How PNG Files Save Space

In the digital age, efficient storage and fast transmission of data are crucial. One common file format that excels in these areas is the PNG (Portable Network Graphics) file. While you might be familiar with PNG files as a popular choice for images on websites, you may not know how they manage to save space while maintaining high quality. In this blog post, we’ll delve into the secrets behind PNG file compression and why this format is so effective at saving space.

1. Lossless Compression: Quality Retained

Unlike other image formats like JPEG, which use lossy compression, PNG files use lossless compression. This means that all the image data is retained without any loss of quality. Lossless compression is particularly important for images with text, sharp edges, or where detail preservation is crucial.

2. Compression Algorithms: DEFLATE and Filtering

PNG files use a combination of the DEFLATE algorithm and filtering techniques to compress image data effectively. The DEFLATE algorithm is a combination of LZ77 (a lossless data compression algorithm) and Huffman coding (an entropy encoding algorithm). Here’s how it works:

• LZ77 Algorithm: This algorithm replaces repeated occurrences of data with references to a single copy of that data existing earlier in the uncompressed data stream. It’s like finding patterns and replacing them with shorter representations.
• Huffman Coding: After applying LZ77, Huffman coding further compresses the data by replacing the most common bytes with shorter codes and less common bytes with longer codes. This reduces the overall size of the data.

3. Filtering Techniques: Preprocessing for Efficiency

Before applying the DEFLATE algorithm, PNG files undergo a filtering process. This preprocessing step transforms the image data to make it more compressible. There are five different filtering methods that PNG files can use, each suited to different types of images:

• None: No filter; the raw data is compressed.
• Sub: Transmits the difference between each byte and the byte to its left.
• Up: Transmits the difference between each byte and the byte directly above it.
• Average: Transmits the average of the differences between the byte to the left and the byte above.
• Paeth: A more complex filter that uses a linear function of the three neighboring pixels (left, above, upper-left) to predict the value of a pixel.

The choice of filter can significantly impact the file size, and PNG encoders typically choose the best filter for each line of the image to optimise compression.

4. Transparency Handling: Alpha Channel

PNG files support an alpha channel, allowing for varying levels of transparency. This feature is particularly useful for web graphics, where transparent backgrounds are common. The alpha channel is compressed along with the image data, ensuring that transparency doesn’t bloat the file size.

5. Interlacing: Faster Loading for Web Images

PNG files can use Adam7 interlacing, which divides the image into a sequence of seven passes. This allows a low-resolution version of the image to be loaded first, providing a quick preview while the full image is still being downloaded. Interlacing can make images appear to load faster, enhancing the user experience without significantly increasing the file size.

6. Palette-Based Images: Optimising Colours

For images with fewer colours (like icons or simple graphics), PNG files can use a palette-based approach. This involves creating a palette of up to 256 colours and referencing them, which can significantly reduce the file size compared to true-colour images (which use 24 bits per pixel).

To Summarise

PNG files are a powerful format for image storage and transmission, balancing quality and file size through sophisticated compression techniques. The use of lossless compression, efficient algorithms, and smart preprocessing ensures that PNG files maintain high visual fidelity while saving space. Whether you’re a web developer, graphic designer, or just someone who appreciates well-compressed images, understanding how PNG files save space can help you make better choices for your digital projects.

By leveraging the strengths of PNG compression, you can ensure your images are both high-quality and storage-efficient, making them ideal for a wide range of applications.