Question

Let’s say that I want to translate a polygon.If before drawing it I use glTranslatef it gets translated, but if I want to draw two polygons and translate only one, how do I do?

#include <OpenGL/OpenGL.h>
#include <GLUT/GLUT.h>
#include "utility.h"

void init()
{
    glLoadIdentity();
    glOrtho(-1, 1, -1, 1, -1, 1);
}

void render()
{
    glClearColor(APPLE_GRAY);
    glClear(GL_COLOR_BUFFER_BIT);
    glMatrixMode(GL_MODELVIEW);
    glTranslatef(0.5, 0.0, 0.0);
    glBegin(GL_POLYGON);
    glColor4f(RED);
    glVertex2f(0,0.5);
    glVertex2f(-0.5,-0.5);
    glVertex2f(0.5, -0.5);
    glBegin(GL_POLYGON);
    glColor4f(BLUE);
    glVertex2f(0.5,0.5);
    glVertex2f(-0.5,0.5);
    glVertex2f(-0.5, -0.5);
    glVertex2f(0.5, -0.5);
    glEnd();
    glFlush();
}

I want to translate only the triangle, not the square.How to do that?

Answer 1

Before you translate, call:

glPushMatrix()

Then after drawing the first polygon, call:

glPopMatrix()

I’ve also noted you have no glEnd() call after your first polygon. Additionally, unless there is more code that’s not listed, I notice that you don’t switch from the modelview to the projection matrix when calling glOrtho(). This isn’t strictly necessary if you are only using the modelview matrix for this example, but the traditional approach is to set up your projection like this:

glMatrixMode(GL_PROJECTION);
glLoadIdentity();
glOrtho(-1, 1, -1, 1, -1, 1);

Then, your render function sets up the modelview matrix for polygon rendering. Again, not a requirement and your example looks like it will do what you need since you are only using one matrix.

Reference: glPushMatrix() OpenGL Documentation