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OpenGL3 中的虚线？

openglglslshaderopengl-3opengl-1.x
13

我目前正在将一个使用线框模型的旧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);

通常情况下，这里的参数按顺序推送到堆栈中，以影响所有后续的绘图操作。

我的问题是：在不再使用该堆栈的OpenGL3中，如何进行此操作？在将坐标传递给glBufferData()之前，我如何设置我的线为虚线？

我认为可以使用片段着色器来完成，但我不太确定具体方法。我认为您需要设置每个顶点的属性，其中包含该点到线的长度，然后着色器可以查看插值的长度并进行计算。 - Andrea
我尝试通过将屏幕空间中的顶点映射到u坐标来实现虚线多边形：https://www.shadertoy.com/view/Dtt3z2，我想这是一个相当天真的方法，但它看起来对我来说足够好，并且不需要预处理几何图形。 - leone ruggiero
1个回答
33
对于单独的线段，这并不是很复杂。例如绘制GL_LINES基元。
诀窍在于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设备坐标传递到片段着色器。一次使用默认插值，一次使用无（flat）插值。这导致在片段着色器中，第一个输入参数包含实际位置的NDC坐标，而后者则是线段起点的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;
}

此外，片元着色器还具有统一变量，除了不同的输入。{{u_resolution}} 包含视口的宽度和高度。{{u_dashSize}} 包含线条的长度，{{u_gapSize}} 包含像素中的间隙长度。
因此，可以计算从起点到实际片元的线条长度：
vec2  dir  = (vertPos.xy-startPos.xy) * u_resolution/2.0;
float dist = length(dir);

而且可以通过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;

片元着色器：

#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);
}

以下是一个简单的演示程序，我使用了GLFW API创建窗口，使用GLEW加载OpenGL以及GLM-OpenGL Mathematics用于数学计算。我没有提供函数CreateProgram的代码，该函数只是从顶点着色器和片段着色器源代码创建程序对象：

#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，情况会变得有些复杂。例如，绘制GL_LINE_STRIP基元。
在不知道所有线条基元的情况下，无法在着色器程序中计算线条长度。即使已知所有基元（例如SSBO），则计算也必须在循环中完成。我决定向着色器程序添加一个额外的属性，其中包含从线的起点到顶点坐标的“距离”。所谓的“距离”是指多边形在视口上的投影长度。
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);
}

片元着色器：

#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。每个顶点坐标都会通过模型、视图、投影矩阵进行变换。最后，它从标准化设备空间转换为窗口空间。计算线条相邻坐标之间的XY距离，并沿着线条累加长度，将其分配给相应的属性值。
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
在OpenGL 3.1中已弃用的glLineStipple
OpenGL ES - 虚线

非常好的回答。我看到你给了我们两套着色器，一套用于GL_LINES，另一套用于GL_LINE_STRIP。 - undefined
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