Re: Detect changes to a PangoLayout

[Behdad Esfahbod <behdad behdad org>]

Hi Neil,

I took a note to add some kind of API for that.  See last comment in:

  http://bugzilla.gnome.org/show_bug.cgi?id=340066

In the mean time here's a trick you can use to detect changes (no signals though):

  - Get the first line of the layout using 
pango_layout_get_line_readonly(layout, 0);

  - Reference that line using pango_layout_line_ref(line) and store it somewhere.

  - When you want to detect changes to the layout, check whether line->layout 
is NULL.  If it is, the layout was changed and as a result it disassociated 
from the old line.

  - If changed, unref the line and repeat.

Hope that helps.

behdad

On 05/20/2009 01:45 PM, Neil Roberts wrote:
> Hi,
>
> In Clutter we currently use Pango to render the ClutterTexts. Every
> time the ClutterText is painted we call pango_renderer_draw_layout
> which creates some geometry to send to the GPU.
>
> It would be nice if we could avoid rebuilding this geometry every
> frame and keep it in a vertex buffer object instead. There is a patch
> to do this here:
>
> http://bugzilla.openedhand.com/show_bug.cgi?id=1572
>
> This works by attaching some data to the pango layout when it is first
> painted using g_object_set_qdata. It works fine for ClutterTexts because
> they abandon the layout and recreate it any time it needs to make some
> changes. However we also expose an interface from Cogl for directly
> rendering pango layouts. In that case it might not work correctly if
> the developer renders the layout, makes some changes to it and then
> renders it again later because the VBO will already have been created
> and attached to the layout.
>
> So my question is: is there any to detect when the PangoLayout has
> been altered so we can recreate the VBO? PangoLayout doesn't seem to
> have any properties or signals that I could attach to.
>
> Thanks for your help.
>
> Regards,
> - Neil
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