结构的 glBufferSubData 偏移量

Question

我实际上正在使用 OpenGL 3.3 渲染引擎，我正试图在我的场景中创建动态数量的灯光。

为此，我正在使用统一缓冲区对象 (UBO)，当我尝试传递数据时遇到问题，而 UBO 将在具有不同类型数据的结构中读取或写入。

我为点光源和定向光源做了这件事，一切都很好，因为我只使用了 vec3 数据。问题是当我定义焦点灯时，它的结构是：

#version 330 core

#define MAX_NUM_TOTAL_LIGHTS 100
...
struct FocalLight{
    vec3 f_light_position;
    vec3 f_light_direction;
    vec3 f_light_diffuse_intensity;
    vec3 f_light_specular_intensity;
    float f_apperture_angle;
    float f_attenuation;
};
layout(std140) uniform focalLights{
    FocalLight f_lights[MAX_NUM_TOTAL_LIGHTS];
};

好吧，位置、方向、漫反射强度和镜面反射强度都很好，我的片段从缓冲区正确接收了这些数据。但我无法写入和读取 f_apperture_angle 和 f_attenuation 的数据。

这是在 CPU 上执行的代码，我用它来写入缓冲区数据，其中 focal_lights 是一个包含 FocalLight class 实例的向量（std::vector<FocalLight> focal_lights) 我检查的内容是正确的：

if(block_focal_lights_id != -1) {
    glUniformBlockBinding(programId, block_focal_lights_id, 2);
    //Loading from light vectors
    glGenBuffers(1, &buffer_focal_lights_id);
    glBindBuffer(GL_UNIFORM_BUFFER, buffer_focal_lights_id);
    glBufferData(GL_UNIFORM_BUFFER, sizeof(float) * 24 * focal_lights.size(), 0, GL_DYNAMIC_DRAW);
    int offset = 0;
    for (unsigned int i=0; i<focal_lights.size(); i++) {
        glBufferSubData(GL_UNIFORM_BUFFER, offset, sizeof(float) * 3, focal_lights[i].position);
        offset += 16;
        glBufferSubData(GL_UNIFORM_BUFFER, offset, sizeof(float) * 3, focal_lights[i].direction);
        offset += 16;
        glBufferSubData(GL_UNIFORM_BUFFER, offset, sizeof(float) * 3, focal_lights[i].diffuse_intensity);
        offset += 16;
        glBufferSubData(GL_UNIFORM_BUFFER, offset, sizeof(float) * 3, focal_lights[i].specular_intensity);
        offset += 16;
        glBufferSubData(GL_UNIFORM_BUFFER, offset, sizeof(float), &focal_lights[i].apperture_angle);
        offset += 16;
        glBufferSubData(GL_UNIFORM_BUFFER, offset, sizeof(float), &focal_lights[i].attenuation);
        offset += 16;
    }
}

我尝试将 f_apperture_angle 的数据类型更改为 vec3，我可以使用之前定义的偏移量读取它，但与使用简单的浮点数无关。我确定缓冲区的绑定是正确的，我知道问题出在 glBufferData 或 glBufferSubdata 代码上。

有人看到问题了吗？

终于成功了，感谢 Rabbid76： 2 focal lights, 2 directional lights and one point light

Answer 1

在将数据绑定到 std140 标准统一块布局时，您必须考虑特殊的对齐规则。

见OpenGL 4.6 API Compatibility Profile Specification; 7.6.2.2 Standard Uniform Block Layout; page 144

When the std140 layout is specified, the offset of each uniform in a uniform block can be derived from the definition of the uniform block by applying the set of rules described below.

If the member is a scalar consuming N basic machine units, the base alignment is N

....

If the member is a three-component vector with components consuming N basic machine units, the base alignment is 4N.

....

If the member is a structure, the base alignment of the structure is N, where N is the largest base alignment value of any of its members, and rounded up to the base alignment of a vec4. The individual members of this substructure are then assigned offsets by applying this set of rules recursively, where the base offset of the first member of the sub-structure is equal to the aligned offset of the structure. The structure may have padding at the end; the base offset of the member following the sub-structure is rounded up to the next multiple of the base alignment of the structure.

If the member is an array of S structures, the S elements of the array are laid out in order, according to rule (9).

当您将此规则应用于您的数据结构时，会产生以下偏移量：

struct FocalLight                    // size 80 (rule 9 and 10)
{
    vec3 f_light_position;           // offset 0  (rule 3 and 10)
    vec3 f_light_direction;          // offset 16 (rule 3)
    vec3 f_light_diffuse_intensity;  // offset 32 (rule 3)
    vec3 f_light_specular_intensity; // offset 48 (rule 3)
    float f_apperture_angle;         // offset 60 (rule 1)
    float f_attenuation;             // offset 64 (rule 1)
};
layout(std140) uniform focalLights{
    FocalLight f_lights[MAX_NUM_TOTAL_LIGHTS];
};

绑定数据：

int offset = 0;
for (unsigned int i=0; i<focal_lights.size(); i++) {

    glBufferSubData(GL_UNIFORM_BUFFER, offset, sizeof(float) * 3, focal_lights[i].position);

    offset += 16; // rule 3
    glBufferSubData(GL_UNIFORM_BUFFER, offset, sizeof(float) * 3, focal_lights[i].direction);

    offset += 16; // rule 3
    glBufferSubData(GL_UNIFORM_BUFFER, offset, sizeof(float) * 3, focal_lights[i].diffuse_intensity);

    offset += 16; // rule 3
    glBufferSubData(GL_UNIFORM_BUFFER, offset, sizeof(float) * 3, focal_lights[i].specular_intensity);

    offset += 12; // rule 1
    glBufferSubData(GL_UNIFORM_BUFFER, offset, sizeof(float), &focal_lights[i].apperture_angle);

    offset += 4; // rule 1
    glBufferSubData(GL_UNIFORM_BUFFER, offset, sizeof(float), &focal_lights[i].attenuation);

    offset += 16; // rules 9 and 10
}

Answer 2

只需调用一次缓冲区数据即可。

glBufferSubData(GL_UNIFORM_BUFFER, 0, sizeof(FocalLight)*focal_lights.size(), focal_lights);

使用多个小缓冲区数据副本会阻塞 API 和驱动程序，导致性能非常差。

要处理 std140（或最好是 std430）的对齐，您只需要在 C++ 端的结构中添加填充或重新排序成员。

struct FocalLight{
  vec3 f_light_position;
  float f_apperture_angle;
  vec3 f_light_direction;
  float f_attenuation;
  vec3 f_light_diffuse_intensity;
  float pad1;
  vec3 f_light_specular_intensity;
  float pad2;
};

或者，您可以使用编译器内置指令进行对齐 __declspec(align(16))

结构的 glBufferSubData 偏移量

glBufferSubData offsets for structs

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opengl

buffer

glsl