Repeating an image many times in OpengGL

Question

I have drawn a circle in OpenGL. What I would like to do is to repeatedly show a part of it. For example, instead of showing just one circle, showing 4 of half circle continuously. I thought it is possible by having two for loops and using a glViewport(parameters) inside. But I failed to do it so.

I have my code here, the for loops are at beginning of circle() function. I would appreciate if someone can help me on this.

#include <Windows.h>
#include <iostream>
#include <stdio.h>
#include <math.h>
#include <stdlib.h>
#include <GL/glut.h>
#include <GL/gl.h>
#include <GL/glu.h>
#include <fstream>


using namespace std;

int pntX1, pntY1, r;

void myInit()
{
    glClearColor(1.0, 1.0, 1.0, 0.0);
    glColor3f(0.0f, 0.0f, 0.0f);
    glPointSize(4.0);
    glMatrixMode(GL_PROJECTION);
    glLoadIdentity();
    gluOrtho2D(0.0, 101.0, 0.0, 101.0);
}

void drawPolyLineFile(char * fileName)
{
    fstream inStream;
    inStream.open(fileName, ios::in);
    if (inStream.fail())
        return;
    glClear(GL_COLOR_BUFFER_BIT);
    GLint numpolys, numlines, x, y;
    inStream >> numpolys;

    for (int j = 0; j < numpolys; j++)
    {
        inStream >> numlines;
        glBegin(GL_LINE_STRIP);
        for (int i = 0; i < numlines; i++)
        {
            inStream >> x >> y;
            glVertex2i(x, y);
        }
        glEnd();
    }
    glFlush();
    inStream.close();
}

void writePixel(GLint x, GLint y)
{
    glBegin(GL_POINTS);
    glVertex2i(x + pntX1, y + pntY1);
    glEnd();
}

void circlePoints(int x, int y) {
    writePixel(x, y);
    writePixel(y, x);
    writePixel(y, -x);
    writePixel(x, -y);
    writePixel(-x, -y);
    writePixel(-y, -x);
    writePixel(-y, x);
    writePixel(-x, y);
}

void setWindow(GLdouble left, GLdouble right, GLdouble bottom, GLdouble top) {
    glClearColor(1.0, 1.0, 1.0, 0.0);
    glColor3f(0.0f, 0.0f, 0.0f);
    glMatrixMode(GL_PROJECTION);
    glLoadIdentity();
    gluOrtho2D(left, right, bottom, top);
}

void setViewPort(GLint left, GLint right, GLint bottom, GLint top)
{
    glViewport(left, bottom, right - left, top - bottom);
}
void Circle() {
    //int x1=100,y1=100,r=50;
    //int x=0,y=r;
    //int d = 3/2 - r;

    //glViewport(50, 50, 50, 50);
    /*
    setWindow(0.0, 101.0, 0.0, 101.0);
    for (int i = 0; i<1; i++)
        for (int j = 0; j < 1; j++)
        {
            //glViewport(i * 50 +, j * 50, 50, 50);
            glViewport(50, 50, 50, 50);
            drawPolyLineFile("dino.dat");
        }
    */

    //glViewport(0, 0, 0, 0);

    glClear(GL_COLOR_BUFFER_BIT);
    glColor3f(1, 0, 0);

    int x = 0;
    int r = 50;
    int y = r;
    int h = (1 - r);
    int deltaE = 3;
    int deltaSE = (-2)*r + 5;

    circlePoints(x, y);

    while (y > x) {

        if (h<0) {
            h += deltaE;
            deltaE += 2;
            deltaSE += 2;
        }
        else {
            h += deltaSE;
            deltaE += 2;
            deltaSE += 4;
            y--;
        }
        x++;
        circlePoints(x, y);
    }

}

void Display() {
    glClear(GL_COLOR_BUFFER_BIT);
    glColor3f(0.0, 0.0, 0.0);
    glPointSize(1.0);

    Circle();

    glFlush();
}


int main(int argc, char *argv[])
{
    pntX1 = 50; pntY1 = 50; r = 50;

    glutInit(&argc, argv);
    glutInitDisplayMode(GLUT_SINGLE | GLUT_RGB);
    glutInitWindowSize(101, 101.0);
    glutInitWindowPosition(100, 150);
    glutCreateWindow("My circles");


    //glViewport(0, 0, 100, 100);

    /*
    setWindow(0, 500.0, 0, 500.0);
    for(int i=0; i<2; i++)
        for(int j=0; j<2; j++)
        {
        glViewport(i*250, j*250, 250, 250);
        //drawPolylineFile("circles.dat");
        }
        */


    glutDisplayFunc(Display);
    myInit();

    glutMainLoop();
    return 0;
}

score 0 · Answer 1 · edited May 23 '17 at 11:53

changing Viewport is not what you need use matrices instead:

for (i=0;i<N;i++) // N is number of objects you want to render
 {
 glMatrixMode(GL_MODELVIEW);
 glPushMatrix();
 glTranslate3(dx[i],dy[i],dz[i]); // dx,dy,dz is offset of object i where you want to draw it
 // render i-th object occurrence
 glPopMatrix();
 }

Also hope you know you can draw circle with GL_LINE_LOOP and GL_TRIANGLE_FAN primitives much faster.

Do not change glViewPort as it is not what you think it is! It maps your screen space and if you are changing it during rendering then you are invalidating depth and any aux buffers so you can not use any advanced stuff anymore.

For more info see:

Understanding 4x4 homogenous transform matrices

Repeating an image many times in OpengGL

1 Answers1