Manual/PartII/Visibility
Wikipedia，自由的百科全书
Pyramid/Frustum Clipping|四面体/平截头体裁剪
To assist in the process of working with complex meshes and scenes a tool for restricting visual data is available. The tool is activated via the ALT-B key. 有一种工具可以方便在编辑复杂网格物体或场景时限制可见的数据。通过点击ALT-B，就可以激活这个工具。
Using this tool you create a 3D clipping volume shaped liked a Pyramid; you could think of it as a Frustum with a top. You specify the base of the Pyramid by creating a 2D rectangular region. 使用这个工具你就可以在3d视图中创建一个正四面体的裁剪区域，当然也可以把它想象为有顶的截头体。 你可以做的就是通过创建一个2维的矩形来指定这个正四面体的底部。 (Region/Volume clipping) is an example of using the clipping tool with a cube. Start by activating the tool with ALT-B, see "Start" in the upper left. This will generate a dashed cross- hair cursor. Click with the LMB and drag out a rectangular region shown in the upper right. Now a region is defined and clipping is now applied against that region in 3D space. Notice that part of the Cube is now invisible or clipped. Use the MMB to rotate the view and you will see that only what is inside the Pyramid volume is visible. All Edge/Face tools still function as normal but only within the Pyramid volume. （区域/体积裁剪）是一个对立方体使用裁剪工具的例子。使用ALT-B激活工具，参照图的左上部分。然后产生一个虚线的十字叉。按下LMB 然后拖动直到出现一个正方形区域，如右上图所示。现在一块区域就已经定义了，裁剪也出现在了3d视图中。注意现在正方体的一部分已经不可见或者说是被裁剪掉了。使用MMB旋转视图，你会发现，只有在正四面体中的部分才被显示出来。所有边/面工具正常运行，但是只是在正四面体内才行。
The gray area surrounding the volume is the Pyramid volume itself. To deactivate the clipping tool toggle it by applying ALT-B a second time. All of 3D space will become visible again. 有体积的灰色区域就是正四面体。要取消裁剪工具，再点击一次ALT-B 就可以。3d空间的所有物体又重新显示。
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