c++ - 在 OpenGL/GLSL 中绘制镶嵌的 LineLoops

Question

我正在球体(行星边界)上渲染一些 LineLoop，并希望将它们镶嵌成较短的线，以防止长线剪裁到球体中。例子:

这是我当前的源代码:C++ 绘图调用:

void Border::Draw() const
{
    glBindVertexArray(_vao);

    glPatchParameteri(GL_PATCH_VERTICES, 2);
    glDrawArrays(GL_PATCHES, 0, _size);

    glBindVertexArray(0);
}

顶点着色器:

#version 400

in layout(location = 0) vec3 position;

void main()
{
    gl_Position = vec4(position, 1.0);
}

曲面 segmentation 控制着色器:

#version 400

layout (vertices = 2) out;

void main()
{
    if(gl_InvocationID == 0)
    {
        //get the distance between two points
        float dist = length(gl_in[0].gl_Position - gl_in[1].gl_Position);
        gl_TessLevelOuter[0] = 1;
        //divide line in segments of size 1.0
        gl_TessLevelOuter[1] = abs(dist);
    }

    gl_out[gl_InvocationID].gl_Position = gl_in[gl_InvocationID].gl_Position;
}

曲面 segmentation 评估着色器

#version 400

layout (isolines, equal_spacing) in;

uniform float planetRadius;
uniform mat4 view;
uniform mat4 projection;

void main()
{
    vec4 p = mix(gl_in[0].gl_Position, gl_in[1].gl_Position, gl_TessCoord.x);

    //offset point so it is on the spheres surface
    p = normalize(p);
    p *= planetRadius;

    gl_Position = projection * view * p;
}

片段着色器:

#version 400

out vec4 outColor;

void main()
{
    outColor = vec4(1.0, 1.0, 1.0, 1.0);
}

镶嵌似乎没有像我预期的那样工作(等值线的文档也很糟糕)。

这是我用 LineLoops 和只有顶点/片段着色器绘制它时的样子:

这就是我实现 segmentation 后的样子:

线条看起来也没有镶嵌，并且与之前的长度相同。

你好强尼

编辑:自己找到了解决方案。请查看下面的答案部分。

Answer 1

我找到了解决方案。问题在于镶嵌着色器无法绘制线条。因此，如果您有一条由点 {p1, p2, p3, p4, p5, p6} 定义的线带，它会在以下之间画线:
p1 -​​> p2
p3 -> p4
p5 -> p6

我通过给着色器每个点 2 次来修复它，所以我的列表如下所示:
{p1, p2, p2, p3, p3, p4, p4, p5, p5, p6, p6, p1}
segmentation 着色器创建以下行:
p1 -​​> p2
p2 -> p3
p3 -> p4
p4 -> p5
p5 -> p6
p6 -> p1

我的着色器现在看起来像这样:

苔丝控制:

#version 430

layout (vertices = 2) out;

void main()
{
    if(gl_InvocationID == 0)
    {
        float dist = length(gl_in[0].gl_Position.xyz - gl_in[1].gl_Position.xyz);

        gl_TessLevelOuter[0] = 1;
        gl_TessLevelOuter[1] = dist;
    }

    gl_out[gl_InvocationID].gl_Position = gl_in[gl_InvocationID].gl_Position;
}

泰斯评估:

#version 430

layout (isolines, equal_spacing) in;

uniform float planetRadius;
uniform mat4 view;
uniform mat4 projection;

void main()
{
    //only interpolate and normalize xyz coordinates. Thanks to @aslg
    vec3 p = mix(gl_in[0].gl_Position.xyz, gl_in[1].gl_Position.xyz, gl_TessCoord.x);

    p = normalize(p);
    p *= planetRadius;

    gl_Position = projection * view * vec4(p, 1.0);
}