c++ - 将三角形带转换为八边形？

Question

我正在使用 gl.h 库，它输出三角形条。该示例显示了这一点:

#include <windows.h>
#include <gl/gl.h>
#include <math.h>
#include <stdio.h>
#define step 8
#define angle 3.1415926 * 2.f / step

LRESULT CALLBACK WindowProc(HWND, UINT, WPARAM, LPARAM);
void EnableOpenGL(HWND hwnd, HDC*, HGLRC*);
void DisableOpenGL(HWND, HDC, HGLRC);


int WINAPI WinMain(HINSTANCE hInstance,
                   HINSTANCE hPrevInstance,
                   LPSTR lpCmdLine,
                   int nCmdShow)
{
    WNDCLASSEX wcex;
    HWND hwnd;
    HDC hDC;
    HGLRC hRC;
    MSG msg;
    BOOL bQuit = FALSE;
    float theta = 0.0f;

    /* register window class */
    wcex.cbSize = sizeof(WNDCLASSEX);
    wcex.style = CS_OWNDC;
    wcex.lpfnWndProc = WindowProc;
    wcex.cbClsExtra = 0;
    wcex.cbWndExtra = 0;
    wcex.hInstance = hInstance;
    wcex.hIcon = LoadIcon(NULL, IDI_APPLICATION);
    wcex.hCursor = LoadCursor(NULL, IDC_ARROW);
    wcex.hbrBackground = (HBRUSH)GetStockObject(BLACK_BRUSH);
    wcex.lpszMenuName = NULL;
    wcex.lpszClassName = "GLSample";
    wcex.hIconSm = LoadIcon(NULL, IDI_APPLICATION);;


    if (!RegisterClassEx(&wcex))
        return 0;

    /* create main window */
    hwnd = CreateWindowEx(0,
                          "GLSample",
                          "OpenGL Sample",
                          WS_OVERLAPPEDWINDOW,
                          CW_USEDEFAULT,
                          CW_USEDEFAULT,
                          600,
                          600,
                          NULL,
                          NULL,
                          hInstance,
                          NULL);

    ShowWindow(hwnd, nCmdShow);

    /* enable OpenGL for the window */
    EnableOpenGL(hwnd, &hDC, &hRC);

    /* program main loop */
    while (!bQuit)
    {
        /* check for messages */
        if (PeekMessage(&msg, NULL, 0, 0, PM_REMOVE))
        {
            /* handle or dispatch messages */
            if (msg.message == WM_QUIT)
            {
                bQuit = TRUE;
            }
            else
            {
                TranslateMessage(&msg);
                DispatchMessage(&msg);
            }
        }
        else
        {
            /* OpenGL animation code goes here */

            glClearColor(0.0f, 0.0f, 0.0f, 0.0f);
            glClear(GL_COLOR_BUFFER_BIT);

            glPushMatrix();
            //glRotatef(theta, 0.0f, 0.0f, 1.0f);

  
                float xPos = 0;
                float yPos = 0;
                float radius = 0.5f;
                float prevX = xPos;
                float prevY = yPos - radius;


            for (int i=0;i<=step;i++)
            {
                float newX =   radius *   sin(angle * i);
                float newY = - radius * cos(angle * i);

                glBegin(GL_TRIANGLES);
                glColor3f(0,0.5f,0);

                glVertex3f(0.0f,0.0f,0.0f);
                glVertex3f(prevX , prevY , 0.0f);
                glVertex3f(newX , newY   , 0.0f);



                prevX = newX;
                prevY = newY;


            }

            glEnd();






            glPopMatrix();
            SwapBuffers(hDC);
            theta += 1.0f;
            Sleep (1);
        }
    }

    /* shutdown OpenGL */
    DisableOpenGL(hwnd, hDC, hRC);

    /* destroy the window explicitly */
    DestroyWindow(hwnd);

    return msg.wParam;
}

LRESULT CALLBACK WindowProc(HWND hwnd, UINT uMsg, WPARAM wParam, LPARAM lParam)
{
    switch (uMsg)
    {
        case WM_CLOSE:
            PostQuitMessage(0);
        break;

        case WM_DESTROY:
            return 0;

        case WM_KEYDOWN:
        {
            switch (wParam)
            {
                case VK_ESCAPE:
                    PostQuitMessage(0);
                break;
            }
        }
        break;

        default:
            return DefWindowProc(hwnd, uMsg, wParam, lParam);
    }

    return 0;
}

void EnableOpenGL(HWND hwnd, HDC* hDC, HGLRC* hRC)
{
    PIXELFORMATDESCRIPTOR pfd;

    int iFormat;

    /* get the device context (DC) */
    *hDC = GetDC(hwnd);

    /* set the pixel format for the DC */
    ZeroMemory(&pfd, sizeof(pfd));

    pfd.nSize = sizeof(pfd);
    pfd.nVersion = 1;
    pfd.dwFlags = PFD_DRAW_TO_WINDOW |
                  PFD_SUPPORT_OPENGL | PFD_DOUBLEBUFFER;
    pfd.iPixelType = PFD_TYPE_RGBA;
    pfd.cColorBits = 24;
    pfd.cDepthBits = 16;
    pfd.iLayerType = PFD_MAIN_PLANE;

    iFormat = ChoosePixelFormat(*hDC, &pfd);

    SetPixelFormat(*hDC, iFormat, &pfd);

    /* create and enable the render context (RC) */
    *hRC = wglCreateContext(*hDC);

    wglMakeCurrent(*hDC, *hRC);
}

void DisableOpenGL (HWND hwnd, HDC hDC, HGLRC hRC)
{
    wglMakeCurrent(NULL, NULL);
    wglDeleteContext(hRC);
    ReleaseDC(hwnd, hDC);
}

                float xPos = 0;
                float yPos = 0;
                float radius = 0.5f;
                float prevX = xPos;
                float prevY = yPos - radius;


        for (int i=0;i<=step;i++)
        {
            float newX =   radius *   sin(angle * i);
            float newY = - radius * cos(angle * i);

            glBegin(GL_TRIANGLES);
            glColor3f(0,0.5f,0);

            glVertex3f(0.0f,0.0f,0.0f);
            glVertex3f(prevX , prevY , 0.0f);
            glVertex3f(newX , newY   , 0.0f);



            prevX = newX;
            prevY = newY;


        }

        glEnd();

问题在于，这会渲染多个三角形带。这对我来说是个问题。我的应用程序使用 IDE 渲染 - code Blocks v17.12，我需要一种方法来修改上述代码，以便它看起来更像这样:

Answer 1

使用多边形光栅化模式GL_LINE渲染线框:

glPolygonMode(GL_FRONT_AND_BACK, GL_LINE);

float xPos = 0.0f;
float yPos = 0.0f;
float radius = 0.5f;
int steps = 8;

float prevX = xPos;
float prevY = yPos - radius;

glBegin(GL_TRIANGLES);
glColor3f(0.0f, 0.5f, 0.0f);

for (int i=0; i <= step; ++i)
{
    float angle = 3.1415926f * 2.0f * i / step;
    float newX =   radius * sin(angle);
    float newY = - radius * cos(angle);

    glVertex3f(0.0f, 0.0f, 0.0f);
    glVertex3f(prevX, prevY, 0.0f);
    glVertex3f(newX, newY, 0.0f);

    prevX = newX;
    prevY = newY;
}
glEnd();

使用 GL_TRIANGLE_FAN Triangle primitive简化您的代码:

glBegin(GL_TRIANGLE_FAN);
glColor3f(0.0f, 0.5f, 0.0f);

for (int i=0; i <= step; ++i)
{
    float angle = 3.1415926f * 2.0f * i / step;
    float newX =   radius * sin(angle);
    float newY = - radius * cos(angle);
    glVertex3f(newX, newY, 0.0f);
}
glEnd();

如果你想使用三角形GL_TRIANGLE_STRIP ，您必须更改顶点的顺序:

glBegin(GL_TRIANGLE_STRIP);
glColor3f(0.0f, 0.5f, 0.0f);

for (int i=0; i <= step; ++i)
{
    int strip_i = i % 2 == 0 ? i/2 : step - i/2;
    float angle = 3.1415926f * 2.0f * strip_i / step;
    float newX =   radius * sin(angle);
    float newY = - radius * cos(angle);
    glVertex3f(newX, newY, 0.0f);
}
glEnd();

如果要在多边形顶部渲染线框，则需要启用depth test并首先渲染多边形。请注意，默认深度测试函数为 GL_LESS :

for (int pass = 0; pass < 2; ++pass)
{
    if (pass == 0)
    {
        glPolygonMode(GL_FRONT_AND_BACK, GL_LINE);
        glColor3f(1.0f, 1.0f, 1.0f);
    }
    else
    {
        glPolygonMode(GL_FRONT_AND_BACK, GL_FILL);
        glColor3f(0.0f, 0.5f, 0.0f);
    }

    glBegin(GL_TRIANGLE_STRIP);
    for (int i=0; i <= step; ++i)
    {
        int strip_i = i % 2 == 0 ? i/2 : step - i/2;
        float angle = 3.1415926f * 2.0f * strip_i / step;
        float newX =   radius * sin(angle);
        float newY = - radius * cos(angle);
        glVertex3f(newX, newY, 0.0f);
    }
    glEnd();
}