ピクセルから色を取得する方法は？ OpenGL

Question

ゲームのピクセルから色を取得し、この色を変数に保存したいと考えています。どのようにOpenGLでそれを行うには？

Answer 1

glReadPixels()：

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

using namespace std; 

void display() 
{ 
    glClear(GL_COLOR_BUFFER_BIT); 

    glMatrixMode(GL_PROJECTION); 
    glLoadIdentity(); 
    glOrtho(-10, 10, -10, 10, -1, 1); 

    glMatrixMode(GL_MODELVIEW); 
    glLoadIdentity(); 

    glPushMatrix(); 
     glScalef(5,5,5); 
     glBegin(GL_TRIANGLES); 
      glColor3ub(255,0,0); 
      glVertex2f(-1,-1); 
      glColor3ub(0,255,0); 
      glVertex2f(1,-1); 
      glColor3ub(0,0,255); 
      glVertex2f(1,1); 
     glEnd(); 
    glPopMatrix(); 

    glutSwapBuffers(); 
} 

void motion(int x, int y) 
{ 
    y = glutGet(GLUT_WINDOW_HEIGHT) - y; 

    unsigned char pixel[4]; 
    glReadPixels(x, y, 1, 1, GL_RGB, GL_UNSIGNED_BYTE, pixel); 
    cout << "R: " << (int)pixel[0] << endl; 
    cout << "G: " << (int)pixel[1] << endl; 
    cout << "B: " << (int)pixel[2] << endl; 
    cout << endl; 
} 

int main(int argc, char **argv) 
{ 
    glutInitWindowSize(640,480); 
    glutInit(&argc, argv); 
    glutInitDisplayMode(GLUT_RGBA | GLUT_DOUBLE); 
    glutCreateWindow("glReadPixels()"); 

    glutDisplayFunc(display); 
    glutPassiveMotionFunc(motion); 
    glutMainLoop(); 
    return 0; 
} 

Answer 2

struct{ GLubyte red, green, blue; } pixel; 
glReadPixels(x, y, 1, 1, GL_RGB, GL_UNSIGNED_BYTE, &pixel); 

Answer 3

#include <math.h> 
#include <gl/glut.h> 

struct Point { 
    GLint x; 
    GLint y; 
}; 

struct Color { 
    GLfloat r; 
    GLfloat g; 
    GLfloat b; 
}; 

void init() { 
    glClearColor(1.0, 1.0, 1.0, 0.0); 
    glColor3f(0.0, 0.0, 0.0); 
    glPointSize(1.0); 
    glMatrixMode(GL_PROJECTION); 
    glLoadIdentity(); 
    gluOrtho2D(0, 640, 0, 480); 
} 
    //this function is used for getting color of pixel 
Color getPixelColor(GLint x, GLint y) { 
    Color color; 
    glReadPixels(x, y, 1, 1, GL_RGB, GL_FLOAT, &color); 
    return color; 
} 

void setPixelColor(GLint x, GLint y, Color color) { 
    glColor3f(color.r, color.g, color.b); 
    glBegin(GL_POINTS); 
    glVertex2i(x, y); 
    glEnd(); 
    glFlush(); 
} 

void floodFill(GLint x, GLint y, Color oldColor, Color newColor) { 
    Color color; 
    color = getPixelColor(x, y); 

    if(color.r == oldColor.r && color.g == oldColor.g && color.b == oldColor.b) 
    { 
     setPixelColor(x, y, newColor); 
     floodFill(x+1, y, oldColor, newColor); 
     floodFill(x, y+1, oldColor, newColor); 
     floodFill(x-1, y, oldColor, newColor); 
     floodFill(x, y-1, oldColor, newColor); 
    } 
    return; 
} 

void onMouseClick(int button, int state, int x, int y) 
{ 
    Color newColor = {1.0f, 0.0f, 0.0f}; 
    Color oldColor = {1.0f, 1.0f, 1.0f}; 

    floodFill(320, 240, oldColor, newColor); 
} 

void draw_circle(Point pC, GLfloat radius) { 
    GLfloat step = 1/radius; 
    GLfloat x, y; 

    for(GLfloat theta = 0; theta <= 360; theta += step) { 
     x = pC.x + (radius * cos(theta)); 
     y = pC.y + (radius * sin(theta)); 
     glVertex2i(x, y); 
    } 
} 

void display(void) { 
    Point pt = {320, 240}; 
    GLfloat radius = 50; 

    glClear(GL_COLOR_BUFFER_BIT); 
    glBegin(GL_POINTS); 
     draw_circle(pt, radius); 
    glEnd(); 
    glFlush(); 
} 

int main(int argc, char** argv) 
{ 
    glutInit(&argc, argv); 
    glutInitDisplayMode(GLUT_SINGLE|GLUT_RGB); 
    glutInitWindowSize(640, 480); 
    glutInitWindowPosition(200, 200); 
    glutCreateWindow("Open GL"); 
    init(); 
    glutDisplayFunc(display); 
    glutMouseFunc(onMouseClick); 
    glutMainLoop(); 
    return 0; 
}