GtkPreview of darkness

[Kevin Turner <Kevin Turner oberlin edu>]

I'm attempting to add a preview window to one of my gimp plug-ins
(antialias).  I've a problem in that my update_preview function always
makes the preview entirely black (with X running at color depth 8 or 16).

Here's the code snippit that I believe is supposed to make it go:

for(y=0; y < fake_rgn.h; y++) {
        printf("%.62s\n",fake_rgn.data + y * fake_rgn.rowstride);
        gtk_preview_draw_row(GTK_PREVIEW(pre_info->preview),
                             fake_rgn.data + y * fake_rgn.rowstride,
                             0,y,fake_rgn.rowstride);
} /* next y */
gtk_widget_draw(pre_info->preview, NULL);

the printf is there just to make sure that there is indeed something
interesting-looking in the memory there, and it tells me that there is.

Any hints as to what could cause the GtkPreview to stay black?

Thanks,
  Kevin

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