18 min read

Lossy and Lossless Compression

How compression makes files smaller, what JPEG quality settings do, and when to avoid lossy compression.

Why Compress?

You calculated that an uncompressed A3 poster is about 52 MB . That is too big to email or put on a website. Compression makes files smaller by storing the data more efficiently.

Lossy vs lossless

$Lossy compression$	$Lossless compression$
$Throws away some detail the eye is unlikely to notice$	$Keeps every pixel exactly — nothing is lost$
$Much smaller files$	$Smaller than uncompressed, but bigger than lossy$
$Quality drops each time you re-save$	$Re-saving does not reduce quality$
$JPG, lossy WebP$	$PNG, GIF, TIFF (LZW or ZIP), PSD, lossless WebP$
$Best for photos$	$Best for text, logos, graphics with sharp edges, and working files$

Typical JPEG results (a 1080 × 1350 px poster image)
Typical JPEG results (a 1080 × 1350 px poster image)

Sizes are examples; they depend on how detailed the image is.

Practice

Which statement about lossy compression is true?

Why Compress?

You calculated that an uncompressed A3 poster is about 52 MB . That is too big to email or put on a website. Compression makes files smaller by storing the data more efficiently.

Lossy vs lossless

$Lossy compression$	$Lossless compression$
$Throws away some detail the eye is unlikely to notice$	$Keeps every pixel exactly — nothing is lost$
$Much smaller files$	$Smaller than uncompressed, but bigger than lossy$
$Quality drops each time you re-save$	$Re-saving does not reduce quality$
$JPG, lossy WebP$	$PNG, GIF, TIFF (LZW or ZIP), PSD, lossless WebP$
$Best for photos$	$Best for text, logos, graphics with sharp edges, and working files$

JPEG Quality Settings

Typical JPEG results (a 1080 × 1350 px poster image)
Typical JPEG results (a 1080 × 1350 px poster image)

Sizes are examples; they depend on how detailed the image is.

Practice

Which statement about lossy compression is true?