anhvth/vision-utilities

Vision Utilities Guide

This skill provides comprehensive guidance for using the vision utilities in speedy_utils.

When to Use This Skill

Use this skill when you need to:

• Load images efficiently, leveraging GPU acceleration (NVIDIA DALI) when available.
• Create memory-mapped datasets (ImageMmap) for extremely fast random access training loops.
• Visualize batches of images in Jupyter notebooks with automatic grid layout.
• Handle various image formats (numpy, torch, file paths) uniformly.

Prerequisites

• speedy_utils installed.
• Pillow and numpy (required).
• matplotlib (for plotting).
• nvidia-dali-cuda110 or similar (optional, for GPU loading).
• torch (optional, for tensor support).

Core Capabilities

Fast Image Loading (read_images)

• Tries GPU (DALI) first, falls back to CPU (Pillow).
• Supports batch processing and resizing.
• Validates images to skip corrupted files.

Memory-Mapped Datasets (ImageMmap, ImageMmapDynamic)

• ImageMmap: For fixed-size images. Pre-processes and resizes images once, then stores them in a single binary file for zero-copy access.
• ImageMmapDynamic: For variable-size images. Stores flattened images and metadata.
• Both support multi-process safe building with file locks.

Notebook Visualization (plot_images_notebook)

• Automatically arranges images in a grid.
• Handles mixed inputs: paths, numpy arrays, torch tensors.
• Supports (H, W, C), (C, H, W), and (B, ...) formats.

Usage Examples

Example 1: Fast Image Loading

Load a batch of images, resizing them to 224x224.

from vision_utils.io_utils import read_images

paths = ["img1.jpg", "img2.jpg", "img3.jpg"]
# Returns dict: {path: np.ndarray}
images = read_images(paths, hw=(224, 224))

Example 2: Creating a Mmap Dataset

Create a dataset that loads instantly on subsequent runs.

from vision_utils.io_utils import ImageMmap

# First run: reads files, resizes, writes .cache/mmap_dataset_...
# Next runs: maps file directly
dataset = ImageMmap(paths, size=(224, 224))

# Access like a list/array
img = dataset[0]  # np.ndarray (224, 224, 3)

Example 3: Visualizing Images

Plot a mix of tensors and paths in a notebook.

from vision_utils.plot import plot_images_notebook
import torch
import numpy as np

images = [
    "img1.jpg",                  # Path
    np.random.rand(100, 100, 3), # Numpy
    torch.rand(3, 64, 64)        # Tensor (C, H, W)
]

plot_images_notebook(images, ncols=3, titles=["File", "Random", "Tensor"])

Guidelines

1. GPU Loading:

   • read_images is most effective for large batches. For single images, CPU overhead is lower.
   • Ensure DALI is installed for GPU speedup.

2. Mmap Datasets:

   • Use ImageMmap for training pipelines where fixed size is required (e.g., ResNet).
   • Use ImageMmapDynamic if you need original resolutions (e.g., for object detection with variable size inputs).
   • The cache is stored in .cache/ by default. Clear it if your source images change content but keep the same filenames (hashing is based on paths).

3. Plotting:

   • plot_images_notebook is designed for notebooks. It uses plt.show().
   • It automatically handles normalization (0-1 vs 0-255) for display.

Limitations

• DALI Installation: Installing DALI can be complex depending on CUDA version. The code gracefully falls back to CPU if DALI is missing.
• Disk Space: Mmap datasets duplicate image data in uncompressed format (raw pixels). This takes significantly more disk space than JPEGs but offers much faster read speeds.