在片段着色器中，为什么我不能使用平面输入整数来索引 sampler2D 的统一数组? [英] In a fragment shader, why can't I use a flat input integer to index a uniform array of sampler2D?

问题描述

我想指定渲染精灵数组时要使用的纹理.所以我在它们的顶点数据中放置了一个纹理索引，并将它作为一个平面值从我的顶点着色器传递到我的片段着色器，但不能像预期的那样使用它来索引采样器数组，因为编译器认为它是非-常数".相反，我不得不求助于下面令人作呕的代码.谁能解释一下这里发生了什么?

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;
    };
} 

推荐答案

[...] 但不能按预期使用它来索引采样器数组，因为编译器将其视为非常量"；[...]

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

在 3.30 版本之前的 GLSL 和 3.00 版本之前的 GLSL ES 中，纹理采样器数组的索引必须是一个常量表达式:

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 规范 - 4.1.7 采样器(第 21 页))
GLSL ES 3.00 规范 - 4.1.7.1 采样器(第 29 页)):

在着色器中聚合到数组中的采样器(使用方括号 [ ])只能使用整数常量表达式 [...] 进行索引

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

---

在更高版本中，采样器数组的索引必须是动态统一的".这意味着索引必须相同".对于所有片段(例如常量或统一变量).

---

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 规范 - 4.1.11.不透明类型(第 31 页)
GLSL ES 3.20 规范 - 4.1.11.不透明类型(第 32 页)

当在着色器中聚合到数组中时，不透明类型只能使用动态统一的整数表达式进行索引.[...]

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

[...] 采样器类型(例如 sampler2D)是不透明类型 [...]

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

GLSL 4.60 规范 - 3.8.2.动态统一表达式(第 20 页)
GLSL ES 3.20 规范 - 3.9.3.动态统一表达式(第 22 页)

如果所有评估它的片段得到相同的结果值，则片段着色器表达式是动态统一的.

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

---

flat 片段着色器输入对于单个图元是不变的，但对于整个网格不是不变的，对于由单个绘制调用"处理的所有图元也不是.分别对于调用组来说是不同的.(flat) 片段着色器输入不是 动态统一.

---

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.

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