffer Object or Texture Buffer Object (same as 1D texture but for accessing by index without interpolation) instead of generic Vertex Attribute. But tricks will be still necessary to determine the triangle index in this array - again, from vertex attribute or from built-in variables like gl_VertexID in Vertex Shader, gl_PrimitiveIDIn in Geometry Shader or gl_PrimitiveID in Fragment Shader (I cannot say, though, how these counters are affected by culling).