移动时扩大正方形

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);
}

我想使正方形向右移动时变大。请参阅下面的第二个GIF。我知道我需要glScalef来扩大正方形，但我不知道如何在正方形移动时使其扩大。

代码预览：

我需要它做类似的事情（对不起，我自己创建了GIF）：

Answer 1

使用glScale来缩放取决于X位置（squareX）的矩形：

float rectScale = 1.0f + (squareX / 400.0f);
glScalef(rectScale, rectScale, 1.0f);

注意squareX的范围是[0.0，400.0]，因此1.0f + (squareX / 400.0f)的范围是[1.0，2.0]。

首先必须将缩放比例应用于矩形。这意味着它必须是在绘制矩形之前应用于模型视图矩阵的最后一个操作。最终功能drawShape可能看起来像这样：

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();
}

预览：