Re: pango_cairo_update_layout()

[Behdad Esfahbod <behdad behdad org>]

Thanks Theo.  I can reproduce this.  Will look into it as time permits.  In
the mean time, mind filing a bug on bugzilla.gnome.org against pango so it
doesn't get lost?

behdad

Theo Veenker wrote:
> Behdad Esfahbod wrote:
>> Theo Veenker wrote:
>>> Hi,
>>
>> Hi,
>>
>>> A question regarding pango_cairo_update_layout(). The example in
>>> http://library.gnome.org/devel/pango/stable/pango-Cairo-Rendering.html
>>> contains the following fragment:
> [snip]
>>> Why is it that it doesn't make a difference whether
>>> pango_cairo_update_layout()
>>> is called or not. The output is the same. Is it really necessary to call
>>> this
>>> function if only the cairo context changes? Why? In my own code I also
>>> don't
>>> notice any diffrence if I just drop the pango_cairo_update_layout()
>>> call.
>>
>> It's very delicate.  For example, if you have hinting enabled, then
>> the size
>> of a layout with identity matrix may be different from one of a
>> rotated matrix
>> as hinting is only done with axis-aligned transforms.  And for pango
>> to pick
>> up the matrix from your cairo_t you need to make that function call.
> 
> Thanks for the info. But I now noticed some condition where calling
> pango_cairo_update_layout() has a big unexpected impact. I've created
> a little X test application that demonstrates it (attached). The program
> draws a string using cairo-pango. It responds to following keys:
>  - page up/down: controls scaling factor for cairo_scale()
>  - up/down: controls rotate angle for cairo_rotate()
>  - 1/0: enable/disable calling pango_cairo_update_layout() before
>         pango_cairo_show_layout()
>  - r: recreate layout object
>  - R: recreate both context and layout object
>  - enter: dump image as xresult.png
> 
> Start the program and scale up to a factor 5 (hit pgup). It now looks
> like in attached result1.png. Hit '1' to enable calling
> pango_cairo_update_layout().
> Now you'll see result2.png, which is rather weird. Turn off calling
> pango_cairo_update_layout() (hit '0') and recreate the layout (hit 'r').
> The result stays bad. After recreating both the context and layout (hit
> 'R')
> the output is OK again. FWIW, there is no difference (in my eyes) when the
> image is rotated a bit.
> 
> What's going on here? I can't believe it is a font question as the output
> looks perfect to me when not calling pango_cairo_update_layout(). Hopefully
> it is just a silly error on my side.
> 
> Could you please take a look at this Behdad?
> 
> Regards,
> Theo
> 
> 
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