不了解Learnopengl.com中的漫射辐射教程中的captureViews

问题描述

我正在> https://learnopengl.com/PBR/IBL/Diffuse-辐照度.

本教程通过创建6个视图将等角矩形转换为立方体贴图.

The tutorial convert a equirectangular to a cubemap by creating 6 views.

视图是以下代码:

glm::mat4 captureViews[] =
{
   glm::lookAt(glm::vec3(0.0f, 0.0f, 0.0f), glm::vec3( 1.0f,  0.0f,  0.0f), glm::vec3(0.0f, -1.0f,  0.0f)),
   glm::lookAt(glm::vec3(0.0f, 0.0f, 0.0f), glm::vec3(-1.0f,  0.0f,  0.0f), glm::vec3(0.0f, -1.0f,  0.0f)),
   glm::lookAt(glm::vec3(0.0f, 0.0f, 0.0f), glm::vec3( 0.0f,  1.0f,  0.0f), glm::vec3(0.0f,  0.0f,  1.0f)),
   glm::lookAt(glm::vec3(0.0f, 0.0f, 0.0f), glm::vec3( 0.0f, -1.0f,  0.0f), glm::vec3(0.0f,  0.0f, -1.0f)),
   glm::lookAt(glm::vec3(0.0f, 0.0f, 0.0f), glm::vec3( 0.0f,  0.0f,  1.0f), glm::vec3(0.0f, -1.0f,  0.0f)),
   glm::lookAt(glm::vec3(0.0f, 0.0f, 0.0f), glm::vec3( 0.0f,  0.0f, -1.0f), glm::vec3(0.0f, -1.0f,  0.0f))
};

我不理解glm::lookAt的第三个参数.

I don't understand the third parameter of glm::lookAt.

glm::lookAt的第三个参数是up向量.我认为captureViews应该是:

glm::lookAt's third parameter is the up vector. I think the captureViews should be:

// zero is    [0, 0, 0]
// right is   [1, 0, 0]
// left is    [-1, 0, 0]
// up is      [0, 1, 0]
// down is    [0, -1, 0]
// back is    [0, 0, 1]
// forward is [0, 0, -1]
glm::mat4 captureViews[] =
{
   glm::lookAt(zero, right, up),
   glm::lookAt(zero, left, up),
   glm::lookAt(zero, up, back),
   glm::lookAt(zero, down, forward),
   glm::lookAt(zero, back, up),
   glm::lookAt(zero, forward, up)
};

但是我完全错了.我不了解本教程的up向量中的魔术.

But I totally wrong. I don't understand the magic in the tutorial's up vector.

有人可以帮我解释一下吗?

Can anyone explain it for me?

推荐答案

使用立方体贴图纹理时，必须将3维方向矢量转换为相对于地图一侧的2维纹理坐标.

When a cubemap texture is used, then a 3 dimensional direction vector has to be transformed, to 2 dimensional texture coordinate relative to one side of the map.

此转换规范的相关部分为 OpenGL 4.6 API核心配置文件规范，8.13多维数据集贴图纹理选择，第253页:

The relevant part of the specification for this transformtion is OpenGL 4.6 API Core Profile Specification, 8.13 Cube Map Texture Selection, page 253:

s = 1/2 * (s_c / |m_a| + 1)
t = 1/2 * (t_c / |m_a| + 1)


Major Axis Direction|        Target             |sc |tc |ma |
--------------------+---------------------------+---+---+---+
       +rx          |TEXTURE_CUBE_MAP_POSITIVE_X|−rz|−ry| rx|
       −rx          |TEXTURE_CUBE_MAP_NEGATIVE_X| rz|−ry| rx|
       +ry          |TEXTURE_CUBE_MAP_POSITIVE_Y| rx| rz| ry|
       −ry          |TEXTURE_CUBE_MAP_NEGATIVE_Y| rx|−rz| ry|
       +rz          |TEXTURE_CUBE_MAP_POSITIVE_Z| rx|−ry| rz|
       −rz          |TEXTURE_CUBE_MAP_NEGATIVE_Z|−rx|−ry| rz|
--------------------+---------------------------+---+---+---+

sc对应于u coordiante，而tc对应于v cooridate.因此，tc必须沿视图空间向上的方向

sc cooresponds the u coordiante and tc to the v cooridnate. So tc has to be in the direction of the view space up vector

查看表格的第一行:

+rx | TEXTURE_CUBE_MAP_POSITIVE_X | −rz | −ry | rx

这意味着，对于立方体贴图的X +侧(右侧)，对应于切线和双法线的方向为

This means, for the X+ side (right side) of the cube map, the directions which correspond to the tangent and binormal are

sc = (0, 0, -1)
tc = (0, -1, 0)

这与表glm::mat4 captureViews[]的第一行完全匹配:

This perfectly matches the 1st row of the table glm::mat4 captureViews[]:

glm::lookAt(glm::vec3(0.0f, 0.0f, 0.0f), glm::vec3(1.0f, 0.0f, 0.0f), glm::vec3(0.0f, -1.0f, 0.0f))

因为主要方向是由视线给出的，所以视线就是从视线到目标位置(los = target - eye)的方向，所以(1，0，0).
向上向量(或ts)为(0，-1，0).
sc由视线和向上矢量(0，0，-1)的叉积给出.

because the major direction is given by the line of sight, which is the directon form the eye position to the target (los = target - eye) and so (1, 0, 0).
The up vector (or ts) is (0, -1, 0).
sc is given by the cross product of the line of sight and the up vector (0, 0, -1).

这篇关于不了解Learnopengl.com中的漫射辐射教程中的captureViews的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持！