[cairo] cairo_set_source_surface questions

[唐博]

    I want to set the surface to itself and scaled the surface. That is , I want to  use just one surface( it has drawed path) to scaled. 

    I hope you can understand what I mean. This is the simple code. But the result (attachment) is very confused to me. wait fot your reply.
    Thank you very much.

    #define edge 600


    uchar * pbit = (uchar *)malloc(32*edge * edge);
    memset(pbit, 0, 32*edge * edge);       

    surface = cairo_image_surface_create_for_data((uchar *)pbit,
                                                  CAIRO_FORMAT_ARGB32,
                                                  edge, edge,
                                                  cairo_format_stride_for_width(CAIRO_FORMAT_ARGB32, edge));
    cr = cairo_create(surface);    

    cairo_set_source_rgba(cr, 1.0, 1.0, 0.0, 1);
    cairo_rectangle(cr, 150, 150, 300, 300);
    cairo_fill_preserve(cr);

    cairo_set_source_rgba(cr, 0.0, 1.0, 1.0, 1);
    cairo_stroke(cr);

    cairo_move_to(cr, 150, 150);
    cairo_line_to(cr, 450, 450);
    cairo_line_to(cr, 450, 150);
    cairo_line_to(cr, 150, 450);
    cairo_stroke(cr);

        cairo_save(cr);
        cairo_scale(cr, 1.0, 1.4);
        cairo_set_source_surface(cr, surface, 0, 0);
        cairo_rectangle(cr , 0, 0, edge, edge);
        cairo_fill(cr);
        cairo_restore(cr);
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