SkRegion Overview

Regions - set operations with rectangles

Regions are a highly compressed way to represent (integer) areas. Skia uses them to represent (internally) the current clip on the canvas. Regions take their inspiration from the data type with the same name on the original Macintosh (thank you Bill).

Regions are opaque structures, but they can be queried via iterators. Best of all, they can be combined with other regions and with rectangles (which can be thought of as “simple” regions. If you remember Set operations from math class (intersection, union, difference, etc.), then you’re all ready to use regions.

bool SkRegion::isEmpty();
bool SkRegion::isRect();
bool SkRegion::isComplex();

Regions can be classified into one of three types: empty, rectangular, or complex.

Empty regions are just that, empty. All empty regions are equal (using operator==). Compare this to rectangles (SkRect or SkIRect). Any rectangle with fLeft >= fRight or fTop >= fBottom is consider empty, but clearly there are different empty rectangles that are not equal.

SkRect a = { 0, 0, 0, 0 };
SkRect b = { 1, 1, 1, 1 };

Both a and b are empty, but they are definitely not equal to each other. However, with regions, all empty regions are equal. If you query its bounds, you will always get { 0, 0, 0, 0 }. Even if you translate it, it will still be all zeros.

SkRegion a, b;   // regions default to empty
assert(a == b);
a.offset(10, 20);
assert(a == b);
assert(a.getBounds() == { 0, 0, 0, 0 });   // not legal C++, but you get the point
assert(b.getBounds() == { 0, 0, 0, 0 });

To initialize a region to something more interesting, use one of the set() methods

SkRegion a, b;
a.setRect(10, 10, 50, 50);
b.setRect(rect);    // see SkIRect
c.setPath(path);   // see SkPath

This is the first step that SkCanvas performs when one of its clip…() methods are called. The clip data is first transformed into device coordinates (see SkMatrix), and then a region is build from the data (either a rect or a path). The final step is to combine this new region with the existing clip using the specified operator.

enum Op {
    kUnion_Op,
    kIntersect_Op,
    kDifference_Op,
    kXor_Op,
    kReverseDifference_Op,
    kReplace_Op
};

By default, intersect op is used when a clip call is made, but the other operators are equally valid.

// returns true if the resulting clip is non-empty (i.e. drawing can
// still occur)
bool SkCanvas::clipRect(const SkRect& rect, SkRegion::Op op) {
    SkRegion rgn;

    // peek at the CTM (current transformation matrix on the canvas)
    const SkMatrix& m = this->getTotalMatrix();

    if (m.rectStaysRect()) {    // check if a transformed rect can be
                                // represented as another rect

        SkRect deviceRect;
        m.mapRect(&deviceRect, rect);
        SkIRect intRect;
        deviceRect.round(&intRect);
        rgn.setRect(intRect);
    } else {  // matrix rotates or skew (or is perspective)
        SkPath path;
        path.addRect(rect);
        path.transform(m);
        rgn.setPath(path);
    }

    // now combine the new region with the current one, using the specified *op*
    return fCurrentClip.op(rgn, op);
}