voxel 与 pixel
中文名称:体素,即顾名思义是体积的像素。用来在三维空间中表示一个显示基本点的单位。类似于二维平面下的pixel(像素)。
voxel是三维空间中定义一个点的图象信息的单位。在平面中定义一个点要两个坐标X和Y就够了,而在三维世界中还要有一个坐标。光有位置还不行,还要有颜色等信息,这就是voxel的含义。
A voxel is a unit of graphic information that defines a point in three-dimensional space. Since a pixel (picture element) defines a point in two dimensional space with its x and y coordinates , a third z coordinate is needed. In 3-D space, each of the coordinates is defined in terms of its position, color, and density. Think of a cube where any point on an outer side is expressed with an x , y coordinate and the third, z coordinate defines a location into the cube from that side, its density, and its color. With this information and 3-D rendering software, a two-dimensional view from various angles of an image can be obtained and viewed at your computer.
Medical practitioners and researchers are now using images defined by voxels and 3-D software to view X-rays, cathode tube scans, and magnetic resonance imaging (MRI) scans from different angles, effectively to see the inside of the body from outside. Geologists can create 3-D views of earth profiles based on sound echoes. Engineers can view complex machinery and material structures to look for weaknesses.
目前的地形渲染技术主要有两种Voxel和LOD,下面我来做个简单的介绍。
Voxel 也就是Volumetric Pixel。也就是所谓的“体素”,它是相对于像素来说的,如果说像素是一个二维的矩形的话,那么Voxel就是一个三维的立方体。它的原理是比较简单的。James Sharman自称他在1995年时就想出了这种方法。前面的提到的Delta Force游戏就是使用了Voxel 技术。关于Voxel的细节技术不是本文的重点,我不准备做深入的介绍。Voxel有一个天生的优点就是渲染的时候它和场景的大小没有关系,而且绝对不会渲染多余的东西(自带裁剪功能)。它的复杂度只和我们需要的视野,以及分辨率有关。而且可以在不使用硬件加速的情况下达到比较理想的速度(Delta Force I就没有使用硬件加速),生成的图象也比较的细腻。它的缺点就是不够的灵活。
LOD 也就是层次细节(Level of Detail)的简称,不同于Voxel技术,它是一种使用多边形的,真正的 3D渲染技术。它根据一定的规则来简化物体的细节,我们可以根据需要来选择不同细节程度的物体表达方式。如离观察者近的选择较高的细节程度、反之选择较底的细节程度。用在地形渲染中,有时我们也称它为多分辨率地形(Muti-resolution terrain)渲染技术。