首先感谢您抽出时间。这是一个有很多答案的问题，但它不起作用，在这里我试图举一个例子，让我们看看我们是否可以解决这个问题，也许还有人将来会再次受益。
所以问题是我们有glreadpixel（）调用圆上的一个点，这是由bresenham函数绘制的。
但事实是它在改变窗口的背景颜色时甚至没有给出0,0,0的RGB值。
这是大代码，享受实验。我尝试了一切。顺便说一下，我正在开发n macOS（OpenGL是硬件无关的）
using namespace std;
#include <GLUT/glut.h>
#include <iostream>
#include <vector>
#include <CoreGraphics/CoreGraphics.h>
int r = 40;
int flag = 0;
int cordinates [50][3]=
{
{50, 50} , //0 station
{400, 450} , //1 station
{750, 250} //2 station
};
int matrix[50][50] = {
{0,1,1},
{1,0,1},
{1,1,0}
};
int trains[50][50] = {
{3,4,0,1,999},
{11,1,1,2,999},
{0,0,0,999}
};
int x,y;
int *x1, *y3, x2, y2;
int xx,yy,xxx,yyy,p,q,vertexcount,counter;
int xinc,xinc3,i,j,system_time,flag1,time_chekcer_cnt,dda =0;
char buf3[12],buf[12];
float tempx0,tempy0,tempx1,tempy1;
int train0 = 999 ;int start0,speed0,next0,nextx0,nexty0,final0 = 999 ;//999 signifies invalid
int train1 ,start1, speed1,next1,nextx1,nexty1,final1 = 999 ;//999 signifies invalid
int train2 ,start2, speed2,next2,final2 = 999 ;//999 signifies invalid
void init2D()
{
glClearColor(0,0,0,0.0);
glMatrixMode (GL_PROJECTION);
glLoadIdentity();
gluOrtho2D (0.0, 900.0, 0.0, 900.0);
}
;
void bresenham_circle(const int h, const int k,const int r)
{
int x=0,y=r,p=(3-(2*r));
do{
//Read pixel
unsigned char pixelub[3];
// glReadPixels(<#GLint x#>, <#GLint y#>, <#GLsizei width#>, <#GLsizei height#>, <#GLenum format#>, <#GLenum type#>, <#GLvoid *pixels#>)
glPointSize(1);
//draw two points
glColor3f(1.0, 1.0, 0.0);
glBegin(GL_POINTS);
glVertex2i((h+x),(k+y));
glEnd();
//color detection start
glReadPixels
(
(h+x),(k+y),
1, 1,
GLUT_RGB, GL_UNSIGNED_BYTE, &pixelub
);
//print
cout <<"reading pixel : "<<(h+x)<<" "<<(k+y);
printf("r: %u g: %u b: %u\n", pixelub[0], pixelub[1], pixelub[2]);
cout << endl;
//end
glColor3f(1.0, 0.0, 0.0);
glBegin(GL_POINTS);
glVertex2i((h+y),(k+x));
glEnd();
glBegin(GL_POINTS);
glVertex2i((h+y),(k-x));
glEnd();
glBegin(GL_POINTS);
glVertex2i((h+x),(k-y));
glEnd();
glBegin(GL_POINTS);
glVertex2i((h-x),(k-y));
glEnd();
glBegin(GL_POINTS);
glVertex2i((h-y),(k-x));
glEnd();
glBegin(GL_POINTS);
glVertex2i((h-y),(k+x));
glEnd();
glBegin(GL_POINTS);
glVertex2i((h-x),(k+y));
glEnd();
x++;
if(p<0){
p+= ((4*x)+6);
}else {
y--;
p+=(4*(x-y)+10);
}
}
while (x<=y);
}
void drawBitmapText(char *string,float x,float y,float z)
{
char *c;
glRasterPos3f(x, y,z);
for (c=string; *c != NULL; c++)
{
glutBitmapCharacter(GLUT_BITMAP_TIMES_ROMAN_24, *c);
}
}
void systemtime(){
sprintf(buf, "%d", system_time); // puts string into buffer
printf("%s\n", buf);
sprintf(buf3, "%d", system_time-1); // puts string into buffer
if(system_time >1){
glColor3f(1.0, 1.0, 1.0);
drawBitmapText(buf3,200,200,0);
glColor3f(0.0, 1.0, 0.0);
drawBitmapText(buf,200,200,0);
system_time =system_time+1;
}else {
drawBitmapText(buf,200,200,0);
system_time =system_time+1;
}
};
void draw_pixel(int x, int y){
glPointSize(5);
glColor3f(1.0, 1.0, 0.0);
glBegin(GL_POINTS);
cout << "\n THIS IS PRINTING X and Y :"<<x<<" & "<<y<<endl;
glVertex2i(x, y);
glEnd();
glFlush();
}
//bresenham ..... i vant DDA
void draw_dda( float *x0, float *y0, int x1, int y1,int speed) {
//cout << "\n LOOPING FOR START CORDIANTES : X0 and Y0 "<<*x0<<*y0<<endl;
int dx = x1 - *x0;
int dy = y1 - *y0;
//GLfloat x1 = p1.x; GLfloat y1 = p1.y;
GLfloat step = 0;
if(abs(dx) > abs(dy)) {
step = abs(dx);
} else {
step = abs(dy);
}
GLfloat xInc = dx/step;
GLfloat yInc = dy/step;
for (int speed_count = 0 ; speed_count < speed ; speed_count++){
glPointSize(5);
glColor3f(1.0, 0.0, 0.0);
glBegin(GL_POINTS);
//cout << "\n THIS IS PRINTING X and Y :"<<xInc<<" & "<<yInc<<endl;
glVertex2i(*x0, *y0);
glEnd();
int cy1 = *y0;
int cx1 = *x0;
//cout << "\n calling bresenham";
glColor3f(1.0, 1.0, 1.0);
bresenham_circle(cx1, cy1, r);
//cout << " \n" <<cx1<<" "<<cy1<<" "<<r;
while (flag != 1){
r--;
//cout <<"\n r is "<<r<<endl;
if (r == 0){
flag = 1;
}
}
r =40;flag= 0;
*x0 = *x0+xInc;
*y0 = *y0+yInc;
glColor3f(1.0, 0.0, 0.0);
int cy = *y0;
int cx = *x0;
//cout << "\n calling bresenham";
bresenham_circle(cx, cy, r);
//cout << " \n" <<cx<<" "<<cy<<" "<<r;
while (flag != 1){
r--;
//cout <<"\n r is "<<r<<endl;
if (r == 0){
flag = 1;
}
}
r =40;flag= 0;
//cout << "\n THIS IS PRINTING X and Y :"<<xInc<<" & "<<yInc<<endl;
}
}
void drawline(float *x0, float *y0, int x1, int y1)
{ cout << "\n LOOPING FOR START CORDIANTES : ("<<*x0<<","<<*y0<<") To ("<<x1<<","<<y1<<")"<<endl;
int dx, dy, p, x, y;
dx=x1-*x0;
dy=y1-*y0;
x=*x0;
y=*y0;
p=2*dy-dx;
//cout << "X and X1 "<<x<<x1<<endl;
if (x<x1)
{
if(p>=0)
{
draw_pixel(x,y);
//cout << ">>>>>. Y INCREMENTED >>>>> "<<y<<endl;
y=y+1;
p=p+2*dy-2*dx;
}
else
{ //cout << "\n >>>>>NOT Y INCREMENTED >>>>> "<<y<<endl;
draw_pixel(x,y);
//cout << "X and Y "<<x<<y<<endl;
p=p+2*dy;
}
x=x+1;
}
*x0=x;
*y0=y;
//cout << "\n X "<<x<<" and Y "<<y<<endl;
//cout << "\n X0 "<<*x0<<" and Y0 "<<*y0<<endl;
}
void display_ndots()
{glClear(GL_COLOR_BUFFER_BIT);
glPointSize(5);
glColor3f(1.0, 0.0, 0.0);
//draw two points
glBegin(GL_POINTS);
for(int i = 0; i < 10; i++)
{ xx = cordinates[i][0];
yy = cordinates[i][1];
glVertex2i(xx,yy);
//printf("%i",xx);
}
glEnd();
glColor3f(0.0, 1.0, 0.0);
//draw a line
glLineWidth(3);
glBegin(GL_LINES);
for(counter = 0 ; counter < 3 ; counter ++){
for (p = 0 ; p < 3 ; p ++){
for (q = 0 ; q < 3 ; q ++){
if( matrix[p][q] == 1){
xx = cordinates[p][0];
yy = cordinates[p][1];
xxx = cordinates[q][0];
yyy = cordinates[q][1];
glVertex2i(xx,yy);
glVertex2i(xxx,yyy);
}
}
}
}
glEnd();
systemtime();
time_chekcer_cnt = 0;
for(time_chekcer_cnt = 0 ; time_chekcer_cnt <10 ; time_chekcer_cnt ++){
if (system_time == trains[time_chekcer_cnt][0]){
switch (time_chekcer_cnt)
{
case 0: printf("Trian number %i have been started server time is %i \n",time_chekcer_cnt,system_time);
/*Initializing Train Data*/
train0 = time_chekcer_cnt;
speed0 = trains[time_chekcer_cnt][1];
start0 = trains[time_chekcer_cnt][2];
cout<<"\n >>>>>>>>>>>>>"<<start0<<endl;
tempx0 = cordinates[start0][0];
tempy0 = cordinates[start0][1];
next0 = trains[time_chekcer_cnt][3];
nextx0 = cordinates[next0][0];
nexty0 = cordinates[next0][1];
;
i=0;
while (trains[time_chekcer_cnt][i] != 999) {
i = i+1;
}
final0 = trains[time_chekcer_cnt][i-1];//999 signifies invalid
printf("\nfinal station is %i \n",final0);
//tempx01 = cordinates[start0][0]+10;
//tempy01 = cordinates[start0][1];
break;
case 1:
printf("Trian number %i have been started server time is %i \n",time_chekcer_cnt,system_time);
/*Initializing Train Data*/
train1 = time_chekcer_cnt;
speed1 = trains[time_chekcer_cnt][1];
start1 = trains[time_chekcer_cnt][2];
cout<<"\n >>>>>>>>>>>>>"<<start0<<endl;
tempx1 = cordinates[start0][0];
tempy1 = cordinates[start0][1];
next1 = trains[time_chekcer_cnt][3];
nextx1 = cordinates[next0][0];
nexty1 = cordinates[next0][1];
;
i=0;
while (trains[time_chekcer_cnt][i] != 999) {
i = i+1;
}
final1 = trains[time_chekcer_cnt][i-1];//999 signifies invalid
printf("\nfinal station is %i \n",final0);
break;
//default: // code to be executed if n doesn't match any cases
}
}
}
if (train0 != 999){
glColor3f(1.0, 1.0, 0.0);
draw_dda( &tempx0, &tempy0 ,nextx0, nexty0,speed0);
cout << "Draving from corinates ("<<tempx0<<","<<tempy0<<") To ("<<nextx0<<","<<nexty0<<")"<<endl;
}
if (train1 != 999){
glColor3f(1.0, 1.0, 0.0);
/// cout << "original x and y are "<<nextx0<<" & "<<nexty0;
draw_dda( &tempx1, &tempy1 ,nextx1, nexty1,speed1);
cout << "Draving from corinates ("<<tempx0<<","<<tempy0<<") To ("<<nextx0<<","<<nexty0<<")"<<endl;
// drawline( &tempx0, &tempy0 ,nextx0, nexty0);
//draw_dda( &tempx01, &tempy01 ,nextx0+10, nexty0);
// cout<<"After call by reference it is :"<<nextx0<<" & "<<nexty0;
}
/*bresenham_circle(100, 100, r);
while (flag != 1){
r--;
cout <<"\n r is "<<r<<endl;
if (r == 0){
flag = 1;
}
}*/
glFlush();
glGetError();
}
void timlycall (int unused) {
glutPostRedisplay();
glutTimerFunc(1000, timlycall, 0);
}
int main(int argc,char *argv[])
{
glutInit(&argc,argv);
glutInitDisplayMode (GLUT_SINGLE | GLUT_RGB);
glutInitWindowSize (900, 900);
glutInitWindowPosition (0, 0);
glutCreateWindow ("points and lines");
init2D();
glutDisplayFunc(display_ndots);
//glutDisplayFunc(init2D);
glutTimerFunc(0, timlycall, 0);
glutMainLoop();
return 0;
}
更新1.0
好的，我们取得了一些进展：
在glreadpixel（）中使用GLUT_RGB给出错误代码1280，即检测到 通过BDL提供的方法。哪个代表不正确的枚举， 所以在glreadpixel（）
现在我们要解决一个有趣的问题，即现在的GlReadPixel
给出r:255 g: 255 b: 255作为输出，即背景颜色
而不是必须是黄色的颜色，因为我们称之为
功能点也位于其后的圆上
绘。
答案 0 :(得分：1)
您将错误的枚举传递给glReadPixels。 GLUT_RGB与GL_RGB不同，因此不能用于此功能。
这会被GL_INVALID_ENUM报告为glGetError()错误，但永远不会检查错误代码。你应该使用类似的东西：
GLenum error = glGetError();
if (error != GL_NO_ERROR)
std::cout << "OpenGL error: " << error << std::endl;