[cairo] drawing inconsistency

Bertram Felgenhauer bertram.felgenhauer at googlemail.com
Mon Jan 21 06:37:41 PST 2008

Adi Oanca wrote:
> > There are a few ways to fix this:
> > - use a line width of 2 (which is, incidentally, cairo's default).
>    AFAIR it gives the same result, only that the line is thicker at
> the left/bottom side.

When I run the program with line width 2, I see a rectangle with a
thicker, white border. When the rectangle is filled later (in red),
the fill paints over the inner half of said border, and only a 1 pixel
border remains.

> > I can't reproduce that. Did you add the cairo_set_line_width(cr, 1)
> > line after testing that, perhaps?
>    Strange.
>    In the code snippet I gave, cairo_set_line_width(cr, 1) is present.
>   To reproduce:
> 1) - take the code from the first mail, add
> cairo_surface_write_to_png(cairoSurface, "image2.png");
> right before
>        cairo_destroy(cr);
>        cairo_surface_destroy(cairoSurface);
> => check image2.png and see the inconsistency.


> 2) let cairo_surface_write_to_png(cairoSurface, "image2.png");
> to be where it was pointed at 1)
> then replace:
>        cairoSurface = cairo_image_surface_create_for_data(
>                (unsigned char*)screen->pixels,
>                CAIRO_FORMAT_ARGB32,
>                gWidth,
>                gHeight,
>                screen->pitch);
> with:
> cairoSurface = cairo_image_surface_create(CAIRO_FORMAT_ARGB32, 640, 480);
> => check the image and see that the result is correct (the rectangle
> is properly shown/visible)

Not for me, I get the same result as in 1). (Using cairo 1.4.12 on
linux. The git version produces the same result.)

> So, this is why I'm puzzled. If I provide a pre-allocated drawing
> buffer, Cairo won't draw properly, but if I tell it to allocate the
> drawing buffer, the result is just fine.

You are probably hitting some bug (although I have no clue where that
bug could be). But in any case, the behaviour in case 1) is the correct
one, even if it is not what you expected.


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