opengl - 几何着色器中的宽线表现得很奇怪

Question

我正在尝试使用几何着色器渲染任意宽线(在屏幕空间中)。起初看起来一切都很好，但在某些 View 位置上，线条渲染不正确:

左侧图像呈现正确的渲染(正 X、Y 和 Z 轴上的三条线，2 像素宽)。

当相机移动到原点附近(实际上是靠近线条)时，线条会像正确的图像一样渲染。着色器看起来很简单，但我不明白我的 GPU 上发生了什么:

--- 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;
}

Answer 1

你的错误在于:

gl_Position = vec4(ndc0.xy + lineScreenOffset, ndc0.z, 1.0 /* WRONG */);

修复它:

vec4 cpos = gl_in[0].gl_Position;
gl_Position = vec4(cpos.xy + lineScreenOffset*cpos.w, cpos.z, cpos.w);

您所做的是:丢失有关 W 的信息，从而使 HW 限幅器失谐，将其从 3D 限幅器降级为 2D 限幅器。