OpenGL绘制图形

坐标系定义

屏幕空间坐标系，以屏幕中心为原点，左下角为(-1,-1)，右上角为(1,1)，符合右手坐标系。在OpenGL中规定，x，y轴的取值范围均为[-1,1]。

当屏幕不是正方形的时候，画圆可能被压成椭圆，这是需要通过宽高比进行调整比例。

绘制图形

在默认情况下，OpenGL会默认把世界空间坐标当作屏幕空间坐标。

使用glBegin(GL_xxx)用来指定绘图模式，用glVertex3f指定在该种模式下的一些顶点坐标，最后调用glEnd()结束绘制。

glBegin(GL_POINTS)
// 在原点处画一个点
glVertex3f(0.0f, 0.0f, 0.0f);
glEnd()

绘图模式包括:

• 点：GL_POINTS
• 线；GL_LINES
• 面：GL_TRIANGLES

调整点的大小

利用glPointSize()设置点的像素大小

使用glEnable()可以启用一些特性，例如GL_POINT_SMOOTH让点的形状变为圆形，使用GL_BLEND开启抗锯齿等。

以上设置可以放在main函数中，一次设置，永久生效，知道再次设置或取消。

调整点的颜色

使用glColor3f指定颜色，格式是按照（R, G, B）三个0到1之间的浮点数指定。如下是一个示例：

glBegin(GL_POINTS);
glColor3f(0.1137f, 0.2980f, 0.3137f);
glVertex3f(0.0f, 0.0f, 0.0f);
glColor3f(0.8275f, 0.6431f, 0.5333f);
glVertex3f(0.0f, 0.4f, 0.0f);
glColor3f(0.7412f, 0.6824f, 0.6784f);
glVertex3f(-0.3f, -0.2f, 0.0f);
glColor3f(0.9412f, 0.6549f, 0.1804f);
glVertex3f(0.3f, -0.2f, 0.0f);
glEnd();

其效果如图：

如果绘制的是三角形，那么如果指定了颜色，则会呈现渐变的效果：

glBegin(GL_TRIANGLES);
glColor3f(0.1137f, 0.2980f, 0.3137f);
glVertex3f(0.0f, 0.6f, 0.0f);
glColor3f(0.7412f, 0.6824f, 0.6784f);
glVertex3f(-0.3f, 0.0f, 0.0f);
glColor3f(0.9412f, 0.6549f, 0.1804f);
glVertex3f(0.3f, 0.0f, 0.0f);

glColor3f(0.8275f, 0.6431f, 0.5333f);
glVertex3f(-0.6f, -0.6f, 0.0f);
glVertex3f(0.6f, -0.6f, 0.0f);
glVertex3f(0.0f, 0.0f, 0.0f);
glEnd();

画点好玩的

1. 加载中...

static float current = 0.0f;
glColor3f(0.0f, 0.5f, 0.4f);
glBegin(GL_POINTS);
constexpr float r = 0.8f;
constexpr int n = 15;
constexpr float v = 0.05f;
constexpr float pi = 3.1415926535897932f;
for (int i = 0; i < n; ++i)
{
    if (std::abs(i-static_cast<int>(current))<1e-7) continue;
    float angle = i / static_cast<float>(n) * pi * 2.0f;
    glVertex3f(r * std::sin(angle) / WdH, r * std::cos(angle), 0.0f);
}
current += v;
if (current > n) current = 0;
glEnd();

2. OpenCV

void paint(float angle, float angle_next, float angle_offset, float x, float y)
{
  angle += angle_offset;
  angle_next += angle_offset;
  constexpr float R = 0.4f;
  constexpr float r = R / 3.0f;
  glVertex3f((x+r * std::cos(angle)) / WdH, y+r * std::sin(angle), 0.0f);
  glVertex3f((x+R * std::cos(angle)) / WdH, y+R * std::sin(angle), 0.0f);
  glVertex3f((x+R * std::cos(angle_next)) / WdH, y+R * std::sin(angle_next), 0.0f);
  glVertex3f((x+r * std::cos(angle)) / WdH, y+r * std::sin(angle), 0.0f);
  glVertex3f((x+r * std::cos(angle_next)) / WdH, y+r * std::sin(angle_next), 0.0f);
  glVertex3f((x+R * std::cos(angle_next)) / WdH, y+R * std::sin(angle_next), 0.0f);
}
void render()
{
  glBegin(GL_TRIANGLES);
  constexpr int n = 200;
  constexpr float pi = 3.1415926535897932f;
  float angle_offset = 0;
  float zoom = 2.6f;
  for (int i = 0; i < n; ++i)
  {
      float angle = i / static_cast<float>(n) * (2 * pi * 5.0f / 6.0f);
      float angle_next = (i + 1) / static_cast<float>(n) * (2 * pi * 5.0f / 6.0f);
      angle_offset = -pi / 3.0f;
      glColor3f(1.0f, 0.0f, 0.0f);
      paint(angle, angle_next, angle_offset, 0.0f, 0.2f*zoom);

      angle_offset = pi / 3.0f;
      glColor3f(0.0f, 1.0f, 0.0f);
      paint(angle, angle_next, angle_offset, -0.1732f*zoom, -0.1f*zoom);

      angle_offset = pi / 3.0f * 2.0f;
      glColor3f(0.0f, 0.0f, 1.0f);
      paint(angle, angle_next, angle_offset, 0.1732f*zoom, -0.1f*zoom);
  }

  glEnd();
}

3. 太极图

glBegin(GL_TRIANGLES);
constexpr int n = 100;
constexpr float R = 0.8f;
constexpr float pi = 3.1415926535897932f;
for(int i = 0; i < n; ++i) {
    float angle = i / static_cast<float>(n) * pi;
    float angle_next = (i+1) / static_cast<float>(n) * pi;
    glColor3f(0.0f, 0.0f, 0.0f);
    glVertex3f(0.0f, 0.0f, 0.0f);
    glVertex3f(R*std::sin(angle)/WdH, R*std::cos(angle), 0.0f);
    glVertex3f(R*std::sin(angle_next)/WdH, R*std::cos(angle_next), 0.0f);

    glColor3f(1.0f, 1.0f, 1.0f);
    glVertex3f(0.0f, 0.0f, 0.0f);
    glVertex3f(R*std::sin(-angle)/WdH, R*std::cos(-angle), 0.0f);
    glVertex3f(R*std::sin(-angle_next)/WdH, R*std::cos(-angle_next), 0.0f);
}

for(int i = 0; i < n; ++i) {
    float angle = i / static_cast<float>(n) * pi;
    float angle_next = (i+1) / static_cast<float>(n) * pi;
    glColor3f(1.0f, 1.0f, 1.0f);
    glVertex3f(0.0f, R/2.0f, 0.0f);
    glVertex3f(R/2*std::sin(angle)/WdH, R/2.0f+R/2*std::cos(angle), 0.0f);
    glVertex3f(R/2*std::sin(angle_next)/WdH, R/2.0f+R/2*std::cos(angle_next), 0.0f);

    glColor3f(0.0f, 0.0f, 0.0f);
    glVertex3f(0.0f, -R/2.0f, 0.0f);
    glVertex3f(R/2*std::sin(-angle)/WdH, -R/2.0f+R/2*std::cos(-angle), 0.0f);
    glVertex3f(R/2*std::sin(-angle_next)/WdH, -R/2.0f+R/2*std::cos(-angle_next), 0.0f);
}

for(int i = 0; i < 2*n; ++i) {
    float angle = i / static_cast<float>(2*n) * 2 * pi;
    float angle_next = (i+1) / static_cast<float>(2*n) * 2 * pi;
    glColor3f(0.0f, 0.0f, 0.0f);
    glVertex3f(0.0f, R/2.0f, 0.0f);
    glVertex3f(R/6*std::sin(angle)/WdH, R/2.0f+R/6*std::cos(angle), 0.0f);
    glVertex3f(R/6*std::sin(angle_next)/WdH, R/2.0f+R/6*std::cos(angle_next), 0.0f);

    glColor3f(1.0f, 1.0f, 1.0f);
    glVertex3f(0.0f, -R/2.0f, 0.0f);
    glVertex3f(R/6*std::sin(-angle)/WdH, -R/2.0f+R/6*std::cos(-angle), 0.0f);
    glVertex3f(R/6*std::sin(-angle_next)/WdH, -R/2.0f+R/6*std::cos(-angle_next), 0.0f);
}

glEnd();