glreadpixel() not giving output - c++

Question

Firstly thanks for giving your time.This is one of question which has a lot of answers but it does not work, here I am trying to take an example and let's see if we can solve this and maybe someone will get benefited again in future.
So the problem is we have glreadpixel() calling on a point on a circle, which is drawn by bresenham function.
But the thing is its not even giving any value than 0,0,0 for RGB on changing the background colour of the window.
Here is the big code, enjoy experimenting. I have tried everything. By the way, I am developing n macOS (OpenGL is hardware independent )
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;
}
Please not macos have library GLUT but windows have GL
Update 1.0
OK folks we have made some progress :
Using GLUT_RGB in glreadpixel() gives error code 1280 i.e. detected
by method BDL have provided. Which stands for incorrect enumeration ,
so use GL_RGB inside glreadpixel()
Now one more interesting thing we have to tackle, i.e now GlReadPixel
is giving r:255 g: 255 b: 255 as output , i.e background color
instead of color that must be yellow , because we are calling the
function on point which lies on the circle that too after its
painted.

Answer 1

You are passing a wrong enumeration to glReadPixels. GLUT_RGB is not the same as GL_RGB and may thus not be used for this function.
This would have been reported as a GL_INVALID_ENUM error by glGetError(), but the error code is never checked. You should use something like:
GLenum error = glGetError();
if (error != GL_NO_ERROR)
std::cout << "OpenGL error: " << error << std::endl;