Hi everyone!
I would like to propose changing the internal representation of
rectangles in platform code.
We currently represent a rectangle by storing the coordinates of its
top-left corner, its width, and its height. I'll refer to this
representation as "x/y/w/h".
I would like to propose storing instead the coordinates of the
top-left corner, and the coordinates of the bottom-right corner. I'll
refer to this representation as "x1/y1/x2/y2".
The x1/y1/x2/y2 representation has several advantages over x/y/w/h:
- Several operations are more efficient with x1/y1/x2/y2, including
intersection,
union, and point-in-rect.
- The representation is more symmetric, since it stores two quantities of the
same kind (two points) rather than a point and a dimension (width/height).
- The representation is less susceptible to overflow. With x/y/w/h,
computation
of x2/y2 can overflow for a large range of values of x/y and w/h. However,
with x1/y1/x2/y2, computation of w/h cannot overflow if the coordinates are
signed and the resulting w/h is unsigned.
A known disadvantage of x1/y1/x2/y2 is that translating the rectangle
requires translating both points, whereas translating x/y/w/h only
requires translating one point. I think this disadvantage is minor in
comparison to the above advantages.
The proposed change would affect the class mozilla::gfx::BaseRect, and
classes that derive from it (such as CSSRect, LayoutRect, etc., and,
notably, nsRect and nsIntRect), but NOT other rectangle classes like
DOMRect.
I would like to propose making the transition as follows:
- Replace direct accesses to the 'width' and 'height' fields throughout
the codebase with calls to getter and setter methods. (There aren't
that many - on the order of a few dozen, last I checked.)
- Make the representation change, which is non-breaking now that
the direct accesses to 'width' and 'height' have been removed.
- Examine remaining calls to the getters and setters for width and
height and see if any can be better expressed using operations
on the points instead.
The Graphics team, which owns this code, is supportive of this change.
However, since this is a fundamental utility type that's used by a
variety of platform code, I would like to ask the wider platform
development community for feedback before proceeding. Please let me
know if you have any!
Thanks,
Botond
[1]
http://searchfox.org/mozilla-central/rev/672c83ed65da286b68be1d02799c35fdd14d0134/gfx/2d/BaseRect.h#46
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