# [cairo] Questions about operators, clipping and etc

Taekyun Kim podain77 at gmail.com
Sun Jun 19 23:10:44 PDT 2011

```Thanks for the replay.

Basically when you use a Bounded operator, you are not touching anything
> outside the extents of your mask, while an XRender operator can change the
> destination even outside the mask.
> For example if you do an "IN" operation, everything outside the mask but
> inside the clip becomes transparent.

> The equation which follows from the description is:
> dst' = ((src IN mask) OP dst) LERP_clip dst
>
> In particular, it explains the (src IN mask) part, i.e.
> the source is cleared outside the mask.

Ok, now I clearly understand why CLEAR and SOURCE should be explicitly
bounded by mask. If (zero OP dst) == dst, we can classify these operators as
"Any" (same for both bounded and unbounded interpretation). For remaining 4
unbounded operators, result of compositing zero with dst is always zero i.e.
(zero OP dst) == zero, where OP = IN, OUT, DEST_IN, DEST_ATOP.

So following equation should hold for 4 unbounded operators.
dst = zero (where mask is not allowed)

> > 3. Limit boxes
> > Boxes, traps, polygon can have there limit boxes. I thought the boxes
> should
> > be disjoint,
> > because I couldn't find any handling of overlapping boxes. Is this true?
>
> No, overlappping boxes are handled by the tessellator and/or the
> rasterizer.

I used the term "boxes" referring to the "limit boxes". I know that boxes
(not limit boxes) are tessellated according to the fill rule by
cairo_bentley_ottmann_tessellate_boxes(). But I couldn't find any accessing
to limits and num_limits inside of tessellator code. Instead, in case of
polygon, an edge is clipped with each of limit boxes before it is added. So
if the limit boxes should be regarded as a set of rectangular paths rather
than rectangular region, we have to consider the fill rule of the limit
boxes. But edge clipping does not consider the fill rule of the limit boxes.
This is also same for cairo_traps_t. So, I concluded that limit boxes should
be disjoint (regional representation) so that we can ignore the fill rule.

--
Best Regards,
Taekyun Kim
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