[Plib-devel] No longer blocking non power-of-two textures

[Paolo L. <p.l...@ci...>]

Dear friends,

As most of you probably know the GL_ARB_texture_non_power_of_two OpenGL
extension, when supported, allows to overcome the subject limit at the =
only
implementation effort of checking if such extension is supported by the
hardware
(http://oss.sgi.com/projects/ogl-sample/registry/ARB/texture_non_power_of=
_tw
o.txt).

GL_ARB_texture_non_power_of_two has no relevant limitation, and is much
better than GL_EXT_texture_rectangle in which, e.g., texture coordinates
were addressed non-parametrically [0..w],[0..h].

Such check could be done in ssgMakeMipMaps at the very beginning just =
before
the power-of-two test.
So in our case, in ssgLoadTexture.cxx:

ssgMakeMipMaps ( GLubyte *image, int xsize, int ysize, int zsize )
{
  bool non_power_of_two_tex_supported =3D ssgIsExtensionSupported(
"GL_ARB_texture_non_power_of_two" );
 =20
  if ( !non_power_of_two_tex_supported &&
      (! ((xsize & (xsize-1))=3D=3D0) ||
       ! ((ysize & (ysize-1))=3D=3D0)) )
  {
    ulSetError ( UL_WARNING, "Map is not a power-of-two in size!" ) ;
    return false ;
  }
  ...

Checking for such extension could be done, in this and future cases, by =
the
following code (code by C=E9sar Blecua Ud=EDas sent me by Roman =
Grigoriev when
testing VBOs):


static bool ssgSearchExtensionString(char *extString, char *extName) {
    // Returns GL_TRUE if the *extName string appears in the *extString
string,
    // surrounded by white spaces, or GL_FALSE otherwise.

    char *p, *end;
    int n, extNameLen;

    if ((extString =3D=3D NULL) || (extName =3D=3D NULL))
        return false;

    extNameLen =3D strlen(extName);

    p=3DextString;
    end =3D p + strlen(p);

    while (p < end) {
        n =3D strcspn(p, " ");
        if ((extNameLen =3D=3D n) && (strncmp(extName, p, n) =3D=3D 0))
            return GL_TRUE;

        p +=3D (n + 1);
    }

    return GL_FALSE;
}

bool ssgIsExtensionSupported(char *extName) {
   // Returns GL_TRUE if the OpenGL Extension whose name is *extName
   // is supported by the system, or GL_FALSE otherwise.
   //
   // The *extName string must follow the OpenGL extensions naming =
scheme
   // (ie: "GL_type_extension", like GL_EXT_convolution)

    return ssgSearchExtensionString((char *)glGetString(GL_EXTENSIONS),
extName);
}

Waiting for reactions -

Paolo Leoncini