contrast.rst

augmenters.contrast

GammaContrast

Adjust image contrast by scaling pixel values to 255*((v/255)**gamma).

Values in the range gamma=(0.5, 2.0) seem to be sensible.

API link: :func:`~imgaug.augmenters.contrast.GammaContrast`

Example. Modify the contrast of images according to 255*((v/255)**gamma), where v is a pixel value and gamma is sampled uniformly from the interval [0.5, 2.0] (once per image):

import imgaug.augmenters as iaa
aug = iaa.GammaContrast((0.5, 2.0))

Example. Same as in the previous example, but gamma is sampled once per image and channel:

aug = iaa.GammaContrast((0.5, 2.0), per_channel=True)

SigmoidContrast

Adjust image contrast to 255*1/(1+exp(gain*(cutoff-I_ij/255))).

Values in the range gain=(5, 20) and cutoff=(0.25, 0.75) seem to be sensible.

API link: :func:`~imgaug.augmenters.contrast.SigmoidContrast`

Example. Modify the contrast of images according to 255*1/(1+exp(gain*(cutoff-v/255))), where v is a pixel value, gain is sampled uniformly from the interval [3, 10] (once per image) and cutoff is sampled uniformly from the interval [0.4, 0.6] (also once per image).

import imgaug.augmenters as iaa
aug = iaa.SigmoidContrast(gain=(3, 10), cutoff=(0.4, 0.6))

Example. Same as in the previous example, but gain and cutoff are each sampled once per image and channel:

aug = iaa.SigmoidContrast(
    gain=(3, 10), cutoff=(0.4, 0.6), per_channel=True)

LogContrast

Adjust image contrast by scaling pixels to 255*gain*log_2(1+v/255).

This augmenter is fairly similar to imgaug.augmenters.arithmetic.Multiply.

API link: :func:`~imgaug.augmenters.contrast.LogContrast`

Example. Modify the contrast of images according to 255*gain*log_2(1+v/255), where v is a pixel value and gain is sampled uniformly from the interval [0.6, 1.4] (once per image):

import imgaug.augmenters as iaa
aug = iaa.LogContrast(gain=(0.6, 1.4))

Example. Same as in the previous example, but gain is sampled once per image and channel:

aug = iaa.LogContrast(gain=(0.6, 1.4), per_channel=True)

LinearContrast

Adjust contrast by scaling each pixel to 127 + alpha*(v-127).

API link: :func:`~imgaug.augmenters.contrast.LinearContrast`

Example. Modify the contrast of images according to 127 + alpha*(v-127)`, where v is a pixel value and alpha is sampled uniformly from the interval [0.4, 1.6] (once per image):

import imgaug.augmenters as iaa
aug = iaa.LinearContrast((0.4, 1.6))

Example. Same as in the previous example, but alpha is sampled once per image and channel:

aug = iaa.LinearContrast((0.4, 1.6), per_channel=True)

AllChannelsCLAHE

Apply CLAHE to all channels of images in their original colorspaces.

CLAHE (Contrast Limited Adaptive Histogram Equalization) performs histogram equilization within image patches, i.e. over local neighbourhoods.

In contrast to imgaug.augmenters.contrast.CLAHE, this augmenter operates directly on all channels of the input images. It does not perform any colorspace transformations and does not focus on specific channels (e.g. L in Lab colorspace).

API link: :class:`~imgaug.augmenters.contrast.AllChannelsCLAHE`

Example. Create an augmenter that applies CLAHE to all channels of input images:

import imgaug.augmenters as iaa
aug = iaa.AllChannelsCLAHE()

Example. Same as in the previous example, but the clip_limit used by CLAHE is uniformly sampled per image from the interval [1, 10]. Some images will therefore have stronger contrast than others (i.e. higher clip limit values).

aug = iaa.AllChannelsCLAHE(clip_limit=(1, 10))

Example. Same as in the previous example, but the clip_limit is sampled per image and channel, leading to different levels of contrast for each channel:

aug = iaa.AllChannelsCLAHE(clip_limit=(1, 10), per_channel=True)

CLAHE

Apply CLAHE to L/V/L channels in HLS/HSV/Lab colorspaces.

This augmenter applies CLAHE (Contrast Limited Adaptive Histogram Equalization) to images, a form of histogram equalization that normalizes within local image patches. The augmenter transforms input images to a target colorspace (e.g. Lab), extracts an intensity-related channel from the converted images (e.g. L for Lab), applies CLAHE to the channel and then converts the resulting image back to the original colorspace.

Grayscale images (images without channel axis or with only one channel axis) are automatically handled, from_colorspace does not have to be adjusted for them. For images with four channels (e.g. RGBA), the fourth channel is ignored in the colorspace conversion (e.g. from an RGBA image, only the RGB part is converted, normalized, converted back and concatenated with the input A channel). Images with unusual channel numbers (2, 5 or more than 5) are normalized channel-by-channel (same behaviour as AllChannelsCLAHE, though a warning will be raised).

If you want to apply CLAHE to each channel of the original input image's colorspace (without any colorspace conversion), use imgaug.augmenters.contrast.AllChannelsCLAHE instead.

API link: :class:`~imgaug.augmenters.contrast.CLAHE`

Example. Create a standard CLAHE augmenter:

import imgaug.augmenters as iaa
aug = iaa.CLAHE()

Example. Create a CLAHE augmenter with a clip limit uniformly sampled from [1..10], where 1 is rather low contrast and 10 is rather high contrast:

aug = iaa.CLAHE(clip_limit=(1, 10))

Example. Create a CLAHE augmenter with kernel sizes of SxS, where S is uniformly sampled from [3..21]. Sampling happens once per image.

aug = iaa.CLAHE(tile_grid_size_px=(3, 21))

Example. Create a CLAHE augmenter with kernel sizes of SxS, where S is sampled from N(7, 2), but does not go below 3:

import imgaug.parameters as iap
aug = iaa.CLAHE(
    tile_grid_size_px=iap.Discretize(iap.Normal(loc=7, scale=2)),
    tile_grid_size_px_min=3)

Example. Create a CLAHE augmenter with kernel sizes of HxW, where H is uniformly sampled from [3..21] and W is randomly picked from the list [3, 5, 7]:

aug = iaa.CLAHE(tile_grid_size_px=((3, 21), [3, 5, 7]))

Example. Create a CLAHE augmenter that converts images from BGR colorspace to HSV colorspace and then applies the local histogram equalization to the V channel of the images (before converting back to BGR). Alternatively, Lab (default) or HLS can be used as the target colorspace. Grayscale images (no channels / one channel) are never converted and are instead directly normalized (i.e. from_colorspace does not have to be changed for them).

aug = iaa.CLAHE(
    from_colorspace=iaa.CLAHE.BGR,
    to_colorspace=iaa.CLAHE.HSV)

AllChannelsHistogramEqualization

Apply Histogram Eq. to all channels of images in their original colorspaces.

In contrast to imgaug.augmenters.contrast.HistogramEqualization, this augmenter operates directly on all channels of the input images. It does not perform any colorspace transformations and does not focus on specific channels (e.g. L in Lab colorspace).

API link: :class:`~imgaug.augmenters.contrast.AllChannelsHistogramEqualization`

Example. Create an augmenter that applies histogram equalization to all channels of input images in the original colorspaces:

import imgaug.augmenters as iaa
aug = iaa.AllChannelsHistogramEqualization()

Example. Same as in the previous example, but alpha-blends the contrast-enhanced augmented images with the original input images using random blend strengths. This leads to random strengths of the contrast adjustment.

aug = iaa.Alpha((0.0, 1.0), iaa.AllChannelsHistogramEqualization())

HistogramEqualization

Apply Histogram Eq. to L/V/L channels of images in HLS/HSV/Lab colorspaces.

This augmenter is similar to imgaug.augmenters.contrast.CLAHE.

The augmenter transforms input images to a target colorspace (e.g. Lab), extracts an intensity-related channel from the converted images (e.g. L for Lab), applies Histogram Equalization to the channel and then converts the resulting image back to the original colorspace.

Grayscale images (images without channel axis or with only one channel axis) are automatically handled, from_colorspace does not have to be adjusted for them. For images with four channels (e.g. RGBA), the fourth channel is ignored in the colorspace conversion (e.g. from an RGBA image, only the RGB part is converted, normalized, converted back and concatenated with the input A channel). Images with unusual channel numbers (2, 5 or more than 5) are normalized channel-by-channel (same behaviour as AllChannelsHistogramEqualization, though a warning will be raised).

If you want to apply HistogramEqualization to each channel of the original input image's colorspace (without any colorspace conversion), use imgaug.augmenters.contrast.AllChannelsHistogramEqualization instead.

API link: :class:`~imgaug.augmenters.contrast.HistogramEqualization`

Example. Create an augmenter that converts images to HLS/HSV/Lab colorspaces, extracts intensity-related channels (i.e. L/V/L), applies histogram equalization to these channels and converts back to the input colorspace:

import imgaug.augmenters as iaa
aug = iaa.HistogramEqualization()

Example. Same as in the previous example, but alpha blends the result, leading to various strengths of contrast normalization:

aug = iaa.Alpha((0.0, 1.0), iaa.HistogramEqualization())

Example. Same as in the first example, but the colorspace of input images has to be BGR (instead of default RGB) and the histogram equalization is applied to the V channel in HSV colorspace:

aug = iaa.HistogramEqualization(
    from_colorspace=iaa.HistogramEqualization.BGR,
    to_colorspace=iaa.HistogramEqualization.HSV)