问题描述

当我第一次加载我的对象时，我用最大和最小 (x,y,z) 点计算初始 AABB.但这是在物体空间中，物体在世界各地移动，更重要的是，旋转.

When I first load my object I calculate the initial AABB with the max and min (x,y,z) points. But this is in object space and the object moves around the world and more importantly, rotates.

如何在每次平移/旋转对象时重新计算新的 AABB?基本上每一帧都会发生这种情况，每帧重新计算新的 AABB 会是一个非常密集的操作吗?如果是这样，替代方案是什么?

How do I recalculate the new AABB every time the object is translated/rotated? This happens basically every frame, is it going to be a very intensive operation to recalculate the new AABB every frame? If so, what would be the alternative?

我知道 AABB 会使我的碰撞检测不那么准确，但它比 OBB 更容易实现碰撞检测代码，我想一次采取这一步.

I know AABBs will make my collision detection less accurate but it's easier to implement the collision detection code than OBBs and I want to take this one step at a time.

根据以下答案的一些见解，这是我当前的代码:

Here's my current code after some insight from the answers below:

typedef struct sAxisAlignedBoundingBox {
    Vector3D bounds[8];
    Vector3D max, min;
} AxisAlignedBoundingBox;

void drawAxisAlignedBoundingBox(AxisAlignedBoundingBox box) {
    glPushAttrib(GL_LIGHTING_BIT | GL_POLYGON_BIT);

    glEnable(GL_COLOR_MATERIAL);
    glDisable(GL_LIGHTING);

    glColor3f(1.0f, 1.0f, 0.0f);

    glBegin(GL_LINE_LOOP);
        glVertex3f(box.bounds[0].x, box.bounds[0].y, box.bounds[0].z);
        glVertex3f(box.bounds[1].x, box.bounds[1].y, box.bounds[1].z);
        glVertex3f(box.bounds[2].x, box.bounds[2].y, box.bounds[2].z);
        glVertex3f(box.bounds[3].x, box.bounds[3].y, box.bounds[3].z);
    glEnd();

    glBegin(GL_LINE_LOOP);
        glVertex3f(box.bounds[4].x, box.bounds[4].y, box.bounds[4].z);
        glVertex3f(box.bounds[5].x, box.bounds[5].y, box.bounds[5].z);
        glVertex3f(box.bounds[6].x, box.bounds[6].y, box.bounds[6].z);
        glVertex3f(box.bounds[7].x, box.bounds[7].y, box.bounds[7].z);
    glEnd();

    glBegin(GL_LINE_LOOP);
        glVertex3f(box.bounds[0].x, box.bounds[0].y, box.bounds[0].z);
        glVertex3f(box.bounds[5].x, box.bounds[5].y, box.bounds[5].z);
        glVertex3f(box.bounds[6].x, box.bounds[6].y, box.bounds[6].z);
        glVertex3f(box.bounds[1].x, box.bounds[1].y, box.bounds[1].z);
    glEnd();

    glBegin(GL_LINE_LOOP);
        glVertex3f(box.bounds[4].x, box.bounds[4].y, box.bounds[4].z);
        glVertex3f(box.bounds[7].x, box.bounds[7].y, box.bounds[7].z);
        glVertex3f(box.bounds[2].x, box.bounds[2].y, box.bounds[2].z);
        glVertex3f(box.bounds[3].x, box.bounds[3].y, box.bounds[3].z);
    glEnd();

    glPopAttrib();
}

void calculateAxisAlignedBoundingBox(GLMmodel *model, float matrix[16]) {
    AxisAlignedBoundingBox box;
    float dimensions[3];

    // This will give me the absolute dimensions of the object
    glmDimensions(model, dimensions);

    // This calculates the max and min points in object space
    box.max.x = dimensions[0] / 2.0f, box.min.x = -1.0f * box.max.x;
    box.max.y = dimensions[1] / 2.0f, box.min.y = -1.0f * box.max.y;
    box.max.z = dimensions[2] / 2.0f, box.min.z = -1.0f * box.max.z;

    // These calculations are probably the culprit but I don't know what I'm doing wrong
    box.max.x = matrix[0] * box.max.x + matrix[4] * box.max.y + matrix[8] * box.max.z + matrix[12];
    box.max.y = matrix[1] * box.max.x + matrix[5] * box.max.y + matrix[9] * box.max.z + matrix[13];
    box.max.z = matrix[2] * box.max.x + matrix[6] * box.max.y + matrix[10] * box.max.z + matrix[14];
    box.min.x = matrix[0] * box.min.x + matrix[4] * box.min.y + matrix[8] * box.min.z + matrix[12];
    box.min.y = matrix[1] * box.min.x + matrix[5] * box.min.y + matrix[9] * box.min.z + matrix[13];
    box.min.z = matrix[2] * box.min.x + matrix[6] * box.min.y + matrix[10] * box.min.z + matrix[14];

    /* NOTE: If I remove the above calculations and do something like this:

             box.max = box.max + objPlayer.position;
             box.min = box.min + objPlayer.position;

             The bounding box will move correctly when I move the player, the same does not
             happen with the calculations above. It makes sense and it's very simple to move
             the box like this. The only problem is when I rotate the player, the box should
             be adapted and increased/decreased in size to properly fit the object as a AABB.
    */

    box.bounds[0] = Vector3D(box.max.x, box.max.y, box.min.z);
    box.bounds[1] = Vector3D(box.min.x, box.max.y, box.min.z);
    box.bounds[2] = Vector3D(box.min.x, box.min.y, box.min.z);
    box.bounds[3] = Vector3D(box.max.x, box.min.y, box.min.z);
    box.bounds[4] = Vector3D(box.max.x, box.min.y, box.max.z);
    box.bounds[5] = Vector3D(box.max.x, box.max.y, box.max.z);
    box.bounds[6] = Vector3D(box.min.x, box.max.y, box.max.z);
    box.bounds[7] = Vector3D(box.min.x, box.min.y, box.max.z);

    // This draw call is for testing porpuses only
    drawAxisAlignedBoundingBox(box);
}

void drawObjectPlayer(void) {
    static float mvMatrix[16];

    if(SceneCamera.GetActiveCameraMode() == CAMERA_MODE_THIRD_PERSON) {
        objPlayer.position = SceneCamera.GetPlayerPosition();
        objPlayer.rotation = SceneCamera.GetRotationAngles();

        objPlayer.position.y += -PLAYER_EYE_HEIGHT + 0.875f;

        /* Only one of the two code blocks below should be active at the same time
           Neither of them is working as expected. The bounding box doesn't is all
           messed up with either code. */

        // Attempt #1
        glPushMatrix();
            glTranslatef(objPlayer.position.x, objPlayer.position.y, objPlayer.position.z);
            glRotatef(objPlayer.rotation.y + 180.0f, 0.0f, 1.0f, 0.0f);
            glCallList(gameDisplayLists.player);
            glGetFloatv(GL_MODELVIEW_MATRIX, mvMatrix);
        glPopMatrix();

        // Attempt #2
        glPushMatrix();
            glLoadIdentity();
            glTranslatef(objPlayer.position.x, objPlayer.position.y, objPlayer.position.z);
            glRotatef(objPlayer.rotation.y + 180.0f, 0.0f, 1.0f, 0.0f);
            glGetFloatv(GL_MODELVIEW_MATRIX, mvMatrix);
        glPopMatrix();

        calculateAxisAlignedBoundingBox(objPlayer.model, mvMatrix);
    }
}

但它不能正常工作......我做错了什么?

But it doesn't work as it should... What I'm doing wrong?

推荐答案

只需重新计算转换后的 AABB 的 AABB.这意味着转换 8 个顶点(8 个顶点-矩阵乘法)和 8 个顶点-顶点比较.

Simply recompute the AABB of the transformed AABB. This means transforming 8 vertices ( 8 vertex - matrix multiplications ) and 8 vertex-vertex comparisons.

所以在初始化时，你在模型空间中计算你的 AABB:对于模型每个顶点的每个 x、y、z，你检查 xmin、xmax、ymin、ymax ......

So at initialisation, you compute your AABB in model space : for each x,y,z of each vertex of the model, you check against xmin, xmax, ymin, ymax, ...

每一帧，你都会生成一个新的变换矩阵.在 OpenGL 中，这是通过 glLoadIdentity 后跟 glTransform/Rotate/Scale(如果使用旧 API)来完成的.正如 lmmilewski 所说，这就是模型矩阵.

Each frame, you generate a new transformation matrix. In OpenGL this is done with glLoadIdentity followed by glTransform/Rotate/Scale (if using the old API). This is the Model Matrix, as lmmilewski said.

您再次计算此变换矩阵(在 Opengl 之外，例如使用 glm).您还可以使用 glGet 获得 OpenGL 的结果矩阵.

You compute this transformation matrix a second time (outside Opengl, for instance using glm). You also can get OpenGL's resulting matrix using glGet.

您将 AABB 的八个顶点中的每一个都乘以这个矩阵.使用 glm 进行矩阵向量乘法.您将获得转换后的 AABB(在世界空间中).它很可能旋转(不再轴对齐)

You multiply each of your AABB's eight vertices by this matrix. Use glm for matrix-vector multiplication. You'll get your transformed AABB (in world space). It it most probably rotated (not axis-aligned anymore)

现在你的算法可能只适用于轴对齐的东西，因此你的问题.所以现在你通过计算转换后的边界框的边界框来近似转换模型的新边界框:

Now your algorithm probably only work with axis-aligned stuff, hence your question. So now you approximate the new bounding box of the transformed model by takinf the bounding box of the transformed bounding box:

对于新 AABB 的每个顶点的每个 x、y、z，您检查 xmin、xmax、ymin、ymax ……这为您提供了一个世界空间 AABB，您可以在裁剪算法中使用它.

for each x,y,z of each vertex of the new AABB, you check against xmin, xmax, ymin, ymax, ... this gives you an world-space AABB that you can use in your clipping algorithm.

这不是最优的(AABB 方面)，您会得到很多空白空间，但在性能方面，重新计算整个网格的 AABB 会好得多.

This is not optimal (AABB-wise), you'll get lots of empty space, but performance-wise, it's much much better that recomputing the AABB of the whole mesh.

至于变换矩阵，在drawObjectPlayer中:

As for the transformation matrix, in drawObjectPlayer:

        gLLoadIdentity();
        glTranslatef(objPlayer.position.x, objPlayer.position.y, objPlayer.position.z);
        glRotatef(objPlayer.rotation.y + 180.0f, 0.0f, 1.0f, 0.0f);
        glGetFloatv(GL_MODELVIEW_MATRIX, mvMatrix);
  // Now you've got your OWN Model Matrix (don't trust the GL_MODELVIEW_MATRIX flag : this is a workaround, and I know what I'm doing ^^ )

        gLLoadIdentity(); // Reset the matrix so that you won't make the transformations twice
        gluLookAt( whatever you wrote here earlier )
        glTranslatef(objPlayer.position.x, objPlayer.position.y, objPlayer.position.z);
        glRotatef(objPlayer.rotation.y + 180.0f, 0.0f, 1.0f, 0.0f);
      // Now OpenGL is happy, he's got his MODELVIEW matrix correct ( gluLookAt is the VIEW part; Translate/Rotate is the MODEL part
        glCallList(gameDisplayLists.player); // Transformed correcty

无法解释更多......正如评论中所说，你必须做两次.顺便说一句，您不会在 OpenGL 3 中遇到这些问题和丑陋的解决方法，因为您将对自己的矩阵负全部责任.在 OpenGL 2 中等效:

Can't explain further than that... as said in the comments, you had to do it twice. You wouldn't have these problems and ugly workarounds in OpenGL 3, btw, because you'd be fully responsible of your own matrices. Equivalent in OpenGL 2 :

glm::mat4 ViewMatrix = glm::LookAt(...);
glm::mat4 ModelMatrix = glm::rotate() * glm::translate(...);
// Use ModelMatrix for whatever you want
glm::mat4 ModelViewMatrix = ViewMatrix * ModelMatrix;
glLoadMatrix4fv( &ModelViewMatrix[0][0] ); // In opengl3 you would use an uniform instead

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