In a fragment shader, why can't I use a flat input integer to index a uniform array of sampler2D?

Question

I want to specify textures to be used when I render an array of sprites. So I put a texture index in their vertex data, and pass it as a flat value from my vertex shader to my fragment shader, but can't use it to index an array of samplers as expected because compiler sees it as "non-constant". Instead I have to resort to the disgusting code below. Can anyone explain what is going on here?

const int numTextures = 2;

uniform sampler2D textures[numTextures];

in vec2 uv;
flat in int tex;
out vec4 colour;

void main(void)
{
    // this caused the compiler error 
        /// "sampler arrays indexed with non-constant expressions"
    // colour = texture( textures[ tex ], uv );

    // hence this (ugh) ...
    switch ( tex ) 
    {
        case 0:
            colour = texture( textures[0], uv );
            break;
        case 1:
            colour = texture( textures[1], uv );
            break;
        default:
            colour = vec4( 0.3f, 0.3f, 0.3f, 1.0f );
            break;
    };
}

using the mipmaps is a cool idea. I think for this applcation I will use large texture pages, maintain separate draw lists for each and switch the active texture inbetwween the draw element calls in my render func. — MarkL
– MarkL, Commented Jan 28, 2019 at 16:12

Rabbid76 · Accepted Answer · 2020-12-29 17:57:51Z

8

[...] but can't use it to index an array of samplers as expected because compiler sees it as "non-constant" [...]

In GLSL up to version 3.30 respectively GLSL ES up to version 3.00, the index of an array of texture samplers has to be a constant expression:

GLSL 3.30 Specification - 4.1.7 Samplers (page 21)
GLSL ES 3.00 Specification - 4.1.7.1 Samplers (page 29):

Samplers aggregated into arrays within a shader (using square brackets [ ]) can only be indexed with integral constant expressions [...]

In later version, the index to an array of samplers has to be "dynamically uniform". This means the index has to be the "same" for all fragments (e.g. a constant or a uniform variable).

GLSL 4.60 Specification - 4.1.11. Opaque Types (page 31)
GLSL ES 3.20 Specification - 4.1.11. Opaque Types (page 32)

When aggregated into arrays within a shader, opaque types can only be indexed with a dynamically uniform integral expression. [...]

[...] Sampler types (e.g. sampler2D) are opaque types [...]

GLSL 4.60 Specification - 3.8.2. Dynamically Uniform Expressions (page 20)
GLSL ES 3.20 Specification - 3.9.3. Dynamically Uniform Expressions (page 22)

A fragment-shader expression is dynamically uniform if all fragments evaluating it get the same resulting value.

A flat fragment shader input is invariant for a single primitive, but not for the entire mesh, not for all the primitives which are processed by a single "draw call" respectively it is not the same for an invocation group. A (flat) fragment shader input is not Dynamically uniform.

edited Dec 29, 2020 at 17:57

answered Jan 27, 2019 at 13:09

Rabbid76

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5 Comments

MarkL Over a year ago

thank you for such a detailed replay. I see that in the newer versions of GLSL I can do what I want. I should probably upgrade my laptop which has an un-upgradeable gpu supporting only GLSL 130.

Rabbid76 Over a year ago

@MarkL No, the index of an array of samplers can never depend on a varying (in) variable.

MarkL Over a year ago

yes, that makes sense, but I'm using a flat int, which is invariant.

Rabbid76 Over a year ago

@MarkL No, it is invariant for a single primitive, but not for the entire mesh, not for all the primitives wich are processed by a single "draw call" respectively it is not the same for an invocation group.

MarkL Over a year ago

OK I get it now .. I had assumed that the shaders were executed per primitive. Obvious nonsense now I come to think about it! Thanks for helping me out with this one.

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