OpenGL3中的虚线？

Posted 2023-04-15

【中文标题】OpenGL3中的虚线？

【英文标题】：Dashed line in OpenGL3?

【发布时间】：2019-03-26 11:44:26

【问题描述】：

我目前正在将使用线框模型的旧 OpenGL 1.1 应用程序移植到 OpenGL 3.0。

在 1.1 中，以下代码用于创建虚线：

glPushAttrib(GL_ENABLE_BIT); 
glLineStipple(1, 0x0F0F);
glEnable(GL_LINE_STIPPLE);

这里像往常一样将参数推送到堆栈中以影响所有后续绘图操作。

我的问题：在不再使用此堆栈的 OpenGL3 中如何做到这一点？如何将我的线条设置为虚线（可能在将坐标交给 glBufferData() 之前？

【问题讨论】：

我认为可以使用片段着色器来完成，但我不太确定如何。我认为您会为每个顶点设置一个属性，其中包含到该点的直线长度，然后着色器可以查看插值长度和点。

【参考方案1】：

对于单独的线段，这根本不是很复杂。例如绘制GL_LINES 原语。 诀窍是知道片段着色器中线段的起点。通过使用flat 插值限定符，这非常简单。

顶点着色器必须将标准化的设备坐标传递给片段着色器。一次使用默认插值，一次没有 (flat) 插值。这导致在片段阴影中，第一个输入参数包含行上实际位置的 NDC 坐标，后面是行开始的 NDC 坐标。

#version 330

layout (location = 0) in vec3 inPos;

flat out vec3 startPos;
out vec3 vertPos;

uniform mat4 u_mvp;

void main()

    vec4 pos    = u_mvp * vec4(inPos, 1.0);
    gl_Position = pos;
    vertPos     = pos.xyz / pos.w;
    startPos    = vertPos;

除了不同的输入，片段着色器还有统一的变量。 u_resolution 包含视口的宽度和高度。 u_dashSize 包含线条的长度，u_gapSize 包含间隙的长度（以像素为单位）。

所以可以计算出从开始到实际片段的线的长度：

vec2  dir  = (vertPos.xy-startPos.xy) * u_resolution/2.0;
float dist = length(dir);

并且可以通过discard 命令丢弃间隙上的片段。

if (fract(dist / (u_dashSize + u_gapSize)) > u_dashSize/(u_dashSize + u_gapSize))
    discard; 

片段着色器：

#version 330

flat in vec3 startPos;
in vec3 vertPos;

out vec4 fragColor;

uniform vec2  u_resolution;
uniform float u_dashSize;
uniform float u_gapSize;

void main()

    vec2  dir  = (vertPos.xy-startPos.xy) * u_resolution/2.0;
    float dist = length(dir);

    if (fract(dist / (u_dashSize + u_gapSize)) > u_dashSize/(u_dashSize + u_gapSize))
        discard; 
    fragColor = vec4(1.0);

对于以下简单的演示程序，我使用GLFW API 创建一个窗口，GLEW 用于加载 OpenGL，GLM -OpenGL Mathematics 用于数学运算。我不提供函数CreateProgram的代码，它只是从顶点着色器和片段着色器源代码中创建一个程序对象：

#include <GL/glew.h>
#include <GL/gl.h>

#include <glm/glm.hpp>
#include <glm/gtc/matrix_transform.hpp>
#include <glm/gtc/type_ptr.hpp>

#include <GLFW/glfw3.h>

#include <vector>

#define _USE_MATH_DEFINES
#include <math.h>

int main(void)

    if (glfwInit() == GLFW_FALSE)
        return 0;
    GLFWwindow *window = glfwCreateWindow(400, 300, "OGL window", nullptr, nullptr);
    if (window == nullptr)
        return 0;
    glfwMakeContextCurrent(window);
    glewExperimental = true;
    if (glewInit() != GLEW_OK)
        return 0;

    GLuint program = CreateProgram(vertShader, fragShader);
    GLint loc_mvp  = glGetUniformLocation(program, "u_mvp");
    GLint loc_res  = glGetUniformLocation(program, "u_resolution");
    GLint loc_dash = glGetUniformLocation(program, "u_dashSize");
    GLint loc_gap  = glGetUniformLocation(program, "u_gapSize");

    glUseProgram(program);
    glUniform1f(loc_dash, 10.0f);
    glUniform1f(loc_gap, 10.0f);

    std::vector<float> varray
        -1, -1, -1,   1, -1, -1,   1, 1, -1,   -1, 1, -1,
        -1, -1,  1,   1, -1,  1,   1, 1,  1,   -1, 1,  1
    ;
    std::vector<unsigned int> iarray
        0, 1, 1, 2, 2, 3, 3, 0, 
        4, 5, 5, 6, 6, 7, 7, 4,
        0, 4, 1, 5, 2, 6, 3, 7
    ;

    GLuint bo[2], vao;
    glGenBuffers(2, bo);
    glGenVertexArrays(1, &vao);
    glBindVertexArray(vao);
    glEnableVertexAttribArray(0); 
    glBindBuffer(GL_ARRAY_BUFFER, bo[0] );
    glBufferData(GL_ARRAY_BUFFER, varray.size()*sizeof(*varray.data()), varray.data(), GL_STATIC_DRAW);
    glVertexAttribPointer(0, 3, GL_FLOAT, GL_FALSE, 0, 0); 
    glBindBuffer(GL_ELEMENT_ARRAY_BUFFER, bo[1]);
    glBufferData(GL_ELEMENT_ARRAY_BUFFER, iarray.size()*sizeof(*iarray.data()), iarray.data(), GL_STATIC_DRAW);

    glClearColor(0.0f, 0.0f, 0.0f, 0.0f);

    glm::mat4 view = glm::lookAt(glm::vec3(0.0f, 0.0f, 5.0f), glm::vec3(0.0f, 0.0f, 0.0f), glm::vec3(0.0f, 1.0f, 0.0f));
    glm::mat4 project;
    int vpSize[2]0, 0;
    while (!glfwWindowShouldClose(window))
    
        int w, h;
        glfwGetFramebufferSize(window, &w, &h);
        if (w != vpSize[0] ||  h != vpSize[1])
        
            vpSize[0] = w; vpSize[1] = h;
            glViewport(0, 0, vpSize[0], vpSize[1]);
            project = glm::perspective(glm::radians(90.0f), (float)w/(float)h, 0.1f, 10.0f);
            glUniform2f(loc_res, (float)w, (float)h);
        

        static float angle = 1.0f;
        glm::mat4 modelview( 1.0f );
        modelview = glm::translate(modelview, glm::vec3(0.0f, 0.0f, -3.0f) );
        modelview = glm::rotate(modelview, glm::radians(angle), glm::vec3(1.0f, 0.0f, 0.0f));
        modelview = glm::rotate(modelview, glm::radians(angle*0.5f), glm::vec3(0.0f, 1.0f, 0.0f));
        angle += 0.5f;
        glm::mat4 mvp = project * modelview;

        glUniformMatrix4fv(loc_mvp, 1, GL_FALSE, glm::value_ptr(mvp));
        glClear(GL_COLOR_BUFFER_BIT);
        glDrawElements(GL_LINES, (GLsizei)iarray.size(), GL_UNSIGNED_INT, nullptr);

        glfwSwapBuffers(window);
        glfwPollEvents();
    
    glfwTerminate();

    return 0;

---

如果目标是沿着多边形绘制虚线，事情会变得有点复杂。例如绘制一个GL_LINE_STRIP 原语。

在不知道线的所有图元的情况下，无法在着色器程序中计算线的长度。即使已知所有原语（例如 SSBO），也必须在循环中完成计算。 我决定在着色器程序中添加一个附加属性，其中包含从线的起点到顶点坐标的“距离”。 “距离”是指投影到视口上的多边形的长度。

这导致顶点着色器和片段着色器更加简单：

顶点着色器：

#version 330

layout (location = 0) in vec3 inPos;
layout (location = 1) in float inDist;

out float dist;

uniform mat4 u_mvp;

void main()

    dist        = inDist;
    gl_Position = u_mvp * vec4(inPos, 1.0);

片段着色器：

#version 330

in float dist;

out vec4 fragColor;

uniform vec2  u_resolution;
uniform float u_dashSize;
uniform float u_gapSize;

void main()

    if (fract(dist / (u_dashSize + u_gapSize)) > u_dashSize/(u_dashSize + u_gapSize))
        discard; 
    fragColor = vec4(1.0);

在演示程序中，inDist 属性是在 CPU 上计算的。每个顶点坐标由模型、视图、投影矩阵变换。最后，它从规范化的设备空间转换为窗口空间。计算线带相邻坐标之间的XY距离，沿线带长度求和并赋值给相应的属性值：

int w = [...], h = [...];               // window widht and height
glm::mat4 mpv = [...];                  // model view projection matrix
std::vector<glm::vec3> varray [...] ; // array of vertex 

std::vector<float> darray(varray.size(), 0.0f); // distance attribute - has to be computed

glm::mat4 wndmat = glm::scale(glm::mat4(1.0f), glm::vec3((float)w/2.0f, (float)h/2.0f, 1.0f));
wndmat = glm::translate(wndmat, glm::vec3(1.0f, 1.0f, 0.0f));

glm::vec2 vpPt(0.0f, 0.0f);
float dist = 0.0f;
for (size_t i=0; i < varray.size(); ++i)

    darray[i] = dist;
    glm::vec4 clip = mvp * glm::vec4(varray[i], 1.0f);
    glm::vec4 ndc  = clip / clip.w;
    glm::vec4 vpC  = wndmat * ndc;
    float len = i==0 ? 0.0f :  glm::length(vpPt - glm::vec2(vpC));
    vpPt = glm::vec2(vpC);
    dist += len;

演示程序：

int main(void)

    if (glfwInit() == GLFW_FALSE)
        return 0;
    GLFWwindow *window = glfwCreateWindow(800, 600, "OGL window", nullptr, nullptr);
    if (window == nullptr)
        return 0;
    glfwMakeContextCurrent(window);
    glewExperimental = true;
    if (glewInit() != GLEW_OK)
        return 0;

    GLuint program = CreateProgram(vertShader, fragShader);
    GLint loc_mvp  = glGetUniformLocation(program, "u_mvp");
    GLint loc_res  = glGetUniformLocation(program, "u_resolution");
    GLint loc_dash = glGetUniformLocation(program, "u_dashSize");
    GLint loc_gap  = glGetUniformLocation(program, "u_gapSize");

    glUseProgram(program);
    glUniform1f(loc_dash, 10.0f);
    glUniform1f(loc_gap, 10.0f);

    std::vector<glm::vec3> varray;
    for (size_t u=0; u <= 360; ++u)
    
        double a = u*M_PI/180.0;
        double c = cos(a), s = sin(a);
        varray.emplace_back(glm::vec3((float)c, (float)s, 0.0f));
    
    std::vector<float> darray(varray.size(), 0.0f);

    GLuint bo[2], vao;
    glGenBuffers(2, bo);
    glGenVertexArrays(1, &vao);
    glBindVertexArray(vao);
    glEnableVertexAttribArray(0); 
    glEnableVertexAttribArray(1); 
    glBindBuffer(GL_ARRAY_BUFFER, bo[0] );
    glBufferData(GL_ARRAY_BUFFER, varray.size()*sizeof(*varray.data()), varray.data(), GL_STATIC_DRAW);
    glVertexAttribPointer(0, 3, GL_FLOAT, GL_FALSE, 0, 0); 
    glBindBuffer(GL_ARRAY_BUFFER, bo[1] );
    glBufferData(GL_ARRAY_BUFFER, darray.size()*sizeof(*darray.data()), darray.data(), GL_STATIC_DRAW);
    glVertexAttribPointer(1, 1, GL_FLOAT, GL_FALSE, 0, 0); 

    glClearColor(0.0f, 0.0f, 0.0f, 0.0f);

    glm::mat4 view = glm::lookAt(glm::vec3(0.0f, 0.0f, 5.0f), glm::vec3(0.0f, 0.0f, 0.0f), glm::vec3(0.0f, 1.0f, 0.0f));
    glm::mat4 project, wndmat;
    int vpSize[2]0, 0;
    while (!glfwWindowShouldClose(window))
    
        int w, h;
        glfwGetFramebufferSize(window, &w, &h);
        if (w != vpSize[0] ||  h != vpSize[1])
        
            vpSize[0] = w; vpSize[1] = h;
            glViewport(0, 0, vpSize[0], vpSize[1]);
            project = glm::perspective(glm::radians(90.0f), (float)w/(float)h, 0.1f, 10.0f);
            glUniform2f(loc_res, (float)w, (float)h);
            wndmat = glm::scale(glm::mat4(1.0f), glm::vec3((float)w/2.0f, (float)h/2.0f, 1.0f));
            wndmat = glm::translate(wndmat, glm::vec3(1.0f, 1.0f, 0.0f));
        

        static float angle = 1.0f;
        glm::mat4 modelview( 1.0f );
        modelview = glm::translate(modelview, glm::vec3(0.0f, 0.0f, -2.0f) );
        modelview = glm::rotate(modelview, glm::radians(angle), glm::vec3(1.0f, 0.0f, 0.0f));
        modelview = glm::rotate(modelview, glm::radians(angle*0.5f), glm::vec3(0.0f, 1.0f, 0.0f));
        angle += 0.5f;
        glm::mat4 mvp = project * modelview;

        glm::vec2 vpPt(0.0f, 0.0f);
        float dist = 0.0f;
        for (size_t i=0; i < varray.size(); ++i)
        
            darray[i] = dist;
            glm::vec4 clip = mvp * glm::vec4(varray[i], 1.0f);
            glm::vec4 ndc  = clip / clip.w;
            glm::vec4 vpC  = wndmat * ndc;
            float len = i==0 ? 0.0f :  glm::length(vpPt - glm::vec2(vpC));
            vpPt = glm::vec2(vpC);
            dist += len;
        
        glBufferSubData(GL_ARRAY_BUFFER, 0, darray.size()*sizeof(*darray.data()), darray.data());

        glUniformMatrix4fv(loc_mvp, 1, GL_FALSE, glm::value_ptr(mvp));
        glClear(GL_COLOR_BUFFER_BIT);
        glDrawArrays(GL_LINE_STRIP, 0, (GLsizei)varray.size());

        glfwSwapBuffers(window);
        glfwPollEvents();
    
    glfwTerminate();

    return 0;

---

另请参阅glLineStipple deprecated in OpenGL 3.1OpenGL ES - Dashed Lines