Displaying in openGL based on a matrix - c

Question

I am trying to draw some shapes in the openGL window. I draw these shapes based on the values in a particular matrix. I am using glut which has a function glutDisplayFunc that takes 1 parameter, a function callback taking no arguments and returns void. But I need to draw an image on the window based on a matrix which I cannot pass to the function callback.
This is an example code
#include<stdio.h>
#include<GL/glut.h>
#include<math.h>
#define pi 3.142857
void mat()
{
int a[2][2];
//
for(int i=0;i<2;i++)
for (int j = 0; j < 2; ++j)
{
scanf("%d",&a[i][j]);
}
}
// function to initialize
void myInit (void)
{
glClearColor(0.0, 0.0, 0.0, 1.0);
glColor3f(0.0, 1.0, 0.0);
glPointSize(1.0);
glMatrixMode(GL_PROJECTION);
glLoadIdentity();
gluOrtho2D(-780, 780, -420, 420);
}
void display (void)
{
glClear(GL_COLOR_BUFFER_BIT);
glBegin(GL_POINTS);
float x, y, i;
for ( i = 0; i < (2 * pi); i += 0.001)
{
x = 200 * cos(i);
y = 200 * sin(i);
glVertex2i(x, y);
}
glEnd();
glFlush();
}
int main (int argc, char** argv)
{
glutInit(&argc, argv);
glutInitDisplayMode(GLUT_SINGLE | GLUT_RGB);
// giving window size in X- and Y- direction
glutInitWindowSize(1366, 768);
glutInitWindowPosition(0, 0);
glutCreateWindow("Circle Drawing");
myInit();
glutDisplayFunc(display);
glutMainLoop();
}
I need to be able to use the matrix a in function mat to define the center of 2 circles. How do I draw the window from within the mat function?
Edit:included code and fixed some typos

Answer 1

void display(void)
{
glClear(GL_COLOR_BUFFER_BIT);
//-----------
float a[4][4] = {
1,0,0,0,
0,1,0,0,
0,0,1,0,
0,0,0,1 };
glMatrixMode(GL_MODELVIEW);
glLoadIdentity();
glLoadMatrixf((float*)a);
//----------
glBegin(GL_POINTS);
float x, y, i;
for (i = 0; i < (2 * pi); i += 0.001)
{
x = 200 * cos(i);
y = 200 * sin(i);
glVertex2i(x, y);
}
glEnd();
glFlush();
}

Answer 2

In general you can load the current model view matrix, by setting the GL_MODELVIEW matrix mode (glMatrixMode), and loading the matrix by glLoadMatrixf.
Optionally the matrix can be multiplied to the current matrix by glMultMatrix.
But in both cases, the matrix has to be 4x4 Transformation matrix. The parameter to both functions is a pointer to an array of 16 floats respectively an 2 dimensional 4x4 float-array.
Init a 4x4 Identity matrix and read the upper left 2x2, to set up a rotation matrix around the z-axis:
Further, I recommend to read an rotation angle in degree and to calculate the rotation axis by the trigonometric functions sin respectively cos.
Finally read the xy translation components:
#define _USE_MATH_DEFINES
#include <math.h>
float a[4][4];
void mat()
{
// init identity matrix
for(int i = 0; i < 4; i++)
for (int j = 0; j < 4; ++j)
a[i][j] = (i==j) ? 1.0f : 0.0f;
// read the angle in degrees
float angle_degree;
scanf("%f", &angle_degree);
// convert the angle to radian
float angle_radiant = angle_degree * (float)M_PI / 180.0f;
// set rotation around z-axis
float cos_ang = cos(angle_radiant);
float sin_ang = sin(angle_radiant);
a[0][0] = cos_ang;
a[0][1] = -sin_ang;
a[1][0] = sin_ang;
a[1][1] = cos_ang;
// read translation
scanf("%f", &a[3][0]);
scanf("%f", &a[3][1]);
}
void display (void)
{
glMatrixMode(GL_MODELVIEW);
glLoadMatrixf(&a[0][0]);
// [...]
}