在什么顺序应该发送我的顶点到OpenGL的拣选 [英] In What Order Should I Send My Vertices To OpenGL for Culling

问题描述

我正在学习一个3d opengl的地方，它走得很好，我有一个漂亮的摄像机移动和一些简单的立方体对象，在这一刻。目前使用顶点数组，但我在这里交换到VBOs相当快。我只是试图启用剔除，但我不知道我应该在哪个顺序，我应该指定我的顶点，现在这是我在做什么：

I'm learning a spot of 3d opengl, and it's going rather well, I've got a nice camera moving about and some simple cube objects, at the moment. Currently using vertex arrays, but I'm swapping to VBOs pretty quick here. I'm just trying to enable culling, however I'm not sure what order in which I ought to specify my vertices, right now this is what I'm doing:

void cube::update_verts(){
GLushort cur=0;

///back face
verts[cur++]=x; verts[cur++]=y; verts[cur++]=z;
verts[cur++]=x+sx; verts[cur++]=y; verts[cur++]=z;
verts[cur++]=x+sx; verts[cur++]=y+sy; verts[cur++]=z;
verts[cur++]=x; verts[cur++]=y+sy; verts[cur++]=z;

///right face
verts[cur++]=x+sx; verts[cur++]=y+sy; verts[cur++]=z;
verts[cur++]=x+sx; verts[cur++]=y; verts[cur++]=z;
verts[cur++]=x+sx; verts[cur++]=y; verts[cur++]=z+sz;
verts[cur++]=x+sx; verts[cur++]=y+sy; verts[cur++]=z+sz;

///top face
verts[cur++]=x+sx; verts[cur++]=y+sy; verts[cur++]=z;
verts[cur++]=x; verts[cur++]=y+sy; verts[cur++]=z;
verts[cur++]=x; verts[cur++]=y+sy; verts[cur++]=z+sz;
verts[cur++]=x+sx; verts[cur++]=y+sy; verts[cur++]=z+sz;

///front face
verts[cur++]=x; verts[cur++]=y; verts[cur++]=z+sz;
verts[cur++]=x+sx; verts[cur++]=y; verts[cur++]=z+sz;
verts[cur++]=x+sx; verts[cur++]=y+sy; verts[cur++]=z+sz;
verts[cur++]=x; verts[cur++]=y+sy; verts[cur++]=z+sz;

///bottom face
verts[cur++]=x+sx; verts[cur++]=y; verts[cur++]=z;
verts[cur++]=x; verts[cur++]=y; verts[cur++]=z;
verts[cur++]=x; verts[cur++]=y; verts[cur++]=z+sz;
verts[cur++]=x+sx; verts[cur++]=y; verts[cur++]=z+sz;

///left face
verts[cur++]=x; verts[cur++]=y+sy; verts[cur++]=z;
verts[cur++]=x; verts[cur++]=y; verts[cur++]=z;
verts[cur++]=x; verts[cur++]=y; verts[cur++]=z+sz;
verts[cur++]=x; verts[cur++]=y+sy; verts[cur++]=z+sz;


}

///Drawing Code:

glVertexPointer(3,GL_FLOAT,0,object.verts);
glColorPointer(3,GL_UNSIGNED_BYTE,0,object.colors);
glDrawArrays(GL_QUADS,0,6*4);

但是这绝对是错误的，因为当我 glEnable（GL_CULL_FACE）; 我的多维数据集不显示正确的面孔（如下所示）。

However it's definitely quite wrong, because when I glEnable(GL_CULL_FACE); my cubes don't show the correct faces (as seen below).

正常

strong>

Problem Child

我应该以什么顺序指定顶点？

In what order should I specify the vertices?

（EDIT）更新工作职能：

void cube::update_verts(){
GLushort cur=0;

///top face
verts[cur++]=x; verts[cur++]=y+sy; verts[cur++]=z;
verts[cur++]=x; verts[cur++]=y+sy; verts[cur++]=z+sz;
verts[cur++]=x+sx; verts[cur++]=y+sy; verts[cur++]=z+sz;
verts[cur++]=x+sx; verts[cur++]=y+sy; verts[cur++]=z;


///bottom face
verts[cur++]=x; verts[cur++]=y; verts[cur++]=z;
verts[cur++]=x+sx; verts[cur++]=y; verts[cur++]=z;
verts[cur++]=x+sx; verts[cur++]=y; verts[cur++]=z+sz;
verts[cur++]=x; verts[cur++]=y; verts[cur++]=z+sz;

///left face
verts[cur++]=x; verts[cur++]=y; verts[cur++]=z;
verts[cur++]=x; verts[cur++]=y; verts[cur++]=z+sz;
verts[cur++]=x; verts[cur++]=y+sy; verts[cur++]=z+sz;
verts[cur++]=x; verts[cur++]=y+sy; verts[cur++]=z;

///right face
verts[cur++]=x+sx; verts[cur++]=y; verts[cur++]=z;
verts[cur++]=x+sx; verts[cur++]=y+sy; verts[cur++]=z;
verts[cur++]=x+sx; verts[cur++]=y+sy; verts[cur++]=z+sz;
verts[cur++]=x+sx; verts[cur++]=y; verts[cur++]=z+sz;

///front face
verts[cur++]=x; verts[cur++]=y; verts[cur++]=z+sz;
verts[cur++]=x+sx; verts[cur++]=y; verts[cur++]=z+sz;
verts[cur++]=x+sx; verts[cur++]=y+sy; verts[cur++]=z+sz;
verts[cur++]=x; verts[cur++]=y+sy; verts[cur++]=z+sz;


///back face
verts[cur++]=x; verts[cur++]=y; verts[cur++]=z;
verts[cur++]=x; verts[cur++]=y+sy; verts[cur++]=z;
verts[cur++]=x+sx; verts[cur++]=y+sy; verts[cur++]=z;
verts[cur++]=x+sx; verts[cur++]=y; verts[cur++]=z;

}

推荐答案

？

考虑面向相机的三角形：

Consider a triangle facing the camera:

A
|\
| \
|  \
B---C

A-> B-> C

A->B->C would be front facing (counter-clockwise order), A->C->B would be rear-facing (clockwise order).

你可以改变OpenGL认为前面的方向（逆时针顺序），A-> C-> 通过 glFrontFace（） ：

You can change which way OpenGL considers "front facing" via glFrontFace():

多边形到窗口坐标的投影据说有
顺时针绕组，如果一个假想对象跟随它的
第一个顶点，它的第二个顶点，等等，到它的最后一个顶点，
最终返回到它的第一个顶点，以顺时针方向绕多边形的内部
移动。如果沿着相同路径的假想对象围绕多边形的内部以逆时针方向移动
，则多边形的绕组被称为
逆时针。
glFrontFace 指定在
窗口坐标中顺时针缠绕的多边形，还是在窗口坐标中逆时针卷绕的多边形
被视为前向。将 GL_CCW 传递到模式选择
逆时针多边形作为前面; GL_CW 选择顺时针
多边形为前置。

The projection of a polygon to window coordinates is said to have clockwise winding if an imaginary object following the path from its first vertex, its second vertex, and so on, to its last vertex, and finally back to its first vertex, moves in a clockwise direction about the interior of the polygon. The polygon's winding is said to be counterclockwise if the imaginary object following the same path moves in a counterclockwise direction about the interior of the polygon. glFrontFace specifies whether polygons with clockwise winding in window coordinates, or counterclockwise winding in window coordinates, are taken to be front-facing. Passing GL_CCW to mode selects counterclockwise polygons as front-facing; GL_CW selects clockwise polygons as front-facing.

默认情况下，逆时针多边形为前向。

对于顶点的排序，考虑一个理想的立方体：

For ordering your vertices, consider an ideal cube:

  6---7
 /|  /|
2---3 |
| 4-|-5
|/  |/ 
0---1

Sides:
2---3  3---7  7---6  6---2
|   |  |   |  |   |  |   |
|   |  |   |  |   |  |   |
0---1  1---5  5---4  4---0

Bottom/Top
0---1  6---7
|   |  |   |
|   |  |   |
4---5  2---3

四边形或三角形对以正确的逆时针顺序：

Then you can just visually read off the quads or triangle pairs in the right counter-clockwise order:

2---3                3         2---3 
|   |  becomes      /|   and   |  / 
|   |             /  |         |/ 
0---1            0---1         0 

Triangles 0-1-3 and 0-3-2
Quad 0-1-3-2

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