C++ 用 OpenGL 画圆
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Drawing Circle with OpenGL
提问by tonyhlav
I'm trying to draw simple circle with C++/OpenGl
我正在尝试用 C++/OpenGl 绘制简单的圆圈
my code is:
我的代码是：
#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!
我是 OpenGL 的初学者，现在我开始学习，谁能解释一下为什么我看不到圆圈（我只看到黑框），谢谢！
采纳答案by user1118321
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().
看起来就像在您绘制圆之后，您立即进入了主要的 glut 循环，在那里您已将Draw()函数设置为每次通过循环进行绘制。所以它可能正在绘制圆圈，然后立即将其擦除并绘制正方形。您可能应该制作DrawCircle()您的glutDisplayFunc()，或DrawCircle()从调用Draw()。
回答by Farah Nazifa
#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.
这就是我所做的。我希望这有帮助。这里有两种类型的圆。填充和未填充。
回答by S.Lapin
There is another way to draw a circle - draw it in fragment shader. Create a quad:
还有另一种绘制圆的方法 - 在片段着色器中绘制它。创建一个四边形：
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:
绑定 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:
并绘制：
#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
顶点着色器
attribute vec2 value;
uniform mat4 viewMatrix;
uniform mat4 projectionMatrix;
varying vec2 val;
void main() {
val = value;
gl_Position = projectionMatrix*viewMatrix*vertex;
}
Fragment shader
片段着色器
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:
不要忘记启用 alpha 混合：
glEnable(GL_BLEND);
glBlendFunc(GL_SRC_ALPHA,GL_ONE_MINUS_SRC_ALPHA);
UPDATE: Read more
更新：阅读更多
回答by Habibur Rahman
We will find the value of X and Y from this image. We know, sinθ=vertical/hypotenuse and cosθ=base/hypotenuse from the image we can say X=base and Y=vertical. Now we can write X=hypotenuse * cosθ and Y=hypotenuse * sinθ.
我们将从这张图片中找到 X 和 Y 的值。我们知道，sinθ=vertical/hypotenuse 和 cosθ=base/hypotenuse 从图像中我们可以说 X=base 和 Y=vertical。现在我们可以写出 X=hypotenuse * cosθ 和 Y=hypotenuse * sinθ。
Now look at this code
现在看看这段代码
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();
}
回答by Ginnelle Gracia
Here is a code to draw a fill elipse, you can use the same method but replacing de xcenter and y center with radius
这是绘制填充椭圆的代码，您可以使用相同的方法，但用半径替换 de xcenter 和 y center
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();
}
}
回答by Codemaker
I have done it using the following code,
我已经使用以下代码完成了，
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();
