trouble with antialiasing

Hi,

I am having trouble getting even the most basic antialiasing to work.
I copied the example right out of the manual and built it in as much
isolation as possible, but am still getting a non-antialiased line.
My test source file is included below, and I am building it with:

    gcc -lgd -o gtest gtest.c

This is on Fedora Core 6, with gd-2.0.35-1.fc6 and
gd-devel-2.0.35-1.fc6.

I was able to get it to antialias by both (a) creating the image
instead with gdImageCreateTrueColor(), and (b) changing the second
99 to a 49, thus making the line be at less than a 45-degree angle.
But these seem arbitrary to me -- I was just thrashing around
randomly -- and I don't see why the original code does not work.

Am I missing something obvious here?

thanks,
Eric

==================================================

/*
 * File: gtest.c
 */

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

int main(int argc, char *argv[])
{
    gdImagePtr im, brush;
    int black;
    int blue;
    FILE *fp = NULL;

    im = gdImageCreate(100, 100);
    /* Background color (first allocated) */
    black = gdImageColorAllocate(im, 0, 0, 0);
    blue = gdImageColorAllocate(im, 0, 0, 255);
    gdImageSetAntiAliased(im, blue);

    /* Draw a smooth line from the upper left corner to the
       lower right corner. */
    gdImageLine(im, 0, 0, 99, 99, gdAntiAliased);

    /* ... Do something with the image, such as
       saving it to a file... */
    fp = fopen(argv[1], "wb");
    gdImagePng(im, fp);
    fclose(fp);

    /* Destroy it */
    gdImageDestroy(im);

    exit(0);
}