Dynamic Mode

Note

This page was generated from the docs/examples/image_processing/brightness_contrast/dynamic_mode.ipynb Jupyter notebook. You can run it locally or open it in Google Colab, see Running the Examples for more information. To prepare Colab for this exact documentation revision, run this cell before the tutorial:

setup_url = (
    "https://raw.githubusercontent.com/NVIDIA/DALI/"
    "5f8bfb8/docs/examples/colab_setup.py"
)
!curl -sSL $setup_url -o colab_setup.py
%run colab_setup.py --ref 5f8bfb8

BrightnessContrast Example

This example demonstrates usage of brightness_contrast operator.

Brightness and Contrast Adjustment

Brightness and contrast can be adjusted using three parameters:

• contrast - makes the dark parts darker and bright parts brighter while keeping the gray (at half dynamic range of the input type) constant

• brightness - multiplies the intensities

• brightness_shift - shifts the pixel intensities

The formula is given as:

\[Out = brightness\_shift \cdot {Max}_{out} + brightness \cdot ({contrast\_center} + {contrast} \cdot ({in} - {contrast\_center}))\]

where \({contrast\_center}\) denotes a value that is unaffected by contrast changes; by default, it’s at half of the input’s dynamic range (e.g. 128 for uint8) and \({Max}_{out}\) is the maximum positive value of the output type (for integer types) or 1 (for floating point) - but it can also be specified as an argument for the operator.

The operator can be used to adjust contrast and brightness as well as to compute negative images (brightness = -1, brightness_shift = 1).

from pathlib import Path

import matplotlib.pyplot as plt
import nvidia.dali.experimental.dynamic as ndd

image_dir = Path("../../data/images")
image_path = next(image_dir.glob("**/*.jpg"))

def display(original: ndd.Tensor, converted: ndd.Tensor):
    fig, ax = plt.subplots(1, 2, figsize=(12, 6))
    ax[0].imshow(original.cpu())
    ax[0].set_title("Original")
    ax[0].axis("off")
    ax[1].imshow(converted.cpu())
    ax[1].set_title("Adjusted")
    ax[1].axis("off")

GPU Example: Color Inversion

Let’s apply brightness/contrast adjustment on the GPU to create a color inversion effect. Setting brightness=-1 and brightness_shift=1 inverts the colors, while contrast=1.5 increases the contrast.

image = ndd.imread(image_path, device="gpu")
converted = ndd.brightness_contrast(
    image,
    contrast=1.5,
    brightness=-1,
    brightness_shift=1,
)
display(image, converted)

CPU Example: Contrast and Brightness Adjustment

Now let’s run on the CPU with different parameters. Here we reduce contrast (contrast=0.4) and shift brightness up (brightness_shift=0.3), with a custom contrast center point.

image_cpu = ndd.imread(image_path, device="cpu")
converted_cpu = ndd.brightness_contrast(
    image_cpu,
    brightness_shift=0.3,
    contrast=0.4,
    contrast_center=100,
)
display(image_cpu, converted_cpu)