Cách lấy màu từ pixel? OpenGL

Question

Tôi muốn lấy màu từ pixel trong trò chơi của mình và lưu màu này thành biến. Làm thế nào để làm điều đó trong opengl?

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; 
}