Making the square bigger when it moves - c++

Question

#include <stdio.h> // this library is for standard input and output
#include "glut.h" // this library is for glut the OpenGL Utility Toolkit
#include <math.h>
float squareX = 0.0f;
float squareY = 200.0f;
static int flag = 1;
void drawShape(void) {
float width = 58.0f;
float height = 40.0f;
glTranslatef(squareX, squareY, 0);
// test
// glScalef(0.0, 0.0, 0.0);
glBegin(GL_POLYGON);
glColor3f(1.0, 0.0, 0.0);
glVertex2f(0, 0);
glVertex2f(width, 0);
glVertex2f(width, height);
glVertex2f(0, height);
glVertex2f(0, 0);
glEnd();
}
void initRendering() {
glEnable(GL_DEPTH_TEST);
}
// called when the window is resized
void handleResize(int w, int h) {
glViewport(0, 0, w, h);
glMatrixMode(GL_PROJECTION);
glLoadIdentity();
glOrtho(0.0f, (float)w, 0.0f, (float)h, -1.0f, 1.0f);
}
int state = 1;
void update(int value) {
if (state == 1) { // 1 : move right
squareX += 1.0f;
if (squareX > 400.0) {
state = 0;
}
}
glutPostRedisplay();
glutTimerFunc(25, update, 0);
}
void display() {
glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
glMatrixMode(GL_MODELVIEW);
glLoadIdentity();
drawShape();
glutSwapBuffers();
}
int main(int argc, char** argv) {
glutInit(&argc, argv);
glutInitDisplayMode(GLUT_DOUBLE | GLUT_RGB | GLUT_DEPTH);
glutInitWindowSize(400, 400);
glutCreateWindow("Moving Square");
initRendering();
glutDisplayFunc(display);
glutReshapeFunc(handleResize);
glutTimerFunc(25, update, 0);
glutMainLoop();
return(0);
}
I want to make the square get bigger while it is moving to the right. See the second GIF below. I know that I need glScalef to make the square bigger but I don't know how to make it bigger while it is moving.
Code preview:
I need it to do something similar to this (sorry about the quality, I created the GIF myself):

Answer 1

Use glScale to scale the rectangle dependent on the X position (squareX):
float rectScale = 1.0f + (squareX / 400.0f);
glScalef(rectScale, rectScale, 1.0f);
Note squareX is in range [0.0, 400.0], so 1.0f + (squareX / 400.0f) is in range [1.0, 2.0].
First the scaling has to be applied to the rectangle. This means it has to be the last operation, which is applied to the model view matrix, before the rectangle is drawn. the final function drawShape may look like this:
void drawShape(void) {
float width = 58.0f;
float height = 40.0f;
glTranslatef(squareX, squareY, 0);
float rectScale = 1.0f + (squareX / 400.0f);
glScalef(rectScale, rectScale, 1.0f);
glBegin(GL_POLYGON);
glColor3f(1.0, 0.0, 0.0);
glVertex2f(0, 0);
glVertex2f(width, 0);
glVertex2f(width, height);
glVertex2f(0, height);
glVertex2f(0, 0);
glEnd();
}
Preview: