Re: [Mesa3d-users] Capture Window pixel values using glReadPixels

[tom f. <tf...@al...>]

Srimal Jayawardena <sr...@gm...> writes:
>         GLvoid* data;
>         data =3D (GLvoid *) malloc (1024*768);
>         glReadPixels (0,0,1024,768,GL_RED, GL_UNSIGNED_BYTE, data);
>         for(int i=3D0; i<1024*768; i++)
>         cout << data[i];
>         free(data);
> 
> But it gives this error:
> 
> $ make
> g++ -Wall -o render main.cpp -lglut
> main.cpp: In function =E2=80=98void drawScene()=E2=80=99:
> main.cpp:154: error: pointer of type =E2=80=98void *=E2=80=99 used in arith=
> metic
> main.cpp:154: error: =E2=80=98GLvoid*=E2=80=99 is not a pointer-to-object t=
> ype
> make: *** [render] Error 1
> 
> Am I missing something here?

Yep.  In short, your types are wrong.  You can't have a void object, so
you can't directly dereference a void pointer.  Try a GLubyte instead.

That said, you might want to read up on C.  I suggest the white book:

  http://cm.bell-labs.com/cm/cs/cbook/

Though your `cout' implies you actually want to write C++, contrary to
malloc/free.  Stroustrup's book is the classic text here:

  http://www.research.att.com/~bs/3rd.html

> Is there a better way to do this?

Somewhere in the Mesa demos (IIRC) there's some code for reading the
current image buffer into a ppm file.  If not there, some simple
googling should bring up code to do this.

-tom