c++ - 将无符号整数输入属性传递给顶点着色器

Question

在典型的顶点数组缓冲区构建中，我试图将一个无符号整数属性与其他经典属性(顶点、法线、纹理坐标...)一起传递。然而，此属性的值最终以某种方式出错:我不确定是值错误还是属性未设置。

从一个简单的例子开始，假设我定义了以下 C++ 输入结构:

struct buffer_data_t
{
    glm::vec3 vertex;
    glm::vec3 normal;
    glm::vec2 texCoords;
};

准备我的顶点数组看起来像这样:

// Assume this 'shader.attribute(..)' is working and returns the attribute's position
unsigned int shadInputs[] = {
    (unsigned int)shader.attribute("VS_Vertex"),
    (unsigned int)shader.attribute("VS_Normal"),
    (unsigned int)shader.attribute("VS_TexCoords"),
};

glGenBuffers(1, &glBuffer);
glBindBuffer(GL_ARRAY_BUFFER, glBuffer);
glBufferData(GL_ARRAY_BUFFER, vertice.size() * sizeof(buffer_data_t), &vertice[0], GL_STATIC_DRAW);
glGenVertexArrays(1, &glArray);
glBindVertexArray(glArray);
{
    glBindBuffer(GL_ARRAY_BUFFER, glBuffer);
    glVertexAttribPointer(shadInputs[0], 3, GL_FLOAT, GL_FALSE, sizeof(buffer_data_t), (void*)(sizeof(glm::vec3) * 0));
    glVertexAttribPointer(shadInputs[1], 3, GL_FLOAT, GL_FALSE, sizeof(buffer_data_t), (void*)(sizeof(glm::vec3) * 1));
    glVertexAttribPointer(shadInputs[2], 2, GL_FLOAT, GL_FALSE, sizeof(buffer_data_t), (void*)(sizeof(glm::vec3) * 2));
    glEnableVertexAttribArray(shadInputs[0]);
    glEnableVertexAttribArray(shadInputs[1]);
    glEnableVertexAttribArray(shadInputs[2]);
}
glBindVertexArray(0);

顶点着色器输入将被定义为:

in vec3 VS_Vertex;
in vec3 VS_Normal;
in vec2 VS_TexCoords;

另外，为了我的示例，假设我的片段着色器中有一个纹理采样器，在其上使用那些输入 TexCoords:

uniform sampler2D ColourMap;

介绍我的问题

到目前为止一切顺利，使用上面的代码我可以成功渲染带纹理的图元。现在我想根据渲染的面部选择不同的颜色贴图。为此，我想引入一个 index 作为顶点属性的一部分。变化是:

C++ 数据结构:

struct buffer_data_t
{
    glm::vec3 vertex;
    glm::vec3 normal;
    glm::vec2 texCoords;
    unsigned int textureId; // <---
};

准备顶点数组:

unsigned int shadInputs[] = {
    (unsigned int)shader.attribute("VS_Vertex"),
    (unsigned int)shader.attribute("VS_Normal"),
    (unsigned int)shader.attribute("VS_TexCoords"),
    (unsigned int)shader.attribute("VS_TextureId"),
};

// ...

glBindBuffer(GL_ARRAY_BUFFER, glBuffer);
glVertexAttribPointer(shadInputs[0], 3, GL_FLOAT, GL_FALSE, sizeof(buffer_data_t), (void*)(sizeof(glm::vec3) * 0));
glVertexAttribPointer(shadInputs[1], 3, GL_FLOAT, GL_FALSE, sizeof(buffer_data_t), (void*)(sizeof(glm::vec3) * 1));
glVertexAttribPointer(shadInputs[2], 2, GL_FLOAT, GL_FALSE, sizeof(buffer_data_t), (void*)(sizeof(glm::vec3) * 2));
glVertexAttribPointer(shadInputs[3], 1, GL_UNSIGNED_INT, GL_FALSE, sizeof(buffer_data_t), (void*)(sizeof(glm::vec3) * 2 + sizeof(glm::vec2))); // <---
glEnableVertexAttribArray(shadInputs[0]);
glEnableVertexAttribArray(shadInputs[1]);
glEnableVertexAttribArray(shadInputs[2]);
glEnableVertexAttribArray(shadInputs[3]);

然后顶点着色器定义新的输入，还有一个“平坦的”输出

in vec3 VS_Vertex;
in vec3 VS_Normal;
in vec2 VS_TexCoords;
in unsigned int VS_TextureId;

...

out flat unsigned int FS_TextureId;

调整片段着色器使输入变平(同样为了示例)颜色图现在是一个我们可以从中选择的数组:

...
uniform sampler2D ColourMaps[2];
in flat unsigned int FS_TextureId;
...
texture2D(ColourMaps[FS_TextureId], ... );

这些更改不起作用，特别是因为顶点着色器输入属性“VS_TextureId”。我能够通过不使用 unsigned int 类型而是求助于 vec2(或 vec3，无论哪种方式都有效)来证明这一点(并找到解决方法)。即:

对比:

in vec2 VS_TextureId;
out flat int FS_TextureId;
FS_TextureId = int(VS_TextureId.x);

财政司司长:

in flat int FS_TextureId;
texture2D(ColourMaps[FS_TextureId], ... );

我的假设

我猜这是错误的线路，虽然我不知道如何/为什么:

glVertexAttribPointer(shadInputs[3], 1, GL_UNSIGNED_INT, GL_FALSE, sizeof(buffer_data_t), (void*)(sizeof(glm::vec3) * 2 + sizeof(glm::vec2)));

注意:我检查了“shader.attribute("VS_TextureId")”的结果，它是正确的，这意味着顶点着色器中的属性已明确定义并找到。

你能看出问题出在哪里吗？

Answer 1

如果你想指定属性数组，具有整数数据类型，那么你必须使用glVertexAttribIPointer (关注函数名称中间的 I)，而不是 glVertexAttribPointer。

参见 OpenGL 4.6 API Core Profile Specification; 10.2. CURRENT VERTEX ATTRIBUTE VALUES; page 348

The VertexAttribI* commands specify signed or unsigned fixed-point valuesthat are stored as signed or unsigned integers, respectively. Such values are referred to as pure integers.

...

All other VertexAttrib* commands specify values that are converted directly to the internal floating-point representation.

这意味着顶点属性的规范必须是:

//glVertexAttribPointer(shadInputs[3], 1, GL_UNSIGNED_INT, GL_FALSE, 
//    sizeof(buffer_data_t), (void*)(sizeof(glm::vec3) * 2 + sizeof(glm::vec2))); 
glVertexAttribIPointer(shadInputs[3], 1, GL_UNSIGNED_INT,
    sizeof(buffer_data_t), (void*)(sizeof(glm::vec3) * 2 + sizeof(glm::vec2)));

---

一般来说，您尝试实现的目标并非如此。采样器数组的索引必须是“动态统一的”。这意味着所有片段的索引必须“相同”(例如常量或统一变量)。

参见 GLSL 4.60 Specification - 4.1.7. Opaque Types(page 33)

Texture-combined sampler types are opaque types, declared and behaving as described above for opaque types. When aggregated into arrays within a shader, they can only be indexed with a dynamically uniform integral expression, otherwise results are undefined. [...]

我建议使用单个 TEXTURE_2D_ARRAY 纹理，而不是一组 TEXTURE_2D 纹理。参见 Texture .
在这种情况下，您可以使用 3 维浮点纹理坐标。