我正在为 OpenGL 中的专有网格和动画格式编写查看器。
在渲染过程中，会为每个骨骼（节点）创建一个变换矩阵，并将其应用于骨骼所连接的顶点。
骨头可能会被标记为“Billboarded”，众所周知，这意味着它应该始终面向相机。
所以这个想法是为那个骨骼生成一个矩阵，当它用于转换它所连接的顶点时，会导致顶点被广告牌。
在我的测试模型上，它应该如下所示：
但是目前它看起来像这样：
请注意，尽管它的方向不正确，但它是广告牌。无论相机朝哪个方向看，这些顶点总是在那个方向上面向那个方向。
我为标记为广告牌的骨骼生成矩阵的代码是：
mat4 view;
glGetFloatv(GL_MODELVIEW_MATRIX, (float*)&view);
vec4 camPos = vec4(-view[3].x, -view[3].y, -view[3].z,1);
vec3 camUp = vec3(view[0].y, view[1].y, view[2].y);
// zero the translation in the matrix, so we can use the matrix to transform
// camera postion to world coordinates using the view matrix
view[3].x = view[3].y = view[3].z = 0;
// the view matrix is how to get to the gluLookAt pos from what we gave as
// input for the camera position, so to go the other way we need to reverse
// the rotation. Transposing the matrix will do this.
{
float * matrix = (float*)&view;
float temp[16];
// copy this into temp
memcpy(temp, matrix, sizeof(float) * 16);
matrix[1] = temp[4]; matrix[4] = temp[1];
matrix[2] = temp[8]; matrix[8] = temp[2];
matrix[6] = temp[9]; matrix[9] = temp[6];
}
// get the correct position of the camera in world space
camPos = view * camPos;
//vec3 pos = pivot;
vec3 look = glm::normalize(vec3(camPos.x-pos.x,camPos.y-pos.y,camPos.z-pos.z));
vec3 right = glm::cross(camUp,look);
vec3 up = glm::cross(look,right);
mat4 bmatrix;
bmatrix[0].x = right.x;
bmatrix[0].y = right.y;
bmatrix[0].z = right.z;
bmatrix[0].w = 0;
bmatrix[1].x = up.x;
bmatrix[1].y = up.y;
bmatrix[1].z = up.z;
bmatrix[1].w = 0;
bmatrix[2].x = look.x;
bmatrix[2].y = look.y;
bmatrix[2].z = look.z;
bmatrix[2].w = 0;
bmatrix[3].x = pos.x;
bmatrix[3].y = pos.y;
bmatrix[3].z = pos.z;
bmatrix[3].w = 1;
我正在使用 GLM 进行相关的数学运算。
尽管这部分代码基于此处的教程，但代码的其他部分基于类似于我正在构建的开源程序。然而，该程序是为 DirectX 编写的，我没有太多运气直接转换。用于广告牌的（工作）directX 代码如下所示：
D3DXMatrixRotationY(&CameraRotationMatrixY, -Camera.GetPitch());
D3DXMatrixRotationZ(&CameraRotationMatrixZ, Camera.GetYaw());
D3DXMatrixMultiply(&CameraRotationMatrix, &CameraRotationMatrixY, &CameraRotationMatrixZ);
D3DXQuaternionRotationMatrix(&CameraRotation, &CameraRotationMatrix);
D3DXMatrixTransformation(&CameraRotationMatrix, NULL, NULL, NULL, &ModelBaseData->PivotPoint, &CameraRotation, NULL);
D3DXMatrixDecompose(&Scaling, &Rotation, &Translation, &BaseMatrix);
D3DXMatrixTransformation(&RotationMatrix, NULL, NULL, NULL, &ModelBaseData->PivotPoint, &Rotation, NULL);
D3DXMatrixMultiply(&TempMatrix, &CameraRotationMatrix, &RotationMatrix);
D3DXMatrixMultiply(&BaseMatrix, &TempMatrix, &BaseMatrix);
请注意，结果存储在 directX 版本的 baseMatrix 中。
EDIT2：这是我根据 datenwolf 的建议尝试修改代码时提出的代码。我很确定我仍然犯了一些错误。这种尝试会产生严重扭曲的结果，对象的一端直接位于相机中。
mat4 view;
glGetFloatv(GL_MODELVIEW_MATRIX, (float*)&view);
vec3 pos = vec3(calculatedMatrix[3].x,calculatedMatrix[3].y,calculatedMatrix[3].z);
mat4 inverted = glm::inverse(view);
vec4 plook = inverted * vec4(0,0,0,1);
vec3 look = vec3(plook.x,plook.y,plook.z);
vec3 right = orthogonalize(vec3(view[0].x,view[1].x,view[2].x),look);
vec3 up = orthogonalize(vec3(view[0].y,view[1].y,view[2].y),look);
mat4 bmatrix;
bmatrix[0].x = right.x;
bmatrix[0].y = right.y;
bmatrix[0].z = right.z;
bmatrix[0].w = 0;
bmatrix[1].x = up.x;
bmatrix[1].y = up.y;
bmatrix[1].z = up.z;
bmatrix[1].w = 0;
bmatrix[2].x = look.x;
bmatrix[2].y = look.y;
bmatrix[2].z = look.z;
bmatrix[2].w = 0;
bmatrix[3].x = pos.x;
bmatrix[3].y = pos.y;
bmatrix[3].z = pos.z;
bmatrix[3].w = 1;
calculatedMatrix = bmatrix;
vec3 orthogonalize(vec3 toOrtho, vec3 orthoAgainst) {
float bottom = (orthoAgainst.x*orthoAgainst.x)+(orthoAgainst.y*orthoAgainst.y)+(orthoAgainst.z*orthoAgainst.z);
float top = (toOrtho.x*orthoAgainst.x)+(toOrtho.y*orthoAgainst.y)+(toOrtho.z*orthoAgainst.z);
return toOrtho - top/bottom*orthoAgainst;
}