.. py:module:: PIL.ImageDraw
.. py:currentmodule:: PIL.ImageDraw
:py:mod:`~PIL.ImageDraw` Module
The :py:mod:`~PIL.ImageDraw` module provides simple 2D graphics for :py:class:`~PIL.Image.Image` objects. You can use this module to create new images, annotate or retouch existing images, and to generate graphics on the fly for web use.
For a more advanced drawing library for PIL, see the aggdraw module.
import sys
from PIL import Image, ImageDraw
with Image.open("hopper.jpg") as im:
draw = ImageDraw.Draw(im)
draw.line((0, 0) + im.size, fill=128)
draw.line((0, im.size[1], im.size[0], 0), fill=128)
# write to stdout
im.save(sys.stdout, "PNG")The graphics interface uses the same coordinate system as PIL itself, with (0, 0) in the upper left corner. Any pixels drawn outside of the image bounds will be discarded.
To specify colors, you can use numbers or tuples just as you would use with :py:meth:`PIL.Image.new` or :py:meth:`PIL.Image.Image.putpixel`. For “1”, “L”, and “I” images, use integers. For “RGB” images, use a 3-tuple containing integer values. For “F” images, use integer or floating point values.
For palette images (mode “P”), use integers as color indexes. In 1.1.4 and later, you can also use RGB 3-tuples or color names (see below). The drawing layer will automatically assign color indexes, as long as you don’t draw with more than 256 colors.
See :ref:`color-names` for the color names supported by Pillow.
PIL can use bitmap fonts or OpenType/TrueType fonts.
Bitmap fonts are stored in PIL’s own format, where each font typically consists of two files, one named .pil and the other usually named .pbm. The former contains font metrics, the latter raster data.
To load a bitmap font, use the load functions in the :py:mod:`~PIL.ImageFont` module.
To load a OpenType/TrueType font, use the truetype function in the :py:mod:`~PIL.ImageFont` module. Note that this function depends on third-party libraries, and may not available in all PIL builds.
from PIL import Image, ImageDraw, ImageFont
# get an image
with Image.open("Pillow/Tests/images/hopper.png").convert("RGBA") as base:
# make a blank image for the text, initialized to transparent text color
txt = Image.new("RGBA", base.size, (255,255,255,0))
# get a font
fnt = ImageFont.truetype("Pillow/Tests/fonts/FreeMono.ttf", 40)
# get a drawing context
d = ImageDraw.Draw(txt)
# draw text, half opacity
d.text((10,10), "Hello", font=fnt, fill=(255,255,255,128))
# draw text, full opacity
d.text((10,60), "World", font=fnt, fill=(255,255,255,255))
out = Image.alpha_composite(base, txt)
out.show()from PIL import Image, ImageDraw, ImageFont
# create an image
out = Image.new("RGB", (150, 100), (255, 255, 255))
# get a font
fnt = ImageFont.truetype("Pillow/Tests/fonts/FreeMono.ttf", 40)
# get a drawing context
d = ImageDraw.Draw(out)
# draw multiline text
d.multiline_text((10,10), "Hello\nWorld", font=fnt, fill=(0, 0, 0))
out.show().. py:method:: Draw(im, mode=None)
Creates an object that can be used to draw in the given image.
Note that the image will be modified in place.
:param im: The image to draw in.
:param mode: Optional mode to use for color values. For RGB
images, this argument can be RGB or RGBA (to blend the
drawing into the image). For all other modes, this argument
must be the same as the image mode. If omitted, the mode
defaults to the mode of the image.
.. py:method:: ImageDraw.getfont()
Get the current default font.
:returns: An image font.
.. py:method:: ImageDraw.arc(xy, start, end, fill=None, width=0)
Draws an arc (a portion of a circle outline) between the start and end
angles, inside the given bounding box.
:param xy: Two points to define the bounding box. Sequence of ``[(x0, y0),
(x1, y1)]`` or ``[x0, y0, x1, y1]``, where ``x1 >= x0`` and ``y1 >=
y0``.
:param start: Starting angle, in degrees. Angles are measured from 3
o'clock, increasing clockwise.
:param end: Ending angle, in degrees.
:param fill: Color to use for the arc.
:param width: The line width, in pixels.
.. versionadded:: 5.3.0
.. py:method:: ImageDraw.bitmap(xy, bitmap, fill=None)
Draws a bitmap (mask) at the given position, using the current fill color
for the non-zero portions. The bitmap should be a valid transparency mask
(mode “1”) or matte (mode “L” or “RGBA”).
This is equivalent to doing ``image.paste(xy, color, bitmap)``.
To paste pixel data into an image, use the
:py:meth:`~PIL.Image.Image.paste` method on the image itself.
.. py:method:: ImageDraw.chord(xy, start, end, fill=None, outline=None, width=1)
Same as :py:meth:`~PIL.ImageDraw.ImageDraw.arc`, but connects the end points
with a straight line.
:param xy: Two points to define the bounding box. Sequence of ``[(x0, y0),
(x1, y1)]`` or ``[x0, y0, x1, y1]``, where ``x1 >= x0`` and ``y1 >=
y0``.
:param outline: Color to use for the outline.
:param fill: Color to use for the fill.
:param width: The line width, in pixels.
.. versionadded:: 5.3.0
.. py:method:: ImageDraw.ellipse(xy, fill=None, outline=None, width=1)
Draws an ellipse inside the given bounding box.
:param xy: Two points to define the bounding box. Sequence of either
``[(x0, y0), (x1, y1)]`` or ``[x0, y0, x1, y1]``, where ``x1 >= x0``
and ``y1 >= y0``.
:param outline: Color to use for the outline.
:param fill: Color to use for the fill.
:param width: The line width, in pixels.
.. versionadded:: 5.3.0
.. py:method:: ImageDraw.line(xy, fill=None, width=0, joint=None)
Draws a line between the coordinates in the ``xy`` list.
:param xy: Sequence of either 2-tuples like ``[(x, y), (x, y), ...]`` or
numeric values like ``[x, y, x, y, ...]``.
:param fill: Color to use for the line.
:param width: The line width, in pixels.
.. versionadded:: 1.1.5
.. note:: This option was broken until version 1.1.6.
:param joint: Joint type between a sequence of lines. It can be ``"curve"``, for rounded edges, or :data:`None`.
.. versionadded:: 5.3.0
.. py:method:: ImageDraw.pieslice(xy, start, end, fill=None, outline=None, width=1)
Same as arc, but also draws straight lines between the end points and the
center of the bounding box.
:param xy: Two points to define the bounding box. Sequence of ``[(x0, y0),
(x1, y1)]`` or ``[x0, y0, x1, y1]``, where ``x1 >= x0`` and ``y1 >=
y0``.
:param start: Starting angle, in degrees. Angles are measured from 3
o'clock, increasing clockwise.
:param end: Ending angle, in degrees.
:param fill: Color to use for the fill.
:param outline: Color to use for the outline.
:param width: The line width, in pixels.
.. versionadded:: 5.3.0
.. py:method:: ImageDraw.point(xy, fill=None)
Draws points (individual pixels) at the given coordinates.
:param xy: Sequence of either 2-tuples like ``[(x, y), (x, y), ...]`` or
numeric values like ``[x, y, x, y, ...]``.
:param fill: Color to use for the point.
.. py:method:: ImageDraw.polygon(xy, fill=None, outline=None, width=1)
Draws a polygon.
The polygon outline consists of straight lines between the given
coordinates, plus a straight line between the last and the first
coordinate.
:param xy: Sequence of either 2-tuples like ``[(x, y), (x, y), ...]`` or
numeric values like ``[x, y, x, y, ...]``.
:param fill: Color to use for the fill.
:param outline: Color to use for the outline.
:param width: The line width, in pixels.
.. py:method:: ImageDraw.regular_polygon(bounding_circle, n_sides, rotation=0, fill=None, outline=None)
Draws a regular polygon inscribed in ``bounding_circle``,
with ``n_sides``, and rotation of ``rotation`` degrees.
:param bounding_circle: The bounding circle is a tuple defined
by a point and radius.
(e.g. ``bounding_circle=(x, y, r)`` or ``((x, y), r)``).
The polygon is inscribed in this circle.
:param n_sides: Number of sides
(e.g. ``n_sides=3`` for a triangle, ``6`` for a hexagon).
:param rotation: Apply an arbitrary rotation to the polygon
(e.g. ``rotation=90``, applies a 90 degree rotation).
:param fill: Color to use for the fill.
:param outline: Color to use for the outline.
.. py:method:: ImageDraw.rectangle(xy, fill=None, outline=None, width=1)
Draws a rectangle.
:param xy: Two points to define the bounding box. Sequence of either
``[(x0, y0), (x1, y1)]`` or ``[x0, y0, x1, y1]``. The second point
is just outside the drawn rectangle.
:param outline: Color to use for the outline.
:param fill: Color to use for the fill.
:param width: The line width, in pixels.
.. versionadded:: 5.3.0
.. py:method:: ImageDraw.rounded_rectangle(xy, radius=0, fill=None, outline=None, width=1)
Draws a rounded rectangle.
:param xy: Two points to define the bounding box. Sequence of either
``[(x0, y0), (x1, y1)]`` or ``[x0, y0, x1, y1]``. The second point
is just outside the drawn rectangle.
:param radius: Radius of the corners.
:param outline: Color to use for the outline.
:param fill: Color to use for the fill.
:param width: The line width, in pixels.
.. versionadded:: 8.2.0
.. py:method:: ImageDraw.shape(shape, fill=None, outline=None)
.. warning:: This method is experimental.
Draw a shape.
.. py:method:: ImageDraw.text(xy, text, fill=None, font=None, anchor=None, spacing=4, align="left", direction=None, features=None, language=None, stroke_width=0, stroke_fill=None, embedded_color=False)
Draws the string at the given position.
:param xy: The anchor coordinates of the text.
:param text: String to be drawn. If it contains any newline characters,
the text is passed on to
:py:meth:`~PIL.ImageDraw.ImageDraw.multiline_text`.
:param fill: Color to use for the text.
:param font: An :py:class:`~PIL.ImageFont.ImageFont` instance.
:param anchor: The text anchor alignment. Determines the relative location of
the anchor to the text. The default alignment is top left.
See :ref:`text-anchors` for valid values. This parameter is
ignored for non-TrueType fonts.
.. note:: This parameter was present in earlier versions
of Pillow, but implemented only in version 8.0.0.
:param spacing: If the text is passed on to
:py:meth:`~PIL.ImageDraw.ImageDraw.multiline_text`,
the number of pixels between lines.
:param align: If the text is passed on to
:py:meth:`~PIL.ImageDraw.ImageDraw.multiline_text`,
``"left"``, ``"center"`` or ``"right"``. Determines the relative alignment of lines.
Use the ``anchor`` parameter to specify the alignment to ``xy``.
:param direction: Direction of the text. It can be ``"rtl"`` (right to
left), ``"ltr"`` (left to right) or ``"ttb"`` (top to bottom).
Requires libraqm.
.. versionadded:: 4.2.0
:param features: A list of OpenType font features to be used during text
layout. This is usually used to turn on optional
font features that are not enabled by default,
for example ``"dlig"`` or ``"ss01"``, but can be also
used to turn off default font features, for
example ``"-liga"`` to disable ligatures or ``"-kern"``
to disable kerning. To get all supported
features, see `OpenType docs`_.
Requires libraqm.
.. versionadded:: 4.2.0
:param language: Language of the text. Different languages may use
different glyph shapes or ligatures. This parameter tells
the font which language the text is in, and to apply the
correct substitutions as appropriate, if available.
It should be a `BCP 47 language code`_.
Requires libraqm.
.. versionadded:: 6.0.0
:param stroke_width: The width of the text stroke.
.. versionadded:: 6.2.0
:param stroke_fill: Color to use for the text stroke. If not given, will default to
the ``fill`` parameter.
.. versionadded:: 6.2.0
:param embedded_color: Whether to use font embedded color glyphs (COLR, CBDT, SBIX).
.. versionadded:: 8.0.0
.. py:method:: ImageDraw.multiline_text(xy, text, fill=None, font=None, anchor=None, spacing=4, align="left", direction=None, features=None, language=None, stroke_width=0, stroke_fill=None, embedded_color=False)
Draws the string at the given position.
:param xy: The anchor coordinates of the text.
:param text: String to be drawn.
:param fill: Color to use for the text.
:param font: An :py:class:`~PIL.ImageFont.ImageFont` instance.
:param anchor: The text anchor alignment. Determines the relative location of
the anchor to the text. The default alignment is top left.
See :ref:`text-anchors` for valid values. This parameter is
ignored for non-TrueType fonts.
.. note:: This parameter was present in earlier versions
of Pillow, but implemented only in version 8.0.0.
:param spacing: The number of pixels between lines.
:param align: ``"left"``, ``"center"`` or ``"right"``. Determines the relative alignment of lines.
Use the ``anchor`` parameter to specify the alignment to ``xy``.
:param direction: Direction of the text. It can be ``"rtl"`` (right to
left), ``"ltr"`` (left to right) or ``"ttb"`` (top to bottom).
Requires libraqm.
.. versionadded:: 4.2.0
:param features: A list of OpenType font features to be used during text
layout. This is usually used to turn on optional
font features that are not enabled by default,
for example ``"dlig"`` or ``"ss01"``, but can be also
used to turn off default font features, for
example ``"-liga"`` to disable ligatures or ``"-kern"``
to disable kerning. To get all supported
features, see `OpenType docs`_.
Requires libraqm.
.. versionadded:: 4.2.0
:param language: Language of the text. Different languages may use
different glyph shapes or ligatures. This parameter tells
the font which language the text is in, and to apply the
correct substitutions as appropriate, if available.
It should be a `BCP 47 language code`_.
Requires libraqm.
.. versionadded:: 6.0.0
:param stroke_width: The width of the text stroke.
.. versionadded:: 6.2.0
:param stroke_fill: Color to use for the text stroke. If not given, will default to
the ``fill`` parameter.
.. versionadded:: 6.2.0
:param embedded_color: Whether to use font embedded color glyphs (COLR, CBDT, SBIX).
.. versionadded:: 8.0.0
.. py:method:: ImageDraw.textsize(text, font=None, spacing=4, direction=None, features=None, language=None, stroke_width=0)
Return the size of the given string, in pixels.
Use :py:meth:`textlength()` to measure the offset of following text with
1/64 pixel precision.
Use :py:meth:`textbbox()` to get the exact bounding box based on an anchor.
.. note:: For historical reasons this function measures text height from
the ascender line instead of the top, see :ref:`text-anchors`.
If you wish to measure text height from the top, it is recommended
to use :meth:`textbbox` with ``anchor='lt'`` instead.
:param text: Text to be measured. If it contains any newline characters,
the text is passed on to :py:meth:`~PIL.ImageDraw.ImageDraw.multiline_textsize`.
:param font: An :py:class:`~PIL.ImageFont.ImageFont` instance.
:param spacing: If the text is passed on to
:py:meth:`~PIL.ImageDraw.ImageDraw.multiline_textsize`,
the number of pixels between lines.
:param direction: Direction of the text. It can be ``"rtl"`` (right to
left), ``"ltr"`` (left to right) or ``"ttb"`` (top to bottom).
Requires libraqm.
.. versionadded:: 4.2.0
:param features: A list of OpenType font features to be used during text
layout. This is usually used to turn on optional
font features that are not enabled by default,
for example ``"dlig"`` or ``"ss01"``, but can be also
used to turn off default font features, for
example ``"-liga"`` to disable ligatures or ``"-kern"``
to disable kerning. To get all supported
features, see `OpenType docs`_.
Requires libraqm.
.. versionadded:: 4.2.0
:param language: Language of the text. Different languages may use
different glyph shapes or ligatures. This parameter tells
the font which language the text is in, and to apply the
correct substitutions as appropriate, if available.
It should be a `BCP 47 language code`_.
Requires libraqm.
.. versionadded:: 6.0.0
:param stroke_width: The width of the text stroke.
.. versionadded:: 6.2.0
.. py:method:: ImageDraw.multiline_textsize(text, font=None, spacing=4, direction=None, features=None, language=None, stroke_width=0)
Return the size of the given string, in pixels.
Use :py:meth:`textlength()` to measure the offset of following text with
1/64 pixel precision.
Use :py:meth:`textbbox()` to get the exact bounding box based on an anchor.
.. note:: For historical reasons this function measures text height as the
distance between the top ascender line and bottom descender line,
not the top and bottom of the text, see :ref:`text-anchors`.
If you wish to measure text height from the top to the bottom of text,
it is recommended to use :meth:`multiline_textbbox` instead.
:param text: Text to be measured.
:param font: An :py:class:`~PIL.ImageFont.ImageFont` instance.
:param spacing: The number of pixels between lines.
:param direction: Direction of the text. It can be ``"rtl"`` (right to
left), ``"ltr"`` (left to right) or ``"ttb"`` (top to bottom).
Requires libraqm.
.. versionadded:: 4.2.0
:param features: A list of OpenType font features to be used during text
layout. This is usually used to turn on optional
font features that are not enabled by default,
for example ``"dlig"`` or ``"ss01"``, but can be also
used to turn off default font features, for
example ``"-liga"`` to disable ligatures or ``"-kern"``
to disable kerning. To get all supported
features, see `OpenType docs`_.
Requires libraqm.
.. versionadded:: 4.2.0
:param language: Language of the text. Different languages may use
different glyph shapes or ligatures. This parameter tells
the font which language the text is in, and to apply the
correct substitutions as appropriate, if available.
It should be a `BCP 47 language code`_.
Requires libraqm.
.. versionadded:: 6.0.0
:param stroke_width: The width of the text stroke.
.. versionadded:: 6.2.0
.. py:method:: ImageDraw.textlength(text, font=None, direction=None, features=None, language=None, embedded_color=False)
Returns length (in pixels with 1/64 precision) of given text when rendered
in font with provided direction, features, and language.
This is the amount by which following text should be offset.
Text bounding box may extend past the length in some fonts,
e.g. when using italics or accents.
The result is returned as a float; it is a whole number if using basic layout.
Note that the sum of two lengths may not equal the length of a concatenated
string due to kerning. If you need to adjust for kerning, include the following
character and subtract its length.
For example, instead of
.. code-block:: python
hello = draw.textlength("Hello", font)
world = draw.textlength("World", font)
hello_world = hello + world # not adjusted for kerning
assert hello_world == draw.textlength("HelloWorld", font) # may fail
use
.. code-block:: python
hello = draw.textlength("HelloW", font) - draw.textlength("W", font) # adjusted for kerning
world = draw.textlength("World", font)
hello_world = hello + world # adjusted for kerning
assert hello_world == draw.textlength("HelloWorld", font) # True
or disable kerning with (requires libraqm)
.. code-block:: python
hello = draw.textlength("Hello", font, features=["-kern"])
world = draw.textlength("World", font, features=["-kern"])
hello_world = hello + world # kerning is disabled, no need to adjust
assert hello_world == draw.textlength("HelloWorld", font, features=["-kern"]) # True
.. versionadded:: 8.0.0
:param text: Text to be measured. May not contain any newline characters.
:param font: An :py:class:`~PIL.ImageFont.ImageFont` instance.
:param direction: Direction of the text. It can be ``"rtl"`` (right to
left), ``"ltr"`` (left to right) or ``"ttb"`` (top to bottom).
Requires libraqm.
:param features: A list of OpenType font features to be used during text
layout. This is usually used to turn on optional
font features that are not enabled by default,
for example ``"dlig"`` or ``"ss01"``, but can be also
used to turn off default font features, for
example ``"-liga"`` to disable ligatures or ``"-kern"``
to disable kerning. To get all supported
features, see `OpenType docs`_.
Requires libraqm.
:param language: Language of the text. Different languages may use
different glyph shapes or ligatures. This parameter tells
the font which language the text is in, and to apply the
correct substitutions as appropriate, if available.
It should be a `BCP 47 language code`_.
Requires libraqm.
:param embedded_color: Whether to use font embedded color glyphs (COLR, CBDT, SBIX).
.. py:method:: ImageDraw.textbbox(xy, text, font=None, anchor=None, spacing=4, align="left", direction=None, features=None, language=None, stroke_width=0, embedded_color=False)
Returns bounding box (in pixels) of given text relative to given anchor
when rendered in font with provided direction, features, and language.
Only supported for TrueType fonts.
Use :py:meth:`textlength` to get the offset of following text with
1/64 pixel precision. The bounding box includes extra margins for
some fonts, e.g. italics or accents.
.. versionadded:: 8.0.0
:param xy: The anchor coordinates of the text.
:param text: Text to be measured. If it contains any newline characters,
the text is passed on to
:py:meth:`~PIL.ImageDraw.ImageDraw.multiline_textbbox`.
:param font: A :py:class:`~PIL.ImageFont.FreeTypeFont` instance.
:param anchor: The text anchor alignment. Determines the relative location of
the anchor to the text. The default alignment is top left.
See :ref:`text-anchors` for valid values. This parameter is
ignored for non-TrueType fonts.
:param spacing: If the text is passed on to
:py:meth:`~PIL.ImageDraw.ImageDraw.multiline_textbbox`,
the number of pixels between lines.
:param align: If the text is passed on to
:py:meth:`~PIL.ImageDraw.ImageDraw.multiline_textbbox`,
``"left"``, ``"center"`` or ``"right"``. Determines the relative alignment of lines.
Use the ``anchor`` parameter to specify the alignment to ``xy``.
:param direction: Direction of the text. It can be ``"rtl"`` (right to
left), ``"ltr"`` (left to right) or ``"ttb"`` (top to bottom).
Requires libraqm.
:param features: A list of OpenType font features to be used during text
layout. This is usually used to turn on optional
font features that are not enabled by default,
for example ``"dlig"`` or ``"ss01"``, but can be also
used to turn off default font features, for
example ``"-liga"`` to disable ligatures or ``"-kern"``
to disable kerning. To get all supported
features, see `OpenType docs`_.
Requires libraqm.
:param language: Language of the text. Different languages may use
different glyph shapes or ligatures. This parameter tells
the font which language the text is in, and to apply the
correct substitutions as appropriate, if available.
It should be a `BCP 47 language code`_.
Requires libraqm.
:param stroke_width: The width of the text stroke.
:param embedded_color: Whether to use font embedded color glyphs (COLR, CBDT, SBIX).
.. py:method:: ImageDraw.multiline_textbbox(xy, text, font=None, anchor=None, spacing=4, align="left", direction=None, features=None, language=None, stroke_width=0, embedded_color=False)
Returns bounding box (in pixels) of given text relative to given anchor
when rendered in font with provided direction, features, and language.
Only supported for TrueType fonts.
Use :py:meth:`textlength` to get the offset of following text with
1/64 pixel precision. The bounding box includes extra margins for
some fonts, e.g. italics or accents.
.. versionadded:: 8.0.0
:param xy: The anchor coordinates of the text.
:param text: Text to be measured.
:param font: A :py:class:`~PIL.ImageFont.FreeTypeFont` instance.
:param anchor: The text anchor alignment. Determines the relative location of
the anchor to the text. The default alignment is top left.
See :ref:`text-anchors` for valid values. This parameter is
ignored for non-TrueType fonts.
:param spacing: The number of pixels between lines.
:param align: ``"left"``, ``"center"`` or ``"right"``. Determines the relative alignment of lines.
Use the ``anchor`` parameter to specify the alignment to ``xy``.
:param direction: Direction of the text. It can be ``"rtl"`` (right to
left), ``"ltr"`` (left to right) or ``"ttb"`` (top to bottom).
Requires libraqm.
:param features: A list of OpenType font features to be used during text
layout. This is usually used to turn on optional
font features that are not enabled by default,
for example ``"dlig"`` or ``"ss01"``, but can be also
used to turn off default font features, for
example ``"-liga"`` to disable ligatures or ``"-kern"``
to disable kerning. To get all supported
features, see `OpenType docs`_.
Requires libraqm.
:param language: Language of the text. Different languages may use
different glyph shapes or ligatures. This parameter tells
the font which language the text is in, and to apply the
correct substitutions as appropriate, if available.
It should be a `BCP 47 language code`_.
Requires libraqm.
:param stroke_width: The width of the text stroke.
:param embedded_color: Whether to use font embedded color glyphs (COLR, CBDT, SBIX).
.. py:method:: getdraw(im=None, hints=None)
.. warning:: This method is experimental.
A more advanced 2D drawing interface for PIL images,
based on the WCK interface.
:param im: The image to draw in.
:param hints: An optional list of hints.
:returns: A (drawing context, drawing resource factory) tuple.
.. py:method:: floodfill(image, xy, value, border=None, thresh=0)
.. warning:: This method is experimental.
Fills a bounded region with a given color.
:param image: Target image.
:param xy: Seed position (a 2-item coordinate tuple).
:param value: Fill color.
:param border: Optional border value. If given, the region consists of
pixels with a color different from the border color. If not given,
the region consists of pixels having the same color as the seed
pixel.
:param thresh: Optional threshold value which specifies a maximum
tolerable difference of a pixel value from the 'background' in
order for it to be replaced. Useful for filling regions of non-
homogeneous, but similar, colors.