Randomly Coloured Polygons

OpenGL OpenGL: Basic Coding
1

I am trying to create 20 randomly coloured polygons each with a random number of sides. I am using the following code but unfortunatley all the polygons are black. It works for lines and points but not polygons. Any ideas would be very helpful:)

Thanks , Mes…

 
void polygons(void )
{ 
glClear(GL_COLOR_BUFFER_BIT);
glBegin(GL_POLYGON);
srand (time(0));
for (int i=0;i<=20;i++)
{

int col = 1.0 * rand () / RAND_MAX ;
glColor3f(col,col,col);
glVertex2f(rand()%640, rand()%480);

}
glEnd();
glFlush();
}
2

You can only send vertices that define a convex polygon in OpenGL.

By randomly generating vertices there is no guarantee that you will honor that convention.

You need some logic in there to ensure that the polygons you are generating are actually convex.

If you don’t know the difference betwixt convex and concave polygons then clicky

3

I could easily be wrong, but as you defined col as an int, the only possible values for col are 0 and 1. 1 will only occur when rand() produces RAND_MAX, which is not very likely. Thus, you are calling glColor3f( 0, 0, 0), which is black.

All you really need to do is declare col as a float, not an int.

You might also be more satisfied with your results if you had col_R, col_G, and col_B, all random, so that your polygons aren’t all shades of gray.

Hope this helps. Good luck!

4

You’re not wrong M4C6YV3R. That is absolutely correct.

5

Uhm sorry Aeluned you are quite wrong and M4C6YV3R is correct the line

int col = 1.0 * rand () / RAND_MAX ;

will for the most part producs 0 as its output.

Try it out

heres a quick test to prove this with output

for (int i= 0;i <= 20; i++){

int col = 1.0 * rand () / RAND_MAX ;
float  colf = 1.0 * rand () / RAND_MAX ;
printf( " Col = %d    , colf = %f 
" , col, colf );

}

As you can see col is always Zero

output is:

Col = 0 , colf = 0.563585
Col = 0 , colf = 0.808740
Col = 0 , colf = 0.479873
Col = 0 , colf = 0.895962
Col = 0 , colf = 0.746605
Col = 0 , colf = 0.858943
Col = 0 , colf = 0.513535
Col = 0 , colf = 0.014985
Col = 0 , colf = 0.364452
Col = 0 , colf = 0.165899
Col = 0 , colf = 0.445692
Col = 0 , colf = 0.004669
Col = 0 , colf = 0.377880
Col = 0 , colf = 0.571184
Col = 0 , colf = 0.607166
Col = 0 , colf = 0.663045
Col = 0 , colf = 0.352123
Col = 0 , colf = 0.607685
Col = 0 , colf = 0.802606
Col = 0 , colf = 0.301950
Col = 0 , colf = 0.726676

Solution here is to change col from an int to float

6

LOL :smiley:

7

Read my post more carefully; I said he was right .

You’re not wrong M4C6YV3R

not wrong = right.

8

DOH

Sorry Aeluned , totally mis-read your reply :frowning: , the not got totally lost :eek:

Need more Beer I think