几何着色器中的宽线表现异常

问题描述

我正在尝试使用几何着色器渲染任意宽线(在屏幕空间中).乍一看似乎一切都很好，但是在某些视图位置上，线条显示不正确:

I'm trying to render arbitrary wide lines (in screen space) using a geometry shader. At first it seems all good, but on certain view position the lines are rendered incorrectly:

左侧的图像呈现正确的渲染效果(在X，Y和Z轴正方向上显示三行，宽度为2像素).

The image on the left present the correct rendering (three lines on positive X, Y and Z axes, 2 pixel wide).

当照相机移动到原点附近(实际上是在线条附近)时，线条将像正确的图像一样呈现.着色器似乎很简单，而且我不了解GPU上正在发生什么:

When the camera moves near the origin (and indeed near the lines), the lines are rendered like the right image. The shader seems straightforward, and I don't understand what's going on my GPU:

--- Vertex Shader

#version 410 core

// Modelview-projection matrix
uniform mat4 ds_ModelViewProjection;
// Vertex position
in vec4 ds_Position;
// Vertex color
in vec4 ds_Color;

// Processed vertex color
out vec4 ds_VertexColor;

void main()
{
    gl_Position = ds_ModelViewProjection * ds_Position;

    ds_VertexColor = ds_Color;
}

--- Geometry Shader

    #version 410 core

    // Viewport size, in pixels
uniform vec2 ds_Viewport;
// Line width, in pixels
uniform float ds_LineWidth = 2.0;
// Processed vertex color (from VS, in clip space)
in vec4 ds_VertexColor[2];
// Processed primitive vertex color
out vec4 ds_GeoColor;

layout (lines) in;
layout (triangle_strip, max_vertices = 4) out;

void main()
{
    vec3 ndc0 = gl_in[0].gl_Position.xyz / gl_in[0].gl_Position.w;
    vec3 ndc1 = gl_in[1].gl_Position.xyz / gl_in[1].gl_Position.w;

    vec2 lineScreenForward = normalize(ndc1.xy - ndc0.xy);
    vec2 lineScreenRight = vec2(-lineScreenForward.y, lineScreenForward.x);
    vec2 lineScreenOffset = (vec2(ds_LineWidth) / ds_ViewportSize) * lineScreenRight;

    gl_Position = vec4(ndc0.xy + lineScreenOffset, ndc0.z, 1.0);
    ds_GeoColor = ds_VertexColor[0];
    EmitVertex();

    gl_Position = vec4(ndc0.xy - lineScreenOffset, ndc0.z, 1.0);
    ds_GeoColor = ds_VertexColor[0];
    EmitVertex();

    gl_Position = vec4(ndc1.xy + lineScreenOffset, ndc1.z, 1.0);
    ds_GeoColor = ds_VertexColor[1];
    EmitVertex();

    gl_Position = vec4(ndc1.xy - lineScreenOffset, ndc1.z, 1.0);
    ds_GeoColor = ds_VertexColor[1];
    EmitVertex();

    EndPrimitive();
}

--- Fragment Shader

// Processed primitive vertex color
in vec4 ds_GeoColor;

// The fragment color.
out vec4 ds_FragColor;

void main()
{
        ds_FragColor = ds_GeoColor;
}

推荐答案

今天，我自己找到了答案.我不太了解，但这解决了这个问题.

Today I've found the answer by myself. I don't understand it entirely, but this solved the question.

当线顶点超出为该场景定义的投影矩阵的近平面(在我的情况下，这三行的所有结束顶点)时，就会出现问题.解决方案是在视锥中手动剪切线顶点(这样，顶点不能超出近平面！).

The problem arise when the line vertices goes beyond the near plane of the projection matrix defined for the scene (in my case, all ending vertices of the three lines). The solution is to manually clip the line vertices within the view frustum (in this way the vertices cannot go beyond the near plane!).

ndc0和ndc1在视锥范围之外会发生什么?查看图像，似乎XY分量的符号已更改(在剪辑空间中变换之后！):这意味着W坐标与普通坐标相反，不是吗?

What happens to ndc0 and ndc1 when they are out of the view frustum? Looking at the images, it seems that the XY components have the sign changed (after they are transformed in clip space!): that would mean that the W coordinate is opposite of the normal one, isn't?

没有几何着色器，光栅化器将负责将那些图元裁剪到视锥范围之外，但是由于我已经引入了几何着色器，因此我需要自己计算那些结果.有人可以建议我有关此事的一些链接吗?

Without the geometry shader, the rasterizer would have be taken the responsability to clip those primitives outside the view frustum, but since I've introduced the geometry shader, I need to compute those result by myself. Do anyone can suggest me some link about this matter?

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