glReadPixels는 OpenGL에서 잘못된 색상을 읽습니다.

Question

실제로 더 큰 글자의 간단한 프로그램이 있는데, 이것을 제쳐두고 간단한 픽셀 읽기/쓰기를 시도했습니다 (Opengl).
픽셀 색상을 읽는 것을 포함하는 첫 번째 섹션은 성공하지만, 픽셀을 쓰고 읽는 작업과 관련된 두 번째 작업이 실패합니다!

두 번째 읽기 절차가 실패의 원인이 무엇 : 여기

#include <iostream> 
#include <glut.h> 

using namespace std; 


void init(void) 
{ 
    glClearColor(1.0,0.0,0.0,0.0); 
    glMatrixMode(GL_PROJECTION); 
    gluOrtho2D(0.0,100.0,0.0,100.0); 
} 


void displayfnc() 
{ 
    glClear(GL_COLOR_BUFFER_BIT); 


    unsigned char current[3]; 
    int r,g,b; 
    //x=0 to 6 is right 
    glBegin (GL_POINTS); 
    glColor3f (0.0,0.0,1.0); 
    glVertex2i (6,0); 
    glEnd(); 

    //works well. 
    glReadPixels(6,0, 1, 1 , GL_RGB , GL_UNSIGNED_BYTE ,current); 
    r = current[0]; 
    g = current[1]; 
    b = current[2]; 
    cout<<"read from x=6::::"<<"r:"<<r<<"g:"<<g<<"b:"<<b<<endl; 

    //x =7 and other isnt right 
    glBegin (GL_POINTS); 
    glColor3f (0.0,0.0,1.0); 
    glVertex2i (7,0); 
    glEnd(); 
    //the problem is here in the reading.why this happen? 
    glReadPixels(7,0, 1, 1 , GL_RGB , GL_UNSIGNED_BYTE ,current); 
    r = current[0]; 
    g = current[1]; 
    b = current[2]; 
    cout<<"Read from x=7::::"<<"r:"<<r<<"g:"<<g<<"b:"<<b<<endl; 

    glFlush(); 
} 

int main(int argc, char** argv) 
{ 

    glutInit(&argc, argv); 
    glutInitDisplayMode(GLUT_SINGLE | GLUT_RGB); 
    glutInitWindowSize(100,100); 
    glutInitWindowPosition(0, 0); 
    glutCreateWindow("BoundaryFill"); 
    init(); 
    glutDisplayFunc(displayfnc); 
    glutMainLoop(); 
    return 0; 
} 

출력입니다 : 여기 문제를 보여줍니다 프로그램이 무엇입니까?

Answer 1

내 시스템에서 적어도 glutInitWindowSize(100,100)은 단지 제안 사항입니다. glutGet(GLUT_WINDOW_WIDTH)은 gluOrtho2D() 전화가 걸리므로 실제로 104을 반환하지만 100이 아닙니다.

대신 동적 투사를 사용해보십시오 :

#include <GL/glut.h> 
#include <iostream> 

using namespace std; 

void displayfnc() 
{ 
    glClearColor(1.0,0.0,0.0,0.0); 
    glClear(GL_COLOR_BUFFER_BIT); 

    glMatrixMode(GL_PROJECTION); 
    glLoadIdentity(); 
    double w = glutGet(GLUT_WINDOW_WIDTH); 
    double h = glutGet(GLUT_WINDOW_HEIGHT); 
    gluOrtho2D(0.0,w,0.0,h); 

    glMatrixMode(GL_MODELVIEW); 
    glLoadIdentity(); 

    unsigned char current[3]; 
    int r,g,b; 
    //x=0 to 6 is right 
    glBegin (GL_POINTS); 
    glColor3f (0.0,0.0,1.0); 
    glVertex2i (6,0); 
    glEnd(); 

    //works well. 
    glReadPixels(6,0, 1, 1 , GL_RGB , GL_UNSIGNED_BYTE ,current); 
    r = current[0]; 
    g = current[1]; 
    b = current[2]; 
    cout<<"read from x=6::::"<<"r:"<<r<<"g:"<<g<<"b:"<<b<<endl; 

    //x =7 and other isnt right 
    glBegin (GL_POINTS); 
    glColor3f (0.0,0.0,1.0); 
    glVertex2i (7,0); 
    glEnd(); 
    //the problem is here in the reading.why this happen? 
    glReadPixels(7,0, 1, 1 , GL_RGB , GL_UNSIGNED_BYTE ,current); 
    r = current[0]; 
    g = current[1]; 
    b = current[2]; 
    cout<<"Read from x=7::::"<<"r:"<<r<<"g:"<<g<<"b:"<<b<<endl; 

    glFlush(); 
} 

int main(int argc, char** argv) 
{ 

    glutInit(&argc, argv); 
    glutInitDisplayMode(GLUT_SINGLE | GLUT_RGB); 
    glutInitWindowSize(100,100); 
    glutInitWindowPosition(0, 0); 
    glutCreateWindow("BoundaryFill"); 
    glutDisplayFunc(displayfnc); 
    glutMainLoop(); 
    return 0; 
}