scikit-image-processing

# scikit-image — Scientific Image Processing

## Overview

scikit-image is a Python library for image processing in the SciPy ecosystem. It provides algorithms for reading/writing images, filtering (noise reduction, edge detection), geometric transforms, segmentation (thresholding, watershed, active contours), object measurement (area, intensity, shape descriptors), and feature detection. Images are represented as NumPy arrays, enabling seamless integration with NumPy, SciPy, matplotlib, and pandas. Widely used for fluorescence microscopy, histology, and general bioimage analysis.

## When to Use

- Preprocessing fluorescence microscopy images: background subtraction, denoising, illumination correction
- Segmenting cells, nuclei, or organelles using thresholding or watershed
- Measuring object properties: area, perimeter, intensity statistics, shape descriptors
- Applying morphological operations: erosion, dilation, opening, closing, fill holes
- Detecting keypoints or local features in biological images
- Converting between image formats and color spaces
- Use `OpenCV` instead for real-time video processing or GPU-accelerated operations
- For deep-learning cell segmentation, use `CellPose` instead (better accuracy for touching cells)
- Use `napari` instead for interactive multi-dimensional image visualization and annotation
- For whole-slide image tiling, use `PathML` or `histolab` instead

## Prerequisites

- **Python packages**: `scikit-image`, `numpy`, `scipy`, `matplotlib`
- **Input requirements**: Images as files (TIFF, PNG, JPEG) or NumPy arrays; fluorescence images as 2D/3D grayscale arrays
- **Environment**: Python 3.9+

```bash
pip install scikit-image numpy scipy matplotlib

# For reading proprietary microscopy formats
pip install tifffile aicsimageio

# Verify
python -c "import skimage; print(skimage.__version__)"
```

## Quick Start

```python
from skimage import io, filters, measure
import numpy as np

# Load → denoise → threshold → measure
img = io.imread("cells.tif")
img_smooth = filters.gaussian(img, sigma=1.5)
threshold = filters.threshold_otsu(img_smooth)
binary = img_smooth > threshold

regions = measure.regionprops(measure.label(binary))
print(f"Found {len(regions)} objects")
print(f"Mean area: {np.mean([r.area for r in regions]):.1f} px²")
```

## Core API

### Module 1: Image I/O and Data Types

```python
from skimage import io, img_as_float, img_as_uint
import numpy as np

# Read single image
img = io.imread("nuclei.tif")
print(f"Shape: {img.shape}, dtype: {img.dtype}")  # (512, 512), uint16

# Read image collection from directory
from skimage import io as ski_io
images = ski_io.ImageCollection("data/*.tif")
print(f"Loaded {len(images)} images")

# Type conversions (critical for correct arithmetic)
img_f = img_as_float(img)      # uint16 → float64, range [0, 1]
img_u8 = (img_f * 255).astype(np.uint8)  # → 8-bit

# Save image
io.imsave("output.tif", img_u8)
```

```python
# Multi-channel fluorescence (TIFF with CZYX or ZCYX dims)
import tifffile

stack = tifffile.imread("multichannel.tif")  # shape: (C, Z, Y, X)
dapi = stack[0]   # DAPI channel
gfp = stack[1]    # GFP channel
print(f"DAPI: {dapi.shape}, GFP: {gfp.shape}")

# Maximum intensity projection along Z
mip = dapi.max(axis=0)
io.imsave("dapi_mip.tif", mip)
```

### Module 2: Filters and Preprocessing

```python
from skimage import filters, restoration
import numpy as np

# Gaussian blur (denoising, smoothing)
from skimage.filters import gaussian
smoothed = gaussian(img, sigma=2.0)

# Median filter (salt-and-pepper noise removal)
from skimage.filters import median
from skimage.morphology import disk
denoised = median(img, footprint=disk(3))

# Top-hat transform (background subtraction for uneven illumination)
from skimage.morphology import white_tophat, disk
background_removed = white_tophat(img, footprint=disk(50))
print(f"Background removed: range [{background_removed.min()}, {background_removed.max()}]")
```

```python
# Edge detection
from skimage.filters import sobel, laplace, prewitt

edges_sobel = sobel(img_as_float(img))
edges_laplace = laplace(img_as_float(img))

# Difference of Gaussians (blob-like structure detection)
from skimage.filters import difference_of_gaussians
blob_enhanced = difference_of_gaussians(img_as_float(img), low_sigma=1, high_sigma=3)

# Contrast enhancement (CLAHE: local histogram equalization)
from skimage.exposure import equalize_adapthist
enhanced = equalize_adapthist(img_as_float(img), clip_limit=0.03)
```

### Module 3: Thresholding and Segmentation

```python
from skimage import filters, morphology, segmentation
from skimage.color import label2rgb
import numpy as np

# Automatic thresholding methods
from skimage.filters import (threshold_otsu, threshold_li,
                              threshold_triangle, threshold_yen)

img_f = img_as_float(img)
print(f"Otsu: {threshold_otsu(img_f):.3f}")
print(f"Li: {threshold_li(img_f):.3f}")

# Apply threshold and clean binary mask
binary = img_f > threshold_otsu(img_f)
binary_clean = morphology.remove_small_objects(binary, min_size=50)
binary_filled = morphology.remove_small_holes(binary_clean, area_threshold=100)
```

```python
# Watershed segmentation (separate touching objects)
from skimage.segmentation import watershed
from skimage.feature import peak_local_max
from scipy import ndimage as ndi

# Distance transform → local maxima → watershed
distance = ndi.distance_transform_edt(binary_filled)
coords = peak_local_max(distance, min_distance=20, labels=binary_filled)
mask = np.zeros(distance.shape, dtype=bool)
mask[tuple(coords.T)] = True
markers = ndi.label(mask)[0]
labels = watershed(-distance, markers, mask=binary_filled)

print(f"Segmented objects: {labels.max()}")
overlay = label2rgb(labels, image=img_f, bg_label=0)
```

### Module 4: Morphological Operations

```python
from skimage.morphology import (erosion, dilation, opening, closing,
                                 disk, ball, binary_erosion, binary_dilation)

# Erosion