Add support for changing viewBox value

svglab/attrparse/length.py

@@ -85,6 +85,10 @@ def serialize(self) -> str:
         value = serialize.serialize(self.value)
         return f"{value}{self.unit or ''}"
 
+    @classmethod
+    def zero(cls) -> Length:
+        return cls(0)
+
     @override
     def __add__(self, other: Length) -> Self:
         other_value = other.to(self.unit).value
@@ -95,6 +99,9 @@ def __add__(self, other: Length) -> Self:
     def __mul__(self, other: float) -> Self:
         return type(self)(value=self.value * other, unit=self.unit)
 
+    def __bool__(self) -> bool:
+        return bool(self.value)
+
     @override
     def __float__(self) -> float:
         return self.to(None).value

svglab/attrparse/point.py

@@ -121,7 +121,7 @@ def __eq__(self, other: object) -> bool:
         )
 
     def __bool__(self) -> bool:
-        return self == self.zero()
+        return self != self.zero()
 
     def __complex__(self) -> complex:
         return complex(self.x, self.y)

svglab/elements/svg.py

@@ -5,6 +5,7 @@
 from typing_extensions import final, overload
 
 from svglab import protocols, serialize
+from svglab.attrparse import point
 from svglab.attrs import groups, regular
 from svglab.elements import traits
 
@@ -106,3 +107,62 @@ def save(
 
             if trailing_newline:
                 file.write("\n")
+
+    def set_viewbox(
+        self, viewbox: tuple[float, float, float, float]
+    ) -> None:
+        """Set a new value for the `viewBox` attribute.
+
+        This method sets a new value for the `viewBox` attribute and scales and
+        translates the SVG content so that the visual representation of the SVG
+        remains unchanged.
+
+        If the `viewBox` is not set, the method uses the `width` and `height`
+        attributes to calculate the initial viewBox. If the `width`
+        and `height` attributes are not set, the method raises an exception.
+
+        The new `viewBox` must have the same aspect ratio as the old `viewBox`.
+        If the aspect ratios differ, the method raises an exception.
+
+        Any attributes of type `Length` in the SVG must be convertible to
+        user units. If an attribute is not convertible, the method raises an
+        exception.
+
+        Args:
+        viewbox: A tuple of four numbers representing the new viewBox.
+
+        Raises:
+        ValueError: If `viewBox` is not set and `width` and `height` are not
+            set or if the aspect ratios of the old and new viewBox differ.
+        SvgUnitConversionError: If an attribute is not convertible to user
+            units.
+
+        """
+        if self.viewBox is None:
+            if self.width is None or self.height is None:
+                raise ValueError(
+                    "Either viewBox or width and height must be set"
+                )
+            old_viewbox = (0, 0, self.width, self.height)
+        else:
+            old_viewbox = self.viewBox
+
+        old_min_x = float(old_viewbox[0])
+        old_min_y = float(old_viewbox[1])
+        old_width = float(old_viewbox[2])
+        old_height = float(old_viewbox[3])
+
+        min_x, min_y, width, height = viewbox
+
+        translate = point.Point(min_x - old_min_x, min_y - old_min_y)
+
+        x_scale = width / old_width
+        y_scale = height / old_height
+
+        if x_scale != y_scale:
+            raise ValueError("Aspect ratios of old and new viewBox differ")
+
+        self.scale(x_scale)
+        self.translate(translate)
+
+        self.viewBox = (min_x, min_y, width, height)

svglab/elements/transforms.py

@@ -1,5 +1,6 @@
 from typing_extensions import TypeVar, cast
 
+from svglab import utils
 from svglab.attrparse import length, point, transform
 from svglab.attrs import common, presentation, regular
 
@@ -93,6 +94,9 @@ def scale_distance_along_a_path_attrs(tag: object, by: float) -> None:
 
 
 def scale(tag: object, by: float) -> None:  # noqa: C901, PLR0912
+    if utils.is_close(by, 1):
+        return
+
     if isinstance(tag, regular.Width):
         tag.width = _scale_attr(tag.width, by)
     if isinstance(tag, regular.Height):
@@ -121,6 +125,7 @@ def scale(tag: object, by: float) -> None:  # noqa: C901, PLR0912
         tag.points = list(transform.Scale(by) @ tag.points)
     if isinstance(tag, regular.D) and tag.d is not None:
         tag.d = transform.Scale(by) @ tag.d
+    # TODO: handle transform attribute
 
     # these assignments have to be mutually exclusive, because the
     # type checker doesn't know that x being a <number> implies that x is
@@ -147,9 +152,14 @@ def scale(tag: object, by: float) -> None:  # noqa: C901, PLR0912
         scale_distance_along_a_path_attrs(tag, by)
 
 
-def translate(tag: object, by: point.Point) -> None:  # noqa: C901
+def translate(tag: object, by: point.Point) -> None:  # noqa: C901, PLR0912
+    if not by:
+        return
+
     x, y = by
+    zero = length.Length.zero()
 
+    # these attributes are mandatory for the respective elements
     if isinstance(tag, regular.X1):
         tag.x1 = _translate_attr(tag.x1, x)
     if isinstance(tag, regular.Y1):
@@ -158,21 +168,25 @@ def translate(tag: object, by: point.Point) -> None:  # noqa: C901
         tag.x2 = _translate_attr(tag.x2, x)
     if isinstance(tag, regular.Y2):
         tag.y2 = _translate_attr(tag.y2, y)
+
+    # but these are not, so if they are not present, we initialize them to 0
     if isinstance(tag, regular.Cx):
-        tag.cx = _translate_attr(tag.cx, x)
+        tag.cx = _translate_attr(tag.cx or zero, x)
     if isinstance(tag, regular.Cy):
-        tag.cy = _translate_attr(tag.cy, y)
-    if isinstance(tag, regular.Points) and tag.points is not None:
-        tag.points = list(transform.Translate(x, y) @ tag.points)
-    if isinstance(tag, regular.D) and tag.d is not None:
-        tag.d += by
+        tag.cy = _translate_attr(tag.cy or zero, y)
 
-    if isinstance(tag, regular.XNumber):  # noqa: SIM114
-        tag.x = _translate_attr(tag.x, x)
+    if isinstance(tag, regular.XNumber):
+        tag.x = _translate_attr(tag.x or 0, x)
     elif isinstance(tag, regular.XCoordinate):
-        tag.x = _translate_attr(tag.x, x)
+        tag.x = _translate_attr(tag.x or zero, x)
 
-    if isinstance(tag, regular.YNumber):  # noqa: SIM114
-        tag.y = _translate_attr(tag.y, y)
+    if isinstance(tag, regular.YNumber):
+        tag.y = _translate_attr(tag.y or 0, y)
     elif isinstance(tag, regular.YCoordinate):
-        tag.y = _translate_attr(tag.y, y)
+        tag.y = _translate_attr(tag.y or zero, y)
+
+    if isinstance(tag, regular.Points) and tag.points is not None:
+        tag.points = list(transform.Translate(x, y) @ tag.points)
+    if isinstance(tag, regular.D) and tag.d is not None:
+        tag.d = tag.d + by
+    # TODO: handle transform attribute