[cairo] Help for using cairo_region and clip

[Adhir Garg]

Hi Carlos


On Wed, Oct 10, 2012 at 4:34 PM, Carlos López González
<genetita at gmail.com> wrote:
> Hi Adhir,
>
> 2012/10/10 Adhir Garg <adhir.garg at gmail.com>:
>> Hello Carlos,
>>
>> Thanks for your time for replying.
>>
>> However, the question still remains unanswered. I am trying to explain
>> problem in following steps
>>
>> 1. Create a big, filled Rectangle 1000x1000 - cairo_rectangle (cr, 0,
>> 0, 1000, 1000);
>> 2. Create a BIG Rectangle Region 100x100  using
>> cairo_region_create_rectangle(cr, rectangle*);
>> 3. Exclude a part (region) of it using subtract -
>> cairo_region_subtract_rectangle
>> Now I assume that my Region consists of reduced size < 100X100
>> 4. I try to create with another shape (say Rectangle) -  cairo_rectangle
>> 5. If I do cairo_clip(), I only get the rectangle of Step 4
>
> Can you be more explicit with this code? Please paste the code that
> currently you build and run to check that it doesn't do what you want.
> I think that you cannot subtract a cairo_region_t from a path or
> viceversa. Cairo regions are for raster surfaces (where those integers
> coincide with pixels) but in general a rectangle represents pixels or
> not, depending on the surface that the cairo context was initialized.
>
>>
>> My final expected output is intersection of Region at (3) and Clip in (4).
>> However, I am unable to find any API that makes use of cairo_region_t.
>>
>
> Possibly you don't need to use cairo_region_t at all, and just work
> with paths and clips.
yes, I can work with Paths and Clips, and every time, we can query for
rectangles, create again a path and then clip it.
But if we have a function that would be good.

>> Prior to 1.10.0, we had API set_clip_region(), This was responsible
>> for setting the current clip region to current region.
>>
>> In case, we have such API, where we are actually making use of
>> cairo_region, it would be great.
>
>
> Does this help?
> http://www.gtkforums.com/viewtopic.php?f=3&t=54873
Exactly, this is a roundabout approach for the same issue.
We should have the function
cairo_bool_t
cairo_clip_region (cairo_t        *cr,
                   cairo_region_t *region)
in Cairo API itself.

Thanks a ton Carlos !!
>>
>> Regards,
>> Adhir
>>
>
> Regards,
>
>> On Wed, Oct 10, 2012 at 3:33 PM, Carlos López González
>> <genetita at gmail.com> wrote:
>>> Hi Adhir,
>>> According to the manual:
>>> "Regions are a simple graphical data type representing an area of
>>> integer-aligned rectangles. They are often used on raster surfaces to
>>> track areas of interest, such as change or clip areas."
>>>
>>> Also:
>>>
>>> "cairo_clip ()
>>> void   cairo_clip (cairo_t *cr);
>>>
>>> Establishes a new clip region by intersecting the current clip region
>>> with the current path as it would be filled by cairo_fill() and
>>> according to the current fill rule (see cairo_set_fill_rule())."
>>>
>>> Do not confuse the "region" word from the cairo_clip description with
>>> the cairo_region_t type. They are pretty different.  The first is like
>>> a path and the second is a pixel aligned rectangle. In general paths
>>> can be of any kind (arcs, strokes, rectangles).
>>>
>>> So if you want to clip your path with a rectangle, just use a
>>> rectangle path as clipping path
>>>
>>> /* prepare your context and surface here*/
>>> cairo_rectangle(cr, x, y, width, height);
>>> /* add more paths here to the clip*/
>>> cairo_clip(cr);
>>> /*your stroke painting stuff here*/
>>> cairo_move_to, cairo_line_to, etc.
>>>
>>> See:
>>> http://cairographics.org/samples/clip/
>>>
>>> I hope it helped.
>>> Carlos
>>>
>>> 2012/10/10 Adhir Garg <adhir.garg at gmail.com>
>>>>
>>>> Hello,
>>>>
>>>> I am pretty new to Cairo and have a query regarding use of
>>>> cairo_region_t and clip.
>>>>
>>>> What I need is
>>>> 1. Create a region - created using cairo_region_create_rectangle
>>>> 2. Create a Path and clip it with region - Here if I use cairo_clip(),
>>>> then the region is not taken into consideration.
>>>>
>>>> Somehow, I couldn't locate correct API for intersection of region with clip.
>>>> On extended platforms I could get set_clip_region, but could not find
>>>> the same in Cairo listed APIs.
>>>>
>>>> Thanks in advance for your help.
>>>>
>>>> Regards,
>>>> Adhir
>>>> --
>>>> cairo mailing list
>>>> cairo at cairographics.org
>>>> http://lists.cairographics.org/mailman/listinfo/cairo
>>>
>>>
>>>
>>>
>>> --
>>> Carlos
>>> http://synfig.org
>
>
>
> --
> Carlos
> http://synfig.org