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

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, &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 >= R || dist <= 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

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

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

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

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

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  lt   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

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

[c++] Drawing Circle with OpenGL

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