Distingish alphatransparent pixels in PNGs

Hello,

Is there a chance to distinguish alphatransparent pixels from simple
transparent pixels in PNGs? More precisely, is there a chance to check,
wheater a PNG uses alphatransparent pixels or not?
The down below testcase reports alphatransparent pixels, even if the
source file has only 1bit transparency.

Thanks a lot
Marcus



#include <stdio.h>
#include <stdlib.h>
#include <gd.h>

int main( int argc, char *argv[] )
{
     system( "wget -O /tmp/test.png http://doppelbauer.name/test.png" );

     FILE *fp = fopen( "/tmp/test.png", "r" );
     gdImagePtr image = gdImageCreateFromPng( fp );
     fclose( fp );

     int x, y;
     for( x=0; x < gdImageSX(image); ++x )
     {
         for( y=0; y < gdImageSY(image); ++y )
         {
             int color = gdImageGetPixel(image, x, y);
             int alpha = gdImageAlpha(image,color);
             if( alpha == 0 || alpha == gdTransparent )
                 continue;
             printf( "PNG with 1bit alpha channel contains 
alphatransparent pixel\n" );
             return 0;
         }
     }
     return 0;
}