Drawing Circle with OpenGL

Question

I m trying to draw simple circle with C   OpenGl  my code is    include  lt GL glut h gt   include  lt math h gt   void Draw         glClear GL COLOR BUFFER BIT       glColor3f 1 0  1 0  1 0        glBegin GL QUADS         glColor3f  0 0  0 0  0 0          glVertex3f  0 1  0 1  0 0         glVertex3f  0 9  0 1  0 0         glVertex3f  0 9  0 9  0 0         glVertex3f  0 1  0 9  0 0        glEnd         glFlush       void DrawCircle float cx  float cy  float r  int num segments        glBegin GL LINE LOOP       for int ii   0  ii  lt  num segments  ii                  float theta   2 0f   3 1415926f   float ii    float num segments    get the current angle          float x   r   cosf theta    calculate the x component         float y   r   sinf theta    calculate the y component          glVertex2f x   cx  y   cy    output vertex            glEnd        void Initialize         glClearColor 1 0  1 0  1 0  0 0       glMatrixMode GL PROJECTION       glLoadIdentity        glOrtho 0 0  1 0  0 0  1 0  -1 0  1 0      int main int iArgc  char   cppArgv        glutInit  amp iArgc  cppArgv       glutInitDisplayMode GLUT SINGLE   GLUT RGB       glutInitWindowSize 950  500       glutInitWindowPosition 200  200       glutCreateWindow  Universum        Initialize        glutDisplayFunc Draw       glClear GL COLOR BUFFER BIT       glColor3f 1 0  1 0  1 0       DrawCircle 0 5  0 5  0 2  5        glutMainLoop        return 0       I m beginner with OpenGL and now i m starting to learn   Can someone please explain me why i don t get the circle  i only see the black box    Thanks

User · Answer

We will find the value of X and Y from this image. We know, sin?=vertical/hypotenuse and cos?=base/hypotenuse from the image we can say X=base and Y=vertical. Now we can write X=hypotenuse * cos? and Y=hypotenuse * sin?.

Now look at this code

void display(){
float x,y;
glColor3f(1, 1, 0);
for(double i =0; i <= 360;){
    glBegin(GL_TRIANGLES);
    x=5*cos(i);
    y=5*sin(i);
    glVertex2d(x, y);
    i=i+.5;
    x=5*cos(i);
    y=5*sin(i);
    glVertex2d(x, y);
    glVertex2d(0, 0);
    glEnd();
    i=i+.5;
}
glEnd();

glutSwapBuffers();
}

User · Answer

include  lt Windows h gt   include  lt GL glu h gt   include  lt GL glut h gt   include  lt stdio h gt   include  lt stdlib h gt   include  lt math h gt   define window width  1080    define window height 720  void drawFilledSun          static float angle      glClear GL COLOR BUFFER BIT   GL DEPTH BUFFER BIT       glLoadIdentity        glTranslatef 0  0  -10       int i  x  y      double radius   0 30        glColor3ub 253  184  19            glColor3ub 255  0  0       double twicePi   2 0   3 142      x   0  y   0      glBegin GL TRIANGLE FAN     BEGIN CIRCLE     glVertex2f x  y      center of circle     for  i   0  i  lt   20  i                glVertex2f                x    radius   cos i   twicePi   20      y    radius   sin i   twicePi   20                             glEnd      END   void DrawCircle float cx  float cy  float r  int num segments        glBegin GL LINE LOOP       for  int ii   0  ii  lt  num segments  ii                float theta   2 0f   3 1415926f   float ii    float num segments    get the current angle          float x   r   cosf theta    calculate the x component          float y   r   sinf theta    calculate the y component          glVertex2f x   cx  y   cy    output vertex            glEnd      void main loop function         int c      drawFilledSun        DrawCircle 0  0  0 7  100       glutSwapBuffers        c   getchar      void GL Setup int width  int height        glViewport 0  0  width  height       glMatrixMode GL PROJECTION       glEnable GL DEPTH TEST       gluPerspective 45   float width   height   1  100       glMatrixMode GL MODELVIEW     int main int argc  char   argv        glutInit  amp argc  argv       glutInitWindowSize window width  window height       glutInitDisplayMode GLUT RGB   GLUT DOUBLE       glutCreateWindow  GLUT Example           glutIdleFunc main loop function       GL Setup window width  window height       glutMainLoop        This is what I did  I hope this helps  Two types of circle are here  Filled and unfilled

User · Answer

glBegin GL POLYGON                             Middle circle double radius   0 2  double ori x   0 0                             the origin or center of circle double ori y   0 0  for  int i   0  i  lt   300  i          double angle   2   PI   i   300      double x   cos angle    radius      double y   sin angle    radius      glVertex2d ori x   x  ori y   y     glEnd

User · Answer

Here is a code to draw a fill elipse  you can use the same method but replacing de xcenter and y center with radius void drawFilledelipse GLfloat x  GLfloat y  GLfloat xcenter GLfloat ycenter        int i      int triangleAmount   20      of triangles used to draw circle        GLfloat radius   0 8f    radius     GLfloat twicePi   2 0f   PI       glBegin GL TRIANGLE FAN       glVertex2f x  y      center of circle     for  i   0  i  lt   triangleAmount  i              glVertex2f              x     xcenter 1   cos i   twicePi   triangleAmount                y     ycenter-1   sin i   twicePi   triangleAmount                        glEnd

User · Answer

There is another way to draw a circle - draw it in fragment shader  Create a quad   float right   0 5  float bottom   -0 5  float left   -0 5  float top   0 5  float quad 20            x  y  z  lx  ly     right  bottom  0  1 0  -1 0      right  top  0  1 0  1 0      left  top  0  -1 0  1 0      left  bottom  0  -1 0  -1 0       Bind VBO   unsigned int glBuffer  glGenBuffers 1   amp glBuffer   glBindBuffer GL ARRAY BUFFER  glBuffer   glBufferData GL ARRAY BUFFER  sizeof float  20  quad  GL STATIC DRAW     and draw    define BUFFER OFFSET i    char   NULL    i   glEnableVertexAttribArray ATTRIB VERTEX   glEnableVertexAttribArray ATTRIB VALUE   glVertexAttribPointer ATTRIB VERTEX   3  GL FLOAT  GL FALSE  20  0   glVertexAttribPointer ATTRIB VALUE   2  GL FLOAT  GL FALSE  20  BUFFER OFFSET 12    glDrawArrays GL TRIANGLE FAN  0  4     Vertex shader  attribute vec2 value  uniform mat4 viewMatrix  uniform mat4 projectionMatrix  varying vec2 val  void main         val   value      gl Position   projectionMatrix viewMatrix vertex      Fragment shader  varying vec2 val  void main         float R   1 0      float R2   0 5      float dist   sqrt dot val val        if  dist  gt   R    dist  lt   R2            discard            float sm   smoothstep R R-0 01 dist       float sm2   smoothstep R2 R2 0 01 dist       float alpha   sm sm2      gl FragColor   vec4 0 0  0 0  1 0  alpha       Don t forget to enable alpha blending   glEnable GL BLEND   glBlendFunc GL SRC ALPHA GL ONE MINUS SRC ALPHA     UPDATE  Read more

User · Answer

It looks like immediately after you draw the circle  you go into the main glut loop  where you ve set the Draw   function to draw every time through the loop  So it s probably drawing the circle  then erasing it immediately and drawing the square  You should probably either make DrawCircle   your glutDisplayFunc    or call DrawCircle   from Draw

User · Answer

I have done it using the following code   glBegin GL GL LINE LOOP        for int i  0  i  lt   300  i              double angle   2   Math PI   i   300           double x   Math cos angle            double y   Math sin angle            gl glVertex2d x y          glEnd

[c++] Drawing Circle with OpenGL

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