Arrays in uniform blocks cannot be indexed with vertex attributes, right? - opengl

Question

I was wondering if I can index into a uniform buffer array with a value contained in the vertices I draw, like:
layout (location = 0) in vec3 position;
layout (location = 1) in flat int idx;
layout(binding = 0, std140) uniform uniformValues
{
float values[100];
};
void main()
{
values[idx];
}
My understanding is that this is not possible because
'in flat int idx'
Is most likely not a 'dynamically uniform expression', and according to the documentation cannot be used to index into a uniform buffer array:
There are places where GLSL requires the expression to be dynamically
uniform. All of the following must use a dynamically uniform
expression:
-The index to buffer-backed interface block arrays.
However I came across information from the same source regarding how to access an array of samplers holding texture handles for 'bindless textures', and it says (emphasis mine):
Sampler and image types used in default block uniform variables can be
populated from handles rather than the index of a binding point.
These types can also now be passed as Shader Stage inputs/outputs (using the flat interpolation qualifier where needed). They can be
used as Vertex Attributes, where they are treated as 64-bit integers
on the OpenGL side. And they can be used in Interface Blocks of all
kinds; buffer-backed interface blocks treat them as 64-bit integers.
It's saying, I believe, that instead of doing this:
layout (location = 0) in flat int textureBinding;
layout (binding = 0) uniform sampler2D textures[16];
void main()
{
textures[textureBinding];
}
You do this:
layout (location = 0) in flat int bindlessTextureHandle;
layout (binding = 0) uniform textureBuffer
{
sampler2D textures[200];
}
void main()
{
textures[bindlessTextureHandle];
}
'bindlessTextureHandle' isn't a 'dynamically uniform expression', how can it be used to index into uniform buffer?
All of the following must use a dynamically uniformexpression:
-The index to buffer-backed interface block arrays.
So why is it saying that you can index into 'interface blocks' of all kinds with values from vertex inputs?
Also are you allowed to index into:
'uniform sampler2D[16] textures;'
with a 'non-dynamically uniform expression'?

Answer 1

My understanding is that this is not possible because
in flat int idx
Is most likely not a 'dynamically uniform expression', and according
to the documentation cannot be used to index into a uniform buffer
array.
You are right that you need a dynamically uniform value to index into an uniform buffer array. However, this:
layout(binding = 0, std140) uniform uniformValues
{
float values[100];
};
is not a uniform buffer array. That is an array inside a single uniform buffer object, and you can index with non-uniform values into this array as you like. A uniform buffer array would be:
layout(binding = 0, std140) uniform myUBO
{
float value;
} myUBOArray[4];
The rest of your question gets even more obscure. You sometimes reference "The index to buffer-backed interface block arrays", which your code never uses. This is talking about SSBOs, which use interface blocks of the form layout(...) buffer foo {...}.
So why is it saying that you can index into 'interface blocks' of all kinds with values from vertex inputs?
Because that is how it is. You just need to understand that indexing into an array of interface blocks is not the same as indexing some other array (which might or might not be defined inside an interface block, doesn't matter).
Also are you allowed to index into:
uniform sampler2D[16] textures; with a 'non-dynamically uniform expression'?
No, not in standard GL.
The first thing about bindless textures is that this is not a core feature of any OpenGL version released to date (which is 4.6 at the time of writing this).It is only defined as an extension GL_ARB_bindless_textures which some modern GPUs and drivers expose, but the availability of that feature is quite far from being universal.
Second, the extension spec above explains: "Sampler and image handles passed to texture built-in functions must be dynamically uniform", so it still doesn't get you there. However, the extension GL_NV_gpu_shader5 removes that restriction. So on recent NVIDIA GPUs, you can get a non-dymically uniform index into an array of bindless texture samplers - but performance will still suffer by a significant amount if you do so.

Answer 2

There are a myriad of separate, overlapping issues here.
Indexing an array within a uniform block has never been limited to dynamically uniform expressions (generally, see below). Even in GL 3.x, you can index an array within a buffer-backed block with an arbitrary index.
However, you're not asking about a general array; you're asking about arrays of textures. Or to be more general, the entire sequence of operations leading to the computation of a sampler type through bindless textures.
That entire sequence must be dynamically uniform (unless you're on NVIDIA, which allows arbitrary expressions). It doesn't matter if you're indexing an SSBO array, using an input variable to pass a texture handle directly, or anything else. The value that leads to the acquisition of a sampler type must be dynamically uniform.
So why is it saying that you can index into 'interface blocks' of all kinds with values from vertex inputs?
Because you can.
A common misunderstanding of what "dynamically uniform" means is that it is a static property. That an expression by itself is either dynamically uniform or not. This is close to true, but it's not actually true.
Some expressions are dynamically uniform by their nature. You could call these "statically dynamically uniform" expressions. A constant expression is always dynamically uniform, for example.
However, being dynamically uniform is about the value of the expression. All invocations (within the rendering command) must result in the same value. An in variable for a shader stage can be dynamically uniform so long as it just so happens to always have the same value within the rendering command. For example, a VS could access a value from a uniform array using gl_DrawID (which is dynamically uniform), pass that as an in to the FS, and the FS can use that value as a sampler. Or to access an array of samplers. Or whatever. All FS invocations will get the same value within the draw command, so that value is dynamically uniform.