OpenGL（ES 2.0）动态更改线宽 [英] OpenGL (ES 2.0) Dynamically Change Line Width

问题描述

目前，我正在使用大量的 GL_LINES 来绘制我的模型。我知道， glLineWidth 将改变所有的半径，但每个半径应该有不同的半径。我想知道它是否可能使用 glLineWidth （以不同的方式）或另一个功能？我还应该怎么做？

I am currently drawing my model right now using a ton of GL_LINES all at a uniform radius. I know that glLineWidth will change the radius of the all the lines but they each should have a different radius. I am wondering if its possible using glLineWidth (in a different fashion) or another function? How else should I go about it?

推荐答案

将它们渲染成特殊构造的三角形条。具体来说，将每个线段渲染成一对三角形以形成四边形，四边形的长度与线的长度相匹配，宽度与radius匹配。然而，真正的技巧是模拟GL_LINES，因为线段不一定必须彼此连接。要做到这一点，在三角形条的情况下，您需要连接三角形与零区域三角形，这将不会被渲染（除非您的OpenGL ES实现不一致）。

Render them as a specially constructed triangle strip. Specifically, render each line segment as a pair of triangles to form a quadraliterial, with the length of the quad matching the length of the line, and the width matching your "radius". The real trick, however, is simulating GL_LINES, since line segments don't necessarily have to be connected to one another. To do this in the triangle-strip case, you'll need to connect the pairs of triangles with a zero-area triangle, which won't be rendered (unless your OpenGL ES implementation isn't conformant).

例如，假设我们处于2D（为简单起见，3D几乎相同），说您的一个线段有端点（x1，y1）和（x2，y2），而宽度W，如下所示。


将此替换为quad，我们需要确定角的坐标，这需要一点数学。对于合适的2D向量类， vec2

For example, assuming we're in 2D (for simplicity; 3D's almost the same) say one of your line segment has endpoints (x1,y1) and (x2,y2), and a width of W, as shown below.

To replace this with a quad, we need to determine the coordinates of the corners, which requires a bit of math. For a suitable 2D vector class, vec2

vec2  p1(x1, y1);
vec2  p2(x2, y2);
vec2  v = p2 - p1;

v /= v.length();  // make it a unit vector

vec2  vp(-v.y, v.x);  // compute the vector perpendicular to v

vec2  v[4];

v[0] = p1 + W/2 * vp;
v[1] = p1 - W/2 * vp;
v[2] = p2 + W/2 * vp;
v[3] = p2 - W/2 * vp;    

// Load the v[] array into a vertex-buffer object

这提供了将一条线插入四边形的解决方案。为了将它们连接在一起，我们需要在其中创建具有退化三角形的三角形条。如果我们使用 glDrawElements 绘制，我们可以构造一个简单的索引数组来实现。假设我们将两行转换成四边形，如上所述，给出了顶点v [0] ... v [7]。要使它们成为一个单一的三角形条，请创建一个索引列表

That provides the solution taking a single line into a quad. To connect them together, we need to create the triangle strip with degenerate triangles in it. If we draw this as using glDrawElements, we can construct a simple index array to do this. Say we converted two lines into quads as described above, giving us vertices v[0] ... v[7]. To make them into a single triangle strip, make an index list of

{ 0, 1, 2, 3, 3, 4, 5, 6, 7 }

通过在列表中重复3两次，我们创建一个新的三角形连接在一起，但不显示。如果您重复四边形的最后一个顶点，则可以将整组GL_LINES渲染为单个三角形条。

by repeating '3' in the list twice, we create a new triangle that connects the others together, yet doesn't show up. If you repeat the last vertex of a quad, you can render your entire set of GL_LINES as a single triangle strip.

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