Unconfuse fontsize calculation

manim/constants.py

@@ -63,6 +63,9 @@
     "DEFAULT_POINT_DENSITY_1D",
     "DEFAULT_STROKE_WIDTH",
     "DEFAULT_FONT_SIZE",
+    "DEFAULT_FONT_SIZE_IN_WORLD_SPACE",
+    "DEFAULT_FONTSIZE_IN_WORLD_SPACE",
+    "DEFAULT_FONT_SIZE_IN_WOLRD_SPACE",
     "SCALE_FACTOR_PER_FONT_POINT",
     "PI",
     "TAU",
@@ -177,12 +180,25 @@
 DEFAULT_POINTWISE_FUNCTION_RUN_TIME = 3.0
 DEFAULT_WAIT_TIME = 1.0
 
+# Font calculation
+DEFAULT_FONT_SIZE_IN_WORLD_SPACE = 0.5
+"""Length occupied by an 'EM' character in manim space, when manim default font size is used.
+An example of an 'EM' character is the em dash: '—'.
+The chosen value is an arbitrary convention."""
+DEFAULT_FONT_SIZE_IN_WOLRD_SPACE = DEFAULT_FONT_SIZE_IN_WORLD_SPACE
+DEFAULT_FONTSIZE_IN_WORLD_SPACE = DEFAULT_FONT_SIZE_IN_WORLD_SPACE
+
 # Misc
 DEFAULT_POINT_DENSITY_2D = 25
 DEFAULT_POINT_DENSITY_1D = 10
 DEFAULT_STROKE_WIDTH = 4
 DEFAULT_FONT_SIZE = 48
-SCALE_FACTOR_PER_FONT_POINT = 1 / 960
+SCALE_FACTOR_PER_FONT_POINT = (  # legacy, not used in code anymore
+    DEFAULT_FONT_SIZE_IN_WORLD_SPACE
+    / 10  # LaTeX default font size in pixels
+    / DEFAULT_FONT_SIZE
+)
+"""Downscale factor from LaTeX pixel to world space, divided by DEFAULT_FONT_SIZE"""
 
 # Mathematical constants
 PI = np.pi

manim/mobject/text/tex_mobject.py

@@ -41,6 +41,12 @@
 from ..opengl.opengl_compatibility import ConvertToOpenGL
 
 MATHTEX_SUBSTRING = "substring"
+TEX_SVG_UNITS_PER_PT = 1
+"""Scale factor from TeX SVG output to point units.
+TeX outputs 1 svg unit per point (72 DPI)."""
+TEX_DEFAULT_FONT_SIZE_PT = 10
+"""The fontsize used by default by tex: 10pt.
+This means that one 'EM' character like '—' will be 13.333 svg units, since 1pt=4/3 px"""
 
 
 class SingleStringMathTex(SVGMobject):
@@ -70,7 +76,7 @@ def __init__(
         if color is None:
             color = VMobject().color
 
-        self._font_size = font_size
+        self.initial_font_size = font_size
         self.organize_left_to_right = organize_left_to_right
         self.tex_environment = tex_environment
         if tex_template is None:
@@ -97,12 +103,18 @@ def __init__(
         )
         self.init_colors()
 
+        if height is None:
+            self.scale(
+                font_size
+                / DEFAULT_FONT_SIZE
+                / TEX_DEFAULT_FONT_SIZE_PT  # convert latex svg output to "fontsize" or "em" units
+                * DEFAULT_FONTSIZE_IN_WORLD_SPACE  # then to worldspace
+            )
+
         # used for scaling via font_size.setter
+        # we use the initial size in world space
         self.initial_height = self.height
 
-        if height is None:
-            self.font_size = self._font_size
-
         if self.organize_left_to_right:
             self._organize_submobjects_left_to_right()
 
@@ -112,7 +124,7 @@ def __repr__(self) -> str:
     @property
     def font_size(self) -> float:
         """The font size of the tex mobject."""
-        return self.height / self.initial_height / SCALE_FACTOR_PER_FONT_POINT
+        return self.height / self.initial_height * self.initial_font_size
 
     @font_size.setter
     def font_size(self, font_val: float) -> None:

manim/mobject/text/text_mobject.py

@@ -80,9 +80,13 @@ def construct(self):
 
     from manim.typing import Point3D
 
-TEXT_MOB_SCALE_FACTOR = 0.05
 DEFAULT_LINE_SPACING_SCALE = 0.3
-TEXT2SVG_ADJUSTMENT_FACTOR = 4.8
+PANGO_SVG_UNITS_PER_PT = 4 / 3
+"""Scale factor from Pango SVG output to point units.
+Pango outputs 4/3 svg units per point (72 DPI)."""
+TEXT_FONT_SIZE_PT = 10
+"""The font size we use to render the unscaled text in the SVG.
+Note that the typical EM dash (—) will be 13.333 svg units, since 1pt = 4/3 px"""
 
 __all__ = ["Text", "Paragraph", "MarkupText", "register_font"]
 
@@ -466,7 +470,7 @@ def __init__(
                 else:
                     logger.warning(f"Font {font} not in {fonts_list}.")
         self.font = font
-        self._font_size = float(font_size)
+        self.initial_font_size = float(font_size)
         # needs to be a float or else size is inflated when font_size = 24
         # (unknown cause)
         self.slant = slant
@@ -508,10 +512,13 @@ def __init__(
         self.text = text_without_tabs
         if self.line_spacing == -1:
             self.line_spacing = (
-                self._font_size + self._font_size * DEFAULT_LINE_SPACING_SCALE
+                self.initial_font_size
+                + self.initial_font_size * DEFAULT_LINE_SPACING_SCALE
             )
         else:
-            self.line_spacing = self._font_size + self._font_size * self.line_spacing
+            self.line_spacing = (
+                self.initial_font_size + self.initial_font_size * self.line_spacing
+            )
 
         parsed_color: ManimColor = ManimColor(color) if color else VMobject().color
         file_name = self._text2svg(parsed_color.to_hex())
@@ -590,22 +597,20 @@ def add_line_to(end: Point3D) -> None:
             each.points = np.array(closed_curve_points, ndmin=2)
         # anti-aliasing
         if height is None and width is None:
-            self.scale(TEXT_MOB_SCALE_FACTOR)
+            self.scale(
+                1
+                / PANGO_SVG_UNITS_PER_PT  # convert svg output to "pt" units
+                / TEXT_FONT_SIZE_PT  # then to "fontsize" or "EM" units
+                * DEFAULT_FONT_SIZE_IN_WORLD_SPACE  # then to world space
+            )
         self.initial_height = self.height
 
     def __repr__(self) -> str:
         return f"Text({repr(self.original_text)})"
 
     @property
     def font_size(self) -> float:
-        return (
-            self.height
-            / self.initial_height
-            / TEXT_MOB_SCALE_FACTOR
-            * 2.4
-            * self._font_size
-            / DEFAULT_FONT_SIZE
-        )
+        return self.height / self.initial_height * self.initial_font_size
 
     @font_size.setter
     def font_size(self, font_val: float) -> None:
@@ -657,7 +662,7 @@ def _text2hash(self, color: ParsableManimColor) -> str:
             "PANGO" + self.font + self.slant + self.weight + str(color)
         )  # to differentiate Text and CairoText
         settings += str(self.t2f) + str(self.t2s) + str(self.t2w) + str(self.t2c)
-        settings += str(self.line_spacing) + str(self._font_size)
+        settings += str(self.line_spacing) + str(self.initial_font_size)
         settings += str(self.disable_ligatures)
         settings += str(self.gradient)
         id_str = self.text + settings
@@ -798,10 +803,8 @@ def _text2settings(self, color: ParsableManimColor) -> list[TextSetting]:
 
     def _text2svg(self, color: ParsableManimColor) -> str:
         """Convert the text to SVG using Pango."""
-        size = self._font_size
-        line_spacing = self.line_spacing
-        size /= TEXT2SVG_ADJUSTMENT_FACTOR
-        line_spacing /= TEXT2SVG_ADJUSTMENT_FACTOR
+        size = TEXT_FONT_SIZE_PT * self.initial_font_size / DEFAULT_FONT_SIZE
+        line_spacing = TEXT_FONT_SIZE_PT * self.line_spacing / DEFAULT_FONT_SIZE
 
         dir_name = config.get_dir("text_dir")
         dir_name.mkdir(parents=True, exist_ok=True)
@@ -1181,7 +1184,7 @@ def __init__(
                 else:
                     logger.warning(f"Font {font} not in {fonts_list}.")
         self.font = font
-        self._font_size = float(font_size)
+        self.initial_font_size = float(font_size)
         self.slant = slant
         self.weight = weight
         self.gradient = gradient
@@ -1206,10 +1209,13 @@ def __init__(
 
         if self.line_spacing == -1:
             self.line_spacing = (
-                self._font_size + self._font_size * DEFAULT_LINE_SPACING_SCALE
+                self.initial_font_size
+                + self.initial_font_size * DEFAULT_LINE_SPACING_SCALE
             )
         else:
-            self.line_spacing = self._font_size + self._font_size * self.line_spacing
+            self.line_spacing = (
+                self.initial_font_size + self.initial_font_size * self.line_spacing
+            )
 
         parsed_color: ManimColor = ManimColor(color) if color else VMobject().color
         file_name = self._text2svg(parsed_color)
@@ -1302,20 +1308,18 @@ def add_line_to(end: Point3D) -> None:
             )
         # anti-aliasing
         if height is None and width is None:
-            self.scale(TEXT_MOB_SCALE_FACTOR)
+            self.scale(
+                1
+                / PANGO_SVG_UNITS_PER_PT  # convert svg output to "pt" units
+                / TEXT_FONT_SIZE_PT  # then to "fontsize" or "EM" units
+                * DEFAULT_FONT_SIZE_IN_WORLD_SPACE  # then to world space
+            )
 
         self.initial_height = self.height
 
     @property
     def font_size(self) -> float:
-        return (
-            self.height
-            / self.initial_height
-            / TEXT_MOB_SCALE_FACTOR
-            * 2.4
-            * self._font_size
-            / DEFAULT_FONT_SIZE
-        )
+        return self.height / self.initial_height * self.initial_font_size
 
     @font_size.setter
     def font_size(self, font_val: float) -> None:
@@ -1334,7 +1338,7 @@ def _text2hash(self, color: ParsableManimColor) -> str:
             + self.weight
             + ManimColor(color).to_hex().lower()
         )  # to differentiate from classical Pango Text
-        settings += str(self.line_spacing) + str(self._font_size)
+        settings += str(self.line_spacing) + str(self.initial_font_size)
         settings += str(self.disable_ligatures)
         settings += str(self.justify)
         id_str = self.text + settings
@@ -1345,10 +1349,9 @@ def _text2hash(self, color: ParsableManimColor) -> str:
     def _text2svg(self, color: ParsableManimColor | None) -> str:
         """Convert the text to SVG using Pango."""
         color = ManimColor(color)
-        size = self._font_size
-        line_spacing: float = self.line_spacing
-        size /= TEXT2SVG_ADJUSTMENT_FACTOR
-        line_spacing /= TEXT2SVG_ADJUSTMENT_FACTOR
+        # scale down so that manim font size becomes a specific target size in pt for pango
+        size = TEXT_FONT_SIZE_PT * self.initial_font_size / DEFAULT_FONT_SIZE
+        line_spacing = TEXT_FONT_SIZE_PT * self.line_spacing / DEFAULT_FONT_SIZE
 
         dir_name = config.get_dir("text_dir")
         dir_name.mkdir(parents=True, exist_ok=True)

tests/module/mobject/text/test_texmobject.py

@@ -6,6 +6,7 @@
 import pytest
 
 from manim import MathTex, SingleStringMathTex, Tex, TexTemplate, tempconfig
+from manim.constants import DEFAULT_FONT_SIZE_IN_WORLD_SPACE
 
 
 def test_MathTex(config):
@@ -256,6 +257,11 @@ def test_changing_font_size():
     assert num.height == Tex("0", font_size=48).height
 
 
+def test_mathtex_em_dash_width_default_font_size():
+    em_dash = MathTex(r"\text{—}")
+    assert abs(em_dash.width - DEFAULT_FONT_SIZE_IN_WORLD_SPACE) < 0.005
+
+
 def test_log_error_context(capsys):
     """Test that the environment context of an error is correctly logged if it exists"""
     invalid_tex = r"""

tests/module/mobject/text/test_text_mobject.py

@@ -3,6 +3,7 @@
 from contextlib import redirect_stdout
 from io import StringIO
 
+from manim.constants import DEFAULT_FONT_SIZE_IN_WORLD_SPACE
 from manim.mobject.text.text_mobject import MarkupText, Text
 
 
@@ -32,3 +33,17 @@ def warning_printed(font: str, **kwargs) -> bool:
 
     # check random string (should be warning)
     assert warning_printed("Manim!" * 3, warn_missing_font=True)
+
+
+def test_em_dash_width_default_font_size():
+    from manimpango import list_fonts
+
+    available = list_fonts()
+    # Fonts that render em dash at full font size width
+    candidates = ["Arial", "Liberation Sans"]
+
+    font = next((f for f in candidates if f in available), None)
+    assert font is not None, f"No suitable font found. Available: {available}"
+
+    text_em_dash = Text("—", font=font)
+    assert abs(text_em_dash.width - DEFAULT_FONT_SIZE_IN_WORLD_SPACE) < 0.01