[cairo] Extents under rotation

Uli Schlachter psychon at znc.in
Sun Dec 8 09:21:25 PST 2013

Hi again,

On 07.12.2013 08:31, Donn wrote:
> On 06/12/2013 17:13, Bill Spitzak wrote:
>> The matrices are multiplied before this is done, so this error does not
>> accumulate. For instance if you rotate by 45 degrees and then back by 45
>> degrees, it will act like a rotation of 0 and produce your expected
>> bounding box, rather than enlarging the bounding box twice.
> I'm sure I will face these things soon when I institute picking beyond a
> basic bbox hit approach.
> Bill,
> Do you have a way to draw a tight bbox in user-space around a path?

I was bored, so:

See the attached test program (written in C since I don't know vala). It uses 
three different ways for getting the extents of a circle.

1) cairo_path_extents()
2) cairo_path_extents() after cairo_identity_matrix()
3) Some slow calculations with cairo_copy_path_flat(). See get_path_extents().

The first row is painted without any rotation (I needed this since I made a typo 
at first with x1/y1/x2/y2 that I took a while to find) with the calculations as 
mentioned above.

The second row is rotated by 67° and the individual rectangles are again 
calculated as above.

Hope this helps,
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