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社区首页 >问答首页 >glReadPixels()返回0

问glReadPixels()返回0
EN

Stack Overflow用户
提问于 2014-04-25 07:09:03
回答 1查看 2K关注 0票数 1

我在Windows平台上使用gl和glut

我的问题是，glReadPixels返回所有的0。我想这与我初始化窗口的方式有关，所以它无法得到正确的像素值。

我就是这样初始化窗口的：

代码语言：javascript
运行
复制
glutInit(&argc, argv);
glutInitWindowSize(800,600);

glClear(GL_COLOR_BUFFER_BIT);    

glMatrixMode (GL_PROJECTION); 
glLoadIdentity();
glOrtho(0, 800, 600, 0, 0, 1);
glMatrixMode(GL_MODELVIEW); 
glTranslatef(0.375, 0.375, 0);

glClearColor(0, 0, 0, 1.0);

通过这个，我得到了所有的0：

编辑：

代码语言：javascript
运行
复制
glColor3f(1.0, 0.0, 0.0);
glBegin(GL_QUADS);        
    glVertex2i(x-20, y-20);
    glVertex2i(x-20, y+20);
    glVertex2i(x+20, y+20);
    glVertex2i(x+20, y-20);
glEnd();
unsigned char pixel[4]; 
glReadPixels(x, y, 1, 1, GL_RGBA, GL_UNSIGNED_BYTE, pixel);

在我glClear之后，我确实呈现了一些形状，在(x，y)，然后我使用glReadPixels在(x，y)处得到颜色，但它返回0。我试图glReadPixels整个屏幕，它也返回0。

编辑2:

因此，为了更清楚地了解我的问题，下面是代码:我只是不知道问题的来源在哪里，所以我把所有的代码粘贴在这里。这是乔纳森·S·哈布尔(JonathanS.Habour)著的“游戏编程一体化”一书中的坦克程序。这本书使用快板库，我试着把它转换成openGL。在更新子弹程序中的“寻找一个命中”，我打印出敌人坦克的坐标和那个像素处的颜色，但我得到的只是0。

代码语言：javascript
运行
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#include <GL/glut.h>
#include <stdlib.h>
#include <iostream.h>
#include <windows.h>

//define tank structure
struct tagTank
{
    int x,y;
    int dir,speed;
} tanks[2];

struct tagBullet
{
    int x,y;
    int alive;
    int xspd,yspd;
} bullets[2];

void setuptanks()
{   
    tanks[0].x = 30;
    tanks[0].y = 40;
    tanks[0].dir = 1;
    tanks[0].speed = 5;

    tanks[1].x = 800 - 30;
    tanks[1].y = 600 - 30;
    tanks[1].dir = 3;
    tanks[1].speed = 5;
}

void drawtank(int num)
{
    int x = tanks[num].x;
    int y = tanks[num].y;
    int dir = tanks[num].dir;

    //draw tank body
    glColor3f(1.0, 0.0, 0.0);
    if (num) glColor3f(0.0, 0.0, 1.0);
    glBegin(GL_QUADS);        
        glVertex2i(x-20, y-20);
        glVertex2i(x-20, y+20);
        glVertex2i(x+20, y+20);
        glVertex2i(x+20, y-20);
    glEnd();

    glColor3f(0.5, 0.0, 0.0);
    if (num) glColor3f(0.0, 0.0, 0.5);
    glBegin(GL_QUADS);        
        glVertex2i(x-10, y-10);
        glVertex2i(x-10, y+10);
        glVertex2i(x+10, y+10);
        glVertex2i(x+10, y-10);
    glEnd();

    //draw the turret based on direction
    glColor3f(1.0, 1.0, 1.0);
    switch (dir)
    {
        case 0:
            glBegin(GL_QUADS);        
                glVertex2i(x-2, y-30);
                glVertex2i(x-2, y);
                glVertex2i(x+2, y);
                glVertex2i(x+2, y-30);
            glEnd();
            break;
        case 1:
            glBegin(GL_QUADS);        
                glVertex2i(x, y-2);
                glVertex2i(x, y+2);
                glVertex2i(x+30, y+2);
                glVertex2i(x+30, y-2);
            glEnd();
            break;
        case 2:
            glBegin(GL_QUADS);        
                glVertex2i(x-2, y);
                glVertex2i(x-2, y+30);
                glVertex2i(x+2, y+30);
                glVertex2i(x+2, y);
            glEnd();
            break;
        case 3:
            glBegin(GL_QUADS);        
                glVertex2i(x-30, y-2);
                glVertex2i(x-30, y+2);
                glVertex2i(x, y+2);
                glVertex2i(x, y-2);
            glEnd();
            break;
    }
}

void erasetank(int num)
{
    //calculate box to encompass the tank
    int left = tanks[num].x - 30;
    int top = tanks[num].y - 30;
    int right = tanks[num].x + 30;
    int bottom = tanks[num].y + 30;

    //erase the tank
    glColor3f(0.0, 0.0, 0.0);
    glBegin(GL_QUADS);        
        glVertex2i(left, top);
        glVertex2i(left, bottom);
        glVertex2i(right, bottom);
        glVertex2i(right, top);
    glEnd();
}

void movetank(int num)
{
    int dir = tanks[num].dir;
    int speed = tanks[num].speed;
    //update tank position based on direction
    switch(dir)
    {
    case 0:
        tanks[num].y -= speed;
        break;
    case 1:
        tanks[num].x += speed;
        break;
    case 2:
        tanks[num].y += speed;
        break;
    case 3:
        tanks[num].x -= speed;
        break;
    }

    //keep tank inside the screen
    if (tanks[num].x > 800-30)
    {
        tanks[num].x = 800-30;
        tanks[num].speed = 0;
    }

    else if (tanks[num].x < 30)
    {
        tanks[num].x = 30;
        tanks[num].speed = 0;
    }

    else if (tanks[num].y > 600-30)
    {
        tanks[num].y = 600-30;
        tanks[num].speed = 0;
    }

    else if (tanks[num].y < 30)
    {
        tanks[num].y = 30;
        tanks[num].speed = 0;
    }

    else tanks[num].speed = 5;
}

void explode(int num, int x, int y)
{
    int n;
    //retrieve location of enemy tank
    int tx = tanks[!num].x;
    int ty = tanks[!num].y;

    //is bullet inside the boundary of the enemy tank?
    if (x > tx-30 && x < tx+30 && y > ty-30 && y < ty+30)
        setuptanks();

    //draw some random circles for the "explosion"
    for (n = 0; n < 10; n++)
    {
        glColor3f((rand() % 101)/100.0, (rand() % 101)/100.0, (rand() % 101)/100.0);
        glBegin(GL_QUADS);        
            glVertex2i(x-16, y-16);
            glVertex2i(x-16, y+16);
            glVertex2i(x+16, y+16);
            glVertex2i(x+16, y-16);
        glEnd();

        //Sleep(10);
    }

    //clear the area of debris
    glColor3f(0.0, 0.0, 0.0);
    glBegin(GL_QUADS);        
        glVertex2i(x-16, y-16);
        glVertex2i(x-16, y+16);
        glVertex2i(x+16, y+16);
        glVertex2i(x+16, y-16);
    glEnd();
}

void updatebullet(int num)
{
    int x = bullets[num].x;
    int y = bullets[num].y;
    if (bullets[num].alive)
    {
        //erase bullet
        glColor3f(0.0, 0.0, 0.0);
        glBegin(GL_QUADS);        
            glVertex2i(x-2, y-2);
            glVertex2i(x-2, y+2);
            glVertex2i(x+2, y+2);
            glVertex2i(x+2, y-2);
        glEnd();

        //move bullet
        bullets[num].x += bullets[num].xspd;
        bullets[num].y += bullets[num].yspd;
        x = bullets[num].x;
        y = bullets[num].y;

        //stay within the screen
        if (x < 5 || x > 800 || y < 20 || y > 600)
        {
            bullets[num].alive = 0;
            return;
        }

        //look for a hit
        unsigned char pixel[4]; 
        glReadPixels(tanks[!num].x, tanks[!num].y, 1, 1, GL_RGBA, GL_UNSIGNED_BYTE, pixel);
        cout << tanks[!num].x << ", " << tanks[!num].y << " | " << (int)pixel[0] << ", " << (int)pixel[1] << ", " << (int)pixel[2] << endl;
        if ((int)pixel[0] || (int)pixel[1] || (int)pixel[2])
        {           
            bullets[num].alive = 0;
            explode(num, x, y);
            return;
        }       

        //draw bullet
        x = bullets[num].x;
        y = bullets[num].y;
        glColor3f(1.0, 1.0, 1.0);
        glBegin(GL_QUADS);        
            glVertex2i(x-2, y-2);
            glVertex2i(x-2, y+2);
            glVertex2i(x+2, y+2);
            glVertex2i(x+2, y-2);
        glEnd();
    }
}

void fireweapon(int num)
{
    int x = tanks[num].x;
    int y = tanks[num].y;

    //ready to fire again?
    if (!bullets[num].alive)
    {
        bullets[num].alive = 1;

        //fire bullet in direction tank is facing
        switch (tanks[num].dir)
        {
            //north
            case 0:
                bullets[num].x = x;
                bullets[num].y = y-30;
                bullets[num].xspd = 0;
                bullets[num].yspd = -20;
                break;

            //east
            case 1:
                bullets[num].x = x+30;
                bullets[num].y = y;
                bullets[num].xspd = 20;
                bullets[num].yspd = 0;
                break;

            //south
            case 2:
                bullets[num].x = x;
                bullets[num].y = y+30;
                bullets[num].xspd = 0;
                bullets[num].yspd = 20;
                break;

            //west
            case 3:
                bullets[num].x = x-30;
                bullets[num].y = y;
                bullets[num].xspd = -20;
                bullets[num].yspd = 0;
                break;
        }
    }
}

void up(int num)
{
    tanks[num].dir = 0; 
}

void down(int num)
{
    tanks[num].dir = 2; 
}

void left(int num)
{
    tanks[num].dir = 3; 
}

void right(int num)
{
    tanks[num].dir = 1; 
}

static void display(void)
{       
    erasetank(0);           
    erasetank(1);   

    movetank(0);        
    movetank(1);    

    drawtank(0);
    drawtank(1);    

    updatebullet(0);
    updatebullet(1);

    glFlush();

    Sleep(50);
}


static void key(unsigned char key, int x, int y)
{
    switch (key)
    {
        case 27 :
            exit(0);
            break;
        case 'a':
            left(1);
            break;
        case 'd':
            right(1);
            break;
        case 'w':
            up(1);
            break;
        case 's':
            down(1);
            break;
        case 32:        
            fireweapon(1);
            break;
        case 13:
            fireweapon(0);
            break;
    }
    glutPostRedisplay();
}

static void specialkey(int key, int x, int y)
{
    switch (key)
    {
        case GLUT_KEY_LEFT:
            left(0);
            break;
        case GLUT_KEY_RIGHT:
            right(0);
            break;
        case GLUT_KEY_UP:
            up(0);
            break;
        case GLUT_KEY_DOWN:
            down(0);
            break;
    }
    glutPostRedisplay();
}

static void idle(void)
{
    glutPostRedisplay();
}

int main(int argc, char *argv[])
{
    glutInit(&argc, argv);
    glutInitWindowSize(800,600);
    glutInitWindowPosition(10,10);  

    glutCreateWindow("Tanks");

    glClear(GL_COLOR_BUFFER_BIT);    

    glutDisplayFunc(display);
    glutKeyboardFunc(key);
    glutSpecialFunc(specialkey);
    glutIdleFunc(idle);

    glMatrixMode (GL_PROJECTION); 
    glLoadIdentity();
    glOrtho(0, 800, 600, 0, 0, 1);
    glMatrixMode(GL_MODELVIEW); 
    glTranslatef(0.375, 0.375, 0);

    glClearColor(0, 0, 0, 1.0); 

    setuptanks();

    glutMainLoop();

    return EXIT_SUCCESS;
}
c++
opengl
EN

回答 1

Stack Overflow用户

回答已采纳

发布于 2014-04-25 18:26:43

正如罗格·罗兰指出的那样，你读错了屏幕的部分。在将y坐标传递给glReadPixels()之前，您需要翻转它。

其他事情：

  1. 您不需要erasetank()或erasebullet()，只需清除屏幕上的每一帧
  2. 在显示回调中放置glClear()和矩阵是个好主意
  3. 显式的glutInitDisplayMode()是个好主意
  4. 没有真正的理由不使用GLUT_DOUBLE
  5. 使用glutTimerFunc()而不是sleep()

合在一起：

代码语言：javascript
运行
复制
#include <GL/glut.h>

#include <iostream>
using namespace std;

//define tank structure
struct tagTank
{
    int x,y;
    int dir,speed;
} tanks[2];

struct tagBullet
{
    int x,y;
    int alive;
    int xspd,yspd;
} bullets[2];

void setuptanks()
{   
    tanks[0].x = 30;
    tanks[0].y = 40;
    tanks[0].dir = 1;
    tanks[0].speed = 5;

    tanks[1].x = 800 - 30;
    tanks[1].y = 600 - 30;
    tanks[1].dir = 3;
    tanks[1].speed = 5;
}

void drawtank(int num)
{
    int x = tanks[num].x;
    int y = tanks[num].y;
    int dir = tanks[num].dir;

    //draw tank body
    glColor3f(1.0, 0.0, 0.0);
    if (num) glColor3f(0.0, 0.0, 1.0);
    glBegin(GL_QUADS);        
    glVertex2i(x-20, y-20);
    glVertex2i(x-20, y+20);
    glVertex2i(x+20, y+20);
    glVertex2i(x+20, y-20);
    glEnd();

    glColor3f(0.5, 0.0, 0.0);
    if (num) glColor3f(0.0, 0.0, 0.5);
    glBegin(GL_QUADS);        
    glVertex2i(x-10, y-10);
    glVertex2i(x-10, y+10);
    glVertex2i(x+10, y+10);
    glVertex2i(x+10, y-10);
    glEnd();

    //draw the turret based on direction
    glColor3f(1.0, 1.0, 1.0);
    switch (dir)
    {
    case 0:
        glBegin(GL_QUADS);        
        glVertex2i(x-2, y-30);
        glVertex2i(x-2, y);
        glVertex2i(x+2, y);
        glVertex2i(x+2, y-30);
        glEnd();
        break;
    case 1:
        glBegin(GL_QUADS);        
        glVertex2i(x, y-2);
        glVertex2i(x, y+2);
        glVertex2i(x+30, y+2);
        glVertex2i(x+30, y-2);
        glEnd();
        break;
    case 2:
        glBegin(GL_QUADS);        
        glVertex2i(x-2, y);
        glVertex2i(x-2, y+30);
        glVertex2i(x+2, y+30);
        glVertex2i(x+2, y);
        glEnd();
        break;
    case 3:
        glBegin(GL_QUADS);        
        glVertex2i(x-30, y-2);
        glVertex2i(x-30, y+2);
        glVertex2i(x, y+2);
        glVertex2i(x, y-2);
        glEnd();
        break;
    }
}

void movetank(int num)
{
    int dir = tanks[num].dir;
    int speed = tanks[num].speed;
    //update tank position based on direction
    switch(dir)
    {
    case 0:
        tanks[num].y -= speed;
        break;
    case 1:
        tanks[num].x += speed;
        break;
    case 2:
        tanks[num].y += speed;
        break;
    case 3:
        tanks[num].x -= speed;
        break;
    }

    //keep tank inside the screen
    if (tanks[num].x > 800-30)
    {
        tanks[num].x = 800-30;
        tanks[num].speed = 0;
    }

    else if (tanks[num].x < 30)
    {
        tanks[num].x = 30;
        tanks[num].speed = 0;
    }

    else if (tanks[num].y > 600-30)
    {
        tanks[num].y = 600-30;
        tanks[num].speed = 0;
    }

    else if (tanks[num].y < 30)
    {
        tanks[num].y = 30;
        tanks[num].speed = 0;
    }

    else tanks[num].speed = 5;
}

void explode(int num, int x, int y)
{
    int n;
    //retrieve location of enemy tank
    int tx = tanks[!num].x;
    int ty = tanks[!num].y;

    //is bullet inside the boundary of the enemy tank?
    if (x > tx-30 && x < tx+30 && y > ty-30 && y < ty+30)
        setuptanks();

    //draw some random circles for the "explosion"
    for (n = 0; n < 10; n++)
    {
        glColor3f((rand() % 101)/100.0, (rand() % 101)/100.0, (rand() % 101)/100.0);
        glBegin(GL_QUADS);        
        glVertex2i(x-16, y-16);
        glVertex2i(x-16, y+16);
        glVertex2i(x+16, y+16);
        glVertex2i(x+16, y-16);
        glEnd();
    }
}

void updatebullet(int num)
{
    if (bullets[num].alive)
    {
        //move bullet
        bullets[num].x += bullets[num].xspd;
        bullets[num].y += bullets[num].yspd;
        int x = bullets[num].x;
        int y = bullets[num].y;

        //stay within the screen
        if (x < 5 || x > 800 || y < 20 || y > 600)
        {
            bullets[num].alive = 0;
            return;
        }

        //look for a hit
        unsigned char pixel[4]; 
        int h = glutGet( GLUT_WINDOW_HEIGHT );
        glReadPixels(x, h - y, 1, 1, GL_RGBA, GL_UNSIGNED_BYTE, pixel);
        cout << x << ", " << y << " | " << (int)pixel[0] << ", " << (int)pixel[1] << ", " << (int)pixel[2] << endl;
        if ((int)pixel[0] || (int)pixel[1] || (int)pixel[2])
        {           
            bullets[num].alive = 0;
            explode(num, x, y);
            return;
        }       

        //draw bullet
        glColor3f(1.0, 1.0, 1.0);
        glBegin(GL_QUADS);        
        glVertex2i(x-2, y-2);
        glVertex2i(x-2, y+2);
        glVertex2i(x+2, y+2);
        glVertex2i(x+2, y-2);
        glEnd();
    }
}

void fireweapon(int num)
{
    int x = tanks[num].x;
    int y = tanks[num].y;

    //ready to fire again?
    if (!bullets[num].alive)
    {
        bullets[num].alive = 1;

        //fire bullet in direction tank is facing
        switch (tanks[num].dir)
        {
            //north
        case 0:
            bullets[num].x = x;
            bullets[num].y = y-30;
            bullets[num].xspd = 0;
            bullets[num].yspd = -20;
            break;

            //east
        case 1:
            bullets[num].x = x+30;
            bullets[num].y = y;
            bullets[num].xspd = 20;
            bullets[num].yspd = 0;
            break;

            //south
        case 2:
            bullets[num].x = x;
            bullets[num].y = y+30;
            bullets[num].xspd = 0;
            bullets[num].yspd = 20;
            break;

            //west
        case 3:
            bullets[num].x = x-30;
            bullets[num].y = y;
            bullets[num].xspd = -20;
            bullets[num].yspd = 0;
            break;
        }
    }
}

void up(int num)
{
    tanks[num].dir = 0; 
}

void down(int num)
{
    tanks[num].dir = 2; 
}

void left(int num)
{
    tanks[num].dir = 3; 
}

void right(int num)
{
    tanks[num].dir = 1; 
}

static void display(void)
{       
    glClearColor(0, 0, 0, 1.0); 
    glClear(GL_COLOR_BUFFER_BIT);    

    glMatrixMode (GL_PROJECTION); 
    glLoadIdentity();
    int w = glutGet( GLUT_WINDOW_WIDTH );
    int h = glutGet( GLUT_WINDOW_HEIGHT );
    glOrtho( 0, w, h, 0, -1, 1 );

    glMatrixMode(GL_MODELVIEW); 
    glLoadIdentity();
    glTranslatef(0.375, 0.375, 0);

    movetank(0);        
    movetank(1);    

    drawtank(0);
    drawtank(1);    

    glFlush();
    updatebullet(0);
    updatebullet(1);

    glutSwapBuffers();
}

static void key(unsigned char key, int x, int y)
{
    switch (key)
    {
    case 27 :   exit(0);        break;
    case 'a':   left(1);        break;
    case 'd':   right(1);       break;
    case 'w':   up(1);          break;
    case 's':   down(1);        break;
    case 32:    fireweapon(1);  break;
    case 13:    fireweapon(0);  break;
    }
}

static void specialkey(int key, int x, int y)
{
    switch (key)
    {
    case GLUT_KEY_LEFT:     left(0);    break;
    case GLUT_KEY_RIGHT:    right(0);   break;
    case GLUT_KEY_UP:       up(0);      break;
    case GLUT_KEY_DOWN:     down(0);    break;
    }
}

void timer( int value )
{
    glutTimerFunc( 50, timer, 0 );
    glutPostRedisplay();
}

int main(int argc, char *argv[])
{
    glutInitWindowSize(800,600);
    glutInit(&argc, argv);
    glutInitDisplayMode( GLUT_RGBA | GLUT_DOUBLE );

    glutCreateWindow("Tanks");

    glutDisplayFunc(display);
    glutKeyboardFunc(key);
    glutSpecialFunc(specialkey);
    glutTimerFunc( 0, timer, 0 );

    setuptanks();

    glutMainLoop();

    return EXIT_SUCCESS;
}
票数 3
EN
页面原文内容由Stack Overflow提供。腾讯云小微IT领域专用引擎提供翻译支持
原文链接：

https://stackoverflow.com/questions/23286407

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