Lossless vs Lossy Image Compression Explained

Lossless vs Lossy Compression

Every image format uses one of two fundamental compression approaches — or sometimes both. Understanding the difference is essential for choosing the right format.

Lossy Compression

Lossy compression permanently removes image data that the algorithm deems less important to human perception. The original image cannot be perfectly reconstructed.

How it works (simplified):

1. Divide image into 8x8 pixel blocks
2. Apply Discrete Cosine Transform (DCT)
3. Quantize frequency coefficients (discard high-frequency details)
4. Entropy-encode the remaining data

Formats using lossy compression:

• JPEG (always lossy)
• WebP (lossy mode)
• AVIF (lossy mode)

Characteristics:

• Much smaller file sizes (typically 5-20x vs uncompressed)
• Quality degrades with each re-save (generation loss)
• Artifacts appear at sharp edges and text
• Quality slider controls the aggressiveness

Lossless Compression

Lossless compression reduces file size while preserving every single pixel exactly. The decompressed image is bit-for-bit identical to the original.

How it works (simplified):

1. Apply prediction filters (each pixel predicted from neighbors)
2. Encode prediction errors with entropy coding (Huffman, arithmetic)
3. The smaller the prediction errors, the better the compression

Formats using lossless compression:

• PNG (always lossless)
• BMP (typically uncompressed, no loss)
• WebP (lossless mode)
• AVIF (lossless mode)

Characteristics:

• Larger file sizes than lossy
• No quality degradation ever
• Safe for repeated editing
• Best for graphics, text, screenshots

Hybrid Approach

Modern formats like WebP and AVIF support both modes, letting you choose per-image:

• Use lossy for photographs
• Use lossless for graphics and UI elements