What Is Lossy Compression?

Lossy compression reduces file size by permanently removing data from the image. The algorithm analyzes the image and discards information that the human eye is less likely to notice — subtle color variations in gradients, fine detail in textured areas, and high-frequency information at edges.

JPG is the most common lossy format. When you save a JPG at 80% quality, you’re telling the encoder to discard roughly 20% of the data while attempting to preserve visual appearance. The result is a much smaller file that usually looks nearly identical to the original.

The trade-off is that this data loss is irreversible. Once you compress an image with lossy compression, you cannot restore the discarded information. Repeatedly opening, editing, and re-saving a JPG compounds the damage, gradually introducing visible artifacts around edges and in smooth gradient areas.

What Is Lossless Compression?

Lossless compression reduces file size by finding more efficient ways to represent the same data. Rather than removing information, it uses mathematical algorithms to pack the data tighter. When you decompress a losslessly compressed image, you get back every single pixel exactly as it was in the original.

PNG is the most widely used lossless format for web graphics. It uses DEFLATE compression, the same algorithm used in ZIP files, to reduce file sizes without any quality loss. GIF also uses lossless compression, though its 256-color limitation often causes quality degradation that is unrelated to the compression itself.

The advantage of lossless compression is preservation. You can edit and re-save a PNG hundreds of times without any quality degradation. The disadvantage is file size: lossless compression typically achieves 20-50% reduction, while lossy compression can achieve 80-90% reduction for photographs.

Visual Comparison

At quality settings above 70%, lossy JPG compression is virtually indistinguishable from the original for most photographs. Quality issues become visible below 60%, where blocky artifacts appear around edges and banding occurs in smooth gradients.

Lossless PNG compression preserves every detail but produces larger files. For a typical photograph, PNG might be 5-10 times larger than a comparable JPG. For graphics with large areas of solid color, PNG can actually be smaller than JPG because the uniform colors compress extremely well.

Format Guide

Lossy formats:

  • JPG: Universal support, excellent for photographs
  • Lossy WebP: Modern alternative with better compression than JPG
  • AVIF: Next-generation format with even smaller files

Lossless formats:

  • PNG: Universal support, transparency, perfect quality preservation
  • Lossless WebP: Smaller files than PNG with identical quality
  • GIF: Limited to 256 colors, supports animation

Choosing Between Them

For photographs and complex images where some quality loss is acceptable, lossy formats are the clear choice. The file size savings are enormous, and at appropriate quality settings, the visual difference is negligible.

For screenshots, graphics with text, logos, and images where you need pixel-perfect accuracy, lossless formats are essential. The text in a screenshot compressed with JPG becomes blurry and difficult to read, while PNG preserves every letter crisply.

For archival purposes, always use lossless formats. You can convert lossless to lossy at any time, but you cannot recover quality from a lossy file.