Almost ready image view OpenGL module

opengl.c

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+// the simplest opengl example
+
+#include <GL/glut.h>
+#include <math.h>
+// compile:
+// gcc -Wall -Wextra -Werror -lglut -lGL -lm -std=gnu99 -D_GNU_SOURCE opengl.c -o openglcircle
+
+#define _U_    __attribute__((__unused__))
+// exit by ctrl+q or escape
+void keyPressed(unsigned char key, _U_ int x, _U_ int y){
+	int mod = glutGetModifiers(); // window = glutGetWindow()
+	if(mod == GLUT_ACTIVE_CTRL && key == 'q') exit(0);
+	else if(key == 27) exit(0);
+}
+
+void DrawCircle(float cx, float cy, float r, int num_segments){
+	float theta = 2. * M_PI / (float)num_segments;
+	float c, s, t, x = r, y = 0.;
+	sincosf(theta, &s, &c);
+	glBegin(GL_LINE_LOOP);
+	for(int ii = 0; ii < num_segments; ii++){
+		glVertex2f(x + cx, y + cy);
+		t = x;
+		x = c * x - s * y;
+		y = s * t + c * y;
+	}
+	glEnd();
+}
+
+void display(){
+	glClear(GL_COLOR_BUFFER_BIT);
+	DrawCircle(0.,0.,0.5,100);
+	glFlush();
+}
+
+int main(int argc, char** argv){
+	glutInit(&argc, argv);
+	glutInitDisplayMode(GLUT_SINGLE | GLUT_RGB);
+	glutInitWindowSize(400, 400);
+	glutCreateWindow("A Simple Circle");
+	glutDisplayFunc(display);
+	glutKeyboardFunc(keyPressed);
+	glutMainLoop();
+}