[cairo] here is pycairo +numpy example (Re: one question answered, and another comes (Re: FT_Bitmap_Convert, and FreeType's bitmap format vs Cairo's))

[Hin-Tak Leung]

Sorry this is the 3rd time I answer my own question... I worked out that it is possible to expose the buffer of a clipped region of an existing cairo surface directly as a multi-dimensional numpy array. So one could do a rather fast vector product between that and another region to see if there is there is any overlap. The code is from line 150 onwards:

https://github.com/rougier/freetype-py/pull/55/commits/e94090455dfb871581cc3c242a086d725f743389

and the result of packed drawings:
https://github.com/rougier/freetype-py/pull/55#issuecomment-297804370

I think it might be possible to do such things without using numpy, with a combination of drawing the 2nd shape as a reverse mask and see if cairo_fill_extents() goes to zero... anyway, for another time.

There is also something not-obvious in the cairo documentation: what do drawing operations do to FORMAT_A1/FORMAT_A8 surfaces, if I am just using them as plain gray drawing surfaces? I thought I'd just get 1-bit/8-bit equivalent operations (i.e. things going B/W or gray-scale). The answer is only obvious on hind-sight - A1/A8 means "alpha". Only the alpha channel of any drawing operation has any effect. Can somebody update the documentation to make it a bit more obvious?
(https://cairographics.org/manual/cairo-Image-Surfaces.html
http://pycairo.readthedocs.io/en/latest/reference/surfaces.html
http://pycairo.readthedocs.io/en/latest/reference/constants.html#constants-format )

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On Tue, 25/4/17, Hin-Tak Leung <htl10 at users.sourceforge.net> wrote:

... One of freetype-py's drawing example
 uses a vector sum between a clip of the drawing surface with
 the current drawing pattern to see if we are drawing over
 something that's already drawn in this single line:
 
 if (I[y-h//2:y-h//2+h,
 x-w//2:x-w//2+w,0] * L).sum() == 0:
 
 i.e. this calculates if L will draw on
 anything previously drawn in I. I am trying to figure out
 how to do this in cairo.
 
 How does one check if two shapes
 overlap, or two paths intersect, in cairo?
...