[cairo] clear operator

[Andrea Canciani]

On Wed, Jun 1, 2011 at 10:06 AM, control H <control.h at gmail.com> wrote:
>>>  surf1=cairo_surface_create_similar(cairo_get_target(cr),
>>>                                     CAIRO_CONTENT_COLOR_ALPHA,
>>>                                     w,h);
>>>  cr1=cairo_create(surf1);
>>>  cairo_set_source_rgba(cr1,0.,1.,0.,.8);
>>>  cairo_move_to(cr1,w/2,h/2);
>>>  cairo_arc(cr1,w/2,h/2,MIN(w,h)/3,0,2*G_PI);
>>>  cairo_fill(cr1);
>>>
>>>  surf2=cairo_surface_create_similar(cairo_get_target(cr),
>>>                                     CAIRO_CONTENT_COLOR_ALPHA,
>>>                                     w,h);
>>>  cr2=cairo_create(surf2);
>>>  cairo_set_source_rgba(cr2,1.,1.,0.,.7);
>>>  cairo_arc(cr2,w/2.5,h/2,MIN(w,h)/4,0,2*G_PI);
>>>  cairo_fill(cr2);
>>>
>>>  cairo_set_operator(cr1,CAIRO_OPERATOR_CLEAR);
>>>  //cairo_set_operator(cr1,CAIRO_OPERATOR_DEST_OUT);
>>>  cairo_set_source_surface(cr1,surf2,0,0);
>>>  cairo_paint(cr1);
>>
>> surf1 is now completely transparent
>
> I don't understand. I expect surf1 to be the circle, except for the
> parts where is any pixel in surf2. That is what I want at least, but
> also how I interpret the operators at
> http://cairographics.org/operators/. I do understand the result if I
> change the operator into any other, like DEST_OUT. So could you please
> explain why in this case surf1 becomes _completely_ transparent?

Using http://cairographics.org/operators/ as reference:
 - paint implies a mask which covers the whole surface (1 everywhere)
 - CLEAR ignores the context source
 - you have not clipped to anything

The expected result can be computed based on the "Bounded" formula:
(src CLEAR dst) LERP_(clip IN mask) dst =
(0) LERP_(1 IN 1) dst =
0 LERP_1 dst = dst

>
>>>  cairo_set_source_surface(cr,surf1,0,0);
>>>  cairo_set_operator(cr,CAIRO_OPERATOR_OVER);
>>>  cairo_paint(cr);
>>
>> This is actually a no-op.
>>
>>>
>>>  cairo_surface_destroy(surf1);
>>>  cairo_surface_destroy(surf2);
>>>  cairo_destroy(cr1);
>>>  cairo_destroy(cr2);
>>>
>>>  cairo_destroy (cr);
>>>
>>>  return TRUE;
>>> }
>>>
>>> The result is just a white screen. However, if I change
>>
>> This is expected. When you paint() cr1 with a CLEAR
>> operator, you make it completely transparent.
>> Painting surf1 OVER cr has thus no effect at all
>> and correctly leaves surface white.
>>
>> Andrea
>
> Sure, when surf1 is already transparent that makes sense :)
>