Incorrect Vulkan UniformBuffer Memory Alignment

Question

As per the specification Variables declared inside a Uniform Buffer must be properly aligned.

I have the following GLM variables in my structure:

struct UniformBufferObject_PointLights {
    glm::f32 constant[64]{};
    glm::f32 linear[64]{};
    glm::f32 quadratic[64]{};

    glm::vec3 position[64]{};

    glm::vec3 ambient[64]{};
    glm::vec3 diffuse[64]{};

    glm::int32 count{};
};

• Attempting to access any of the variables from within the shader acts as if their values are all 0. The issue is centered around the glm::f32 and glm::uint32 declarations.

The glm::vec3's are all accessible by simply declaring them above the glm::f32's and glm::uint32, however, the glm::uint32 and glm::f32's are still inaccessible. At this point I figure it must be an alignment issue.

//  After rearrangement.
struct UniformBufferObject_PointLights {
    glm::vec3 position[64]{};

    glm::vec3 ambient[64]{};
    glm::vec3 diffuse[64]{};

    glm::f32 constant[64]{};
    glm::f32 linear[64]{};
    glm::f32 quadratic[64]{};

    glm::uint32 count{};
};

• position,ambient, and diffuse are all accessible after moving them to the top of the struct.

I have set #define GLM_FORCE_DEFAULT_ALIGNED_GENTYPES but it doesn't appear to work for glm::f32 and glm::uint32 and probably others. What do I need to do to get these variables working in my uniform buffer? I've tried placing alignas(4),alignas(8),alignas(16), and alignas(32) before their declarations but no combination works.

Answer 1

Arrays in uniform buffers must be aligned as per std140 which basically states vec4 alignment.

That means your uniform buffer is too small. As for the shader the float foo[64] is actually the same size as vec4 foo[64]. alignas qualifier won't allow you to change that.

Either use storage buffers, which could be slower, or just use vec4's in arrays.