Source code for fontTools.pens.basePen

"""fontTools.pens.basePen.py -- Tools and base classes to build pen objects.

The Pen Protocol

A Pen is a kind of object that standardizes the way how to "draw" outlines:
it is a middle man between an outline and a drawing. In other words:
it is an abstraction for drawing outlines, making sure that outline objects
don't need to know the details about how and where they're being drawn, and
that drawings don't need to know the details of how outlines are stored.

The most basic pattern is this::

	outline.draw(pen)  # 'outline' draws itself onto 'pen'

Pens can be used to render outlines to the screen, but also to construct
new outlines. Eg. an outline object can be both a drawable object (it has a
draw() method) as well as a pen itself: you *build* an outline using pen
methods.

The AbstractPen class defines the Pen protocol. It implements almost
nothing (only no-op closePath() and endPath() methods), but is useful
for documentation purposes. Subclassing it basically tells the reader:
"this class implements the Pen protocol.". An examples of an AbstractPen
subclass is :py:class:`fontTools.pens.transformPen.TransformPen`.

The BasePen class is a base implementation useful for pens that actually
draw (for example a pen renders outlines using a native graphics engine).
BasePen contains a lot of base functionality, making it very easy to build
a pen that fully conforms to the pen protocol. Note that if you subclass
BasePen, you *don't* override moveTo(), lineTo(), etc., but _moveTo(),
_lineTo(), etc. See the BasePen doc string for details. Examples of
BasePen subclasses are fontTools.pens.boundsPen.BoundsPen and
fontTools.pens.cocoaPen.CocoaPen.

Coordinates are usually expressed as (x, y) tuples, but generally any
sequence of length 2 will do.
"""

from typing import Tuple, Dict

from fontTools.misc.loggingTools import LogMixin
from fontTools.misc.transform import DecomposedTransform, Identity

__all__ = [
    "AbstractPen",
    "NullPen",
    "BasePen",
    "PenError",
    "decomposeSuperBezierSegment",
    "decomposeQuadraticSegment",
]


[docs] class PenError(Exception): """Represents an error during penning."""
class OpenContourError(PenError): pass
[docs] class AbstractPen:
[docs] def moveTo(self, pt: Tuple[float, float]) -> None: """Begin a new sub path, set the current point to 'pt'. You must end each sub path with a call to pen.closePath() or pen.endPath(). """ raise NotImplementedError
[docs] def lineTo(self, pt: Tuple[float, float]) -> None: """Draw a straight line from the current point to 'pt'.""" raise NotImplementedError
[docs] def curveTo(self, *points: Tuple[float, float]) -> None: """Draw a cubic bezier with an arbitrary number of control points. The last point specified is on-curve, all others are off-curve (control) points. If the number of control points is > 2, the segment is split into multiple bezier segments. This works like this: Let n be the number of control points (which is the number of arguments to this call minus 1). If n==2, a plain vanilla cubic bezier is drawn. If n==1, we fall back to a quadratic segment and if n==0 we draw a straight line. It gets interesting when n>2: n-1 PostScript-style cubic segments will be drawn as if it were one curve. See decomposeSuperBezierSegment(). The conversion algorithm used for n>2 is inspired by NURB splines, and is conceptually equivalent to the TrueType "implied points" principle. See also decomposeQuadraticSegment(). """ raise NotImplementedError
[docs] def qCurveTo(self, *points: Tuple[float, float]) -> None: """Draw a whole string of quadratic curve segments. The last point specified is on-curve, all others are off-curve points. This method implements TrueType-style curves, breaking up curves using 'implied points': between each two consequtive off-curve points, there is one implied point exactly in the middle between them. See also decomposeQuadraticSegment(). The last argument (normally the on-curve point) may be None. This is to support contours that have NO on-curve points (a rarely seen feature of TrueType outlines). """ raise NotImplementedError
[docs] def closePath(self) -> None: """Close the current sub path. You must call either pen.closePath() or pen.endPath() after each sub path. """ pass
[docs] def endPath(self) -> None: """End the current sub path, but don't close it. You must call either pen.closePath() or pen.endPath() after each sub path. """ pass
[docs] def addComponent( self, glyphName: str, transformation: Tuple[float, float, float, float, float, float], ) -> None: """Add a sub glyph. The 'transformation' argument must be a 6-tuple containing an affine transformation, or a Transform object from the fontTools.misc.transform module. More precisely: it should be a sequence containing 6 numbers. """ raise NotImplementedError
[docs] def addVarComponent( self, glyphName: str, transformation: DecomposedTransform, location: Dict[str, float], ) -> None: """Add a VarComponent sub glyph. The 'transformation' argument must be a DecomposedTransform from the fontTools.misc.transform module, and the 'location' argument must be a dictionary mapping axis tags to their locations. """ # GlyphSet decomposes for us raise AttributeError
[docs] class NullPen(AbstractPen): """A pen that does nothing."""
[docs] def moveTo(self, pt): pass
[docs] def lineTo(self, pt): pass
[docs] def curveTo(self, *points): pass
[docs] def qCurveTo(self, *points): pass
[docs] def closePath(self): pass
[docs] def endPath(self): pass
[docs] def addComponent(self, glyphName, transformation): pass
[docs] def addVarComponent(self, glyphName, transformation, location): pass
class LoggingPen(LogMixin, AbstractPen): """A pen with a ``log`` property (see fontTools.misc.loggingTools.LogMixin)""" pass class MissingComponentError(KeyError): """Indicates a component pointing to a non-existent glyph in the glyphset.""" class DecomposingPen(LoggingPen): """Implements a 'addComponent' method that decomposes components (i.e. draws them onto self as simple contours). It can also be used as a mixin class (e.g. see ContourRecordingPen). You must override moveTo, lineTo, curveTo and qCurveTo. You may additionally override closePath, endPath and addComponent. By default a warning message is logged when a base glyph is missing; set the class variable ``skipMissingComponents`` to False if you want all instances of a sub-class to raise a :class:`MissingComponentError` exception by default. """ skipMissingComponents = True # alias error for convenience MissingComponentError = MissingComponentError def __init__( self, glyphSet, *args, skipMissingComponents=None, reverseFlipped=False, **kwargs, ): """Takes a 'glyphSet' argument (dict), in which the glyphs that are referenced as components are looked up by their name. If the optional 'reverseFlipped' argument is True, components whose transformation matrix has a negative determinant will be decomposed with a reversed path direction to compensate for the flip. The optional 'skipMissingComponents' argument can be set to True/False to override the homonymous class attribute for a given pen instance. """ super(DecomposingPen, self).__init__(*args, **kwargs) self.glyphSet = glyphSet self.skipMissingComponents = ( self.__class__.skipMissingComponents if skipMissingComponents is None else skipMissingComponents ) self.reverseFlipped = reverseFlipped def addComponent(self, glyphName, transformation): """Transform the points of the base glyph and draw it onto self.""" from fontTools.pens.transformPen import TransformPen try: glyph = self.glyphSet[glyphName] except KeyError: if not self.skipMissingComponents: raise MissingComponentError(glyphName) self.log.warning("glyph '%s' is missing from glyphSet; skipped" % glyphName) else: pen = self if transformation != Identity: pen = TransformPen(pen, transformation) if self.reverseFlipped: # if the transformation has a negative determinant, it will # reverse the contour direction of the component a, b, c, d = transformation[:4] det = a * d - b * c if det < 0: from fontTools.pens.reverseContourPen import ReverseContourPen pen = ReverseContourPen(pen) glyph.draw(pen) def addVarComponent(self, glyphName, transformation, location): # GlyphSet decomposes for us raise AttributeError
[docs] class BasePen(DecomposingPen): """Base class for drawing pens. You must override _moveTo, _lineTo and _curveToOne. You may additionally override _closePath, _endPath, addComponent, addVarComponent, and/or _qCurveToOne. You should not override any other methods. """ def __init__(self, glyphSet=None): super(BasePen, self).__init__(glyphSet) self.__currentPoint = None # must override def _moveTo(self, pt): raise NotImplementedError def _lineTo(self, pt): raise NotImplementedError def _curveToOne(self, pt1, pt2, pt3): raise NotImplementedError # may override def _closePath(self): pass def _endPath(self): pass def _qCurveToOne(self, pt1, pt2): """This method implements the basic quadratic curve type. The default implementation delegates the work to the cubic curve function. Optionally override with a native implementation. """ pt0x, pt0y = self.__currentPoint pt1x, pt1y = pt1 pt2x, pt2y = pt2 mid1x = pt0x + 0.66666666666666667 * (pt1x - pt0x) mid1y = pt0y + 0.66666666666666667 * (pt1y - pt0y) mid2x = pt2x + 0.66666666666666667 * (pt1x - pt2x) mid2y = pt2y + 0.66666666666666667 * (pt1y - pt2y) self._curveToOne((mid1x, mid1y), (mid2x, mid2y), pt2) # don't override def _getCurrentPoint(self): """Return the current point. This is not part of the public interface, yet is useful for subclasses. """ return self.__currentPoint
[docs] def closePath(self): self._closePath() self.__currentPoint = None
[docs] def endPath(self): self._endPath() self.__currentPoint = None
[docs] def moveTo(self, pt): self._moveTo(pt) self.__currentPoint = pt
[docs] def lineTo(self, pt): self._lineTo(pt) self.__currentPoint = pt
[docs] def curveTo(self, *points): n = len(points) - 1 # 'n' is the number of control points assert n >= 0 if n == 2: # The common case, we have exactly two BCP's, so this is a standard # cubic bezier. Even though decomposeSuperBezierSegment() handles # this case just fine, we special-case it anyway since it's so # common. self._curveToOne(*points) self.__currentPoint = points[-1] elif n > 2: # n is the number of control points; split curve into n-1 cubic # bezier segments. The algorithm used here is inspired by NURB # splines and the TrueType "implied point" principle, and ensures # the smoothest possible connection between two curve segments, # with no disruption in the curvature. It is practical since it # allows one to construct multiple bezier segments with a much # smaller amount of points. _curveToOne = self._curveToOne for pt1, pt2, pt3 in decomposeSuperBezierSegment(points): _curveToOne(pt1, pt2, pt3) self.__currentPoint = pt3 elif n == 1: self.qCurveTo(*points) elif n == 0: self.lineTo(points[0]) else: raise AssertionError("can't get there from here")
[docs] def qCurveTo(self, *points): n = len(points) - 1 # 'n' is the number of control points assert n >= 0 if points[-1] is None: # Special case for TrueType quadratics: it is possible to # define a contour with NO on-curve points. BasePen supports # this by allowing the final argument (the expected on-curve # point) to be None. We simulate the feature by making the implied # on-curve point between the last and the first off-curve points # explicit. x, y = points[-2] # last off-curve point nx, ny = points[0] # first off-curve point impliedStartPoint = (0.5 * (x + nx), 0.5 * (y + ny)) self.__currentPoint = impliedStartPoint self._moveTo(impliedStartPoint) points = points[:-1] + (impliedStartPoint,) if n > 0: # Split the string of points into discrete quadratic curve # segments. Between any two consecutive off-curve points # there's an implied on-curve point exactly in the middle. # This is where the segment splits. _qCurveToOne = self._qCurveToOne for pt1, pt2 in decomposeQuadraticSegment(points): _qCurveToOne(pt1, pt2) self.__currentPoint = pt2 else: self.lineTo(points[0])
[docs] def decomposeSuperBezierSegment(points): """Split the SuperBezier described by 'points' into a list of regular bezier segments. The 'points' argument must be a sequence with length 3 or greater, containing (x, y) coordinates. The last point is the destination on-curve point, the rest of the points are off-curve points. The start point should not be supplied. This function returns a list of (pt1, pt2, pt3) tuples, which each specify a regular curveto-style bezier segment. """ n = len(points) - 1 assert n > 1 bezierSegments = [] pt1, pt2, pt3 = points[0], None, None for i in range(2, n + 1): # calculate points in between control points. nDivisions = min(i, 3, n - i + 2) for j in range(1, nDivisions): factor = j / nDivisions temp1 = points[i - 1] temp2 = points[i - 2] temp = ( temp2[0] + factor * (temp1[0] - temp2[0]), temp2[1] + factor * (temp1[1] - temp2[1]), ) if pt2 is None: pt2 = temp else: pt3 = (0.5 * (pt2[0] + temp[0]), 0.5 * (pt2[1] + temp[1])) bezierSegments.append((pt1, pt2, pt3)) pt1, pt2, pt3 = temp, None, None bezierSegments.append((pt1, points[-2], points[-1])) return bezierSegments
[docs] def decomposeQuadraticSegment(points): """Split the quadratic curve segment described by 'points' into a list of "atomic" quadratic segments. The 'points' argument must be a sequence with length 2 or greater, containing (x, y) coordinates. The last point is the destination on-curve point, the rest of the points are off-curve points. The start point should not be supplied. This function returns a list of (pt1, pt2) tuples, which each specify a plain quadratic bezier segment. """ n = len(points) - 1 assert n > 0 quadSegments = [] for i in range(n - 1): x, y = points[i] nx, ny = points[i + 1] impliedPt = (0.5 * (x + nx), 0.5 * (y + ny)) quadSegments.append((points[i], impliedPt)) quadSegments.append((points[-2], points[-1])) return quadSegments
class _TestPen(BasePen): """Test class that prints PostScript to stdout.""" def _moveTo(self, pt): print("%s %s moveto" % (pt[0], pt[1])) def _lineTo(self, pt): print("%s %s lineto" % (pt[0], pt[1])) def _curveToOne(self, bcp1, bcp2, pt): print( "%s %s %s %s %s %s curveto" % (bcp1[0], bcp1[1], bcp2[0], bcp2[1], pt[0], pt[1]) ) def _closePath(self): print("closepath") if __name__ == "__main__": pen = _TestPen(None) pen.moveTo((0, 0)) pen.lineTo((0, 100)) pen.curveTo((50, 75), (60, 50), (50, 25), (0, 0)) pen.closePath() pen = _TestPen(None) # testing the "no on-curve point" scenario pen.qCurveTo((0, 0), (0, 100), (100, 100), (100, 0), None) pen.closePath()