Threshold

Adaptive Threshold

New in version 0.5.3.

Also known as Local Adaptive Threshold, each pixel value is adjusted by the surrounding pixels. If the current pixel has greater value than the average of the surrounding pixels, then the pixel becomes white, else black.

The size of the surrounding pixels is defined by passing width & height arguments. Use offset argument to apply a +/- value to the current pixel.

from wand.image import Image

with Image(filename='inca_tern.jpg') as img:
    img.transform_colorspace('gray')
    img.adaptive_threshold(width=16, height=16,
                           offset=-0.08 * img.quantum_range)
    img.save(filename='threshold_adaptive.jpg')

Original	Adaptive Threshold

Note

Requires ImageMagick-7.

Auto Threshold

New in version 0.5.5.

This method applies threshold automatically. You can define which method to use from AUTO_THRESHOLD_METHODS. Defaults to 'kapur'.

from wand.image import Image

with Image(filename='inca_tern.jpg') as img:
    img.transform_colorspace('gray')
    with img.clone() as kapur:
        kapur.auto_threshold(method='kapur')
        kapur.save(filename='threshold_auto_kapur.jpg')
    with img.clone() as otsu:
        otsu.auto_threshold(method='otsu')
        otsu.save(filename='threshold_auto_otsu.jpg')
    with img.clone() as triangle:
        triangle.auto_threshold(method='triangle')
        triangle.save(filename='threshold_auto_triangle.jpg')

Original	Auto Threshold ('kapur')
Auto Threshold ('otsu')	Auto Threshold ('triangle')

Note

Requires ImageMagick-7.0.8-41

Black Threshold

New in version 0.5.3.

Force all pixels below a given pixel value to black. This works on a color channel-by-channel basis, and can be used to reduce unwanted colors.

from wand.image import Image

with Image(filename='inca_tern.jpg') as img:
    img.black_threshold(threshold='#930')
    img.save(filename='threshold_black.jpg')

Original	Black Threshold

Color Threshold

New in version 0.6.4.

Creates a binary image where all pixels between start & stop are forced to white, else black.

from wand.image import Image

with Image(filename='inca_tern.jpg') as img:
    img.color_threshold(start='#333', stop='#cdc')
    img.save(filename='threshold_color.jpg')

Original	Color Threshold

Note

Requires ImageMagick-7.0.10

Ordered Dither

Applies a pre-defined threshold map to create dithering to an image.

The pre-defined thresholds are the following:

Map	Alias	Description
threshold	1x1	Threshold 1x1 (non-dither)
checks	2x1	Checkerboard 2x1 (dither)
o2x2	2x2	Ordered 2x2 (dispersed)
o3x3	3x3	Ordered 3x3 (dispersed)
o4x4	4x4	Ordered 4x4 (dispersed)
o8x8	8x8	Ordered 8x8 (dispersed)
h4x4a	4x1	Halftone 4x4 (angled)
h6x6a	6x1	Halftone 6x6 (angled)
h8x8a	8x1	Halftone 8x8 (angled)
h4x4o		Halftone 4x4 (orthogonal)
h6x6o		Halftone 6x6 (orthogonal)
h8x8o		Halftone 8x8 (orthogonal)
h16x16o		Halftone 16x16 (orthogonal)
c5x5b	c5x5	Circles 5x5 (black)
c5x5w		Circles 5x5 (white)
c6x6b	c6x6	Circles 6x6 (black)
c6x6w		Circles 6x6 (white)
c7x7b	c7x7	Circles 7x7 (black)
c7x7w		Circles 7x7 (white)

from wand.image import Image

with Image(filename='inca_tern.jpg') as img:
    img.transform_colorspace('gray')
    with img.clone() as dispersed:
        dispersed.ordered_dither('o3x3')
        dispersed.save(filename='threshold_ordered_dither_dispersed.jpg')
    with img.clone() as halftone:
        halftone.ordered_dither('h6x6a')
        halftone.save(filename='threshold_ordered_dither_halftone.jpg')
    with img.clone() as circles:
        circles.ordered_dither('c6x6b')
        circles.save(filename='threshold_ordered_dither_circles.jpg')

Original	Ordered Dither (Ordered 3x3)
Ordered Dither (Halftone 4x4)	Ordered Dither (Circles 6x6)

Random Threshold

New in version 0.5.7.

Applies a random threshold between low & high values.

from wand.image import Image

with Image(filename='inca_tern.jpg') as img:
    img.transform_colorspace('gray')
    img.random_threshold(low=0.3 * img.quantum_range,
                         high=0.6 * img.quantum_range)
    img.save(filename='threshold_random.jpg')

Original	Random Threshold

Range Threshold

New in version 0.5.5.

This can either apply a soft, or hard, threshold between two quantum points.

To use a soft threshold, define the low & high range between each white & black point.

from wand.image import Image

with Image(filename='inca_tern.jpg') as img:
    img.transform_colorspace('gray')
    white_point = 0.9 * img.quantum_range
    black_point = 0.5 * img.quantum_range
    delta = 0.05 * img.quantum_range
    img.range_threshold(low_black=black_point - delta,
                        low_white=white_point - delta,
                        high_white=white_point + delta,
                        high_black=black_point + delta)
    img.save(filename='threshold_range_soft.jpg')

Original	Range Threshold (soft)

To use a hard threshold, pass the same values as both low & high range.

from wand.image import Image

with Image(filename='inca_tern.jpg') as img:
    img.transform_colorspace('gray')
    white_point = 0.9 * img.quantum_range
    black_point = 0.5 * img.quantum_range
    img.range_threshold(low_black=black_point,
                        low_white=white_point,
                        high_white=white_point,
                        high_black=black_point)
    img.save(filename='threshold_range_hard.jpg')

Original	Range Threshold (hard)

Note

Requires ImageMagick-7.0.8-41

White Threshold

New in version 0.5.2.

Force all pixels above a given pixel value to white. This works on a color channel-by-channel basis, and can be used to reduce unwanted colors.

from wand.image import Image

with Image(filename='inca_tern.jpg') as img:
    img.white_threshold(threshold='#ace')
    img.save(filename='threshold_white.jpg')

Original	White Threshold