I'm trying to draw simple circle with C++/OpenGl
my code is:
#include <GL/glut.h>
#include <math.h>
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 < 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(&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!
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 <= 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();
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
I have done it using the following code,
glBegin(GL.GL_LINE_LOOP);
for(int i =0; i <= 300; i++){
double angle = 2 * Math.PI * i / 300;
double x = Math.cos(angle);
double y = Math.sin(angle);
gl.glVertex2d(x,y);
}
glEnd();
#include <Windows.h>
#include <GL/glu.h>
#include <GL/glut.h>
#include <stdio.h>
#include <stdlib.h>
#include <math.h>
#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 <= 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 < 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(&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.
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();
}
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 <= triangleAmount; i++) {
glVertex2f(
x + ((xcenter+1)* cos(i * twicePi / triangleAmount)),
y + ((ycenter-1)* sin(i * twicePi / triangleAmount))
);
}
glEnd();
}
Source: Stackoverflow.com