Lossy Compression

Lossy compression reduces file size by permanently discarding information that viewers are less likely to notice. The decoded image is an approximation of the original, not an exact copy.

Image codecs usually transform blocks of pixels into frequency components, as JPEG does with the discrete cosine transform. They then quantize those components, rounding fine detail to coarser values or to zero, and compress the result. Many also store color at lower resolution than brightness, because the eye is less sensitive to color detail.

A quality or compression setting controls how much is discarded. Lossy files are often a fraction of the size of lossless files of the same image.

Convert image formats

To move between lossy and lossless formats, use Image Converter. Converting to lossless stops further loss but cannot recover discarded detail.

How lossy compression works

1. Split the image into blocks.
2. Transform each block into frequency components.
3. Quantize: keep coarse components and round fine ones away.
4. Entropy-code the remaining values.

Typical artifacts are blocking in flat areas, ringing around sharp edges, banding in gradients and color bleeding near colored text.

Lossy formats: JPG, WebP lossy, AVIF

• JPG: DCT on 8×8 blocks, universal support, no alpha.
• WebP lossy: based on VP8 intra-frame coding, with optional alpha.
• AVIF: based on AV1, often the smallest at a given quality, slower to encode.
• MP4 video also uses lossy compression. Temposhot offers High, Medium and Low quality for MP4.

Lossy vs lossless

	Lossy	Lossless
Output	Approximate	Exact
File size	Small	Larger
Best for	Photos, sharing, web	UI text, editing, archives

Common mistakes

• Saving screenshots of small text at low quality.
• Editing and re-saving a lossy file many times, which compounds the loss.
• Using lossy output as input for OCR or pixel comparison.
• Picking one quality value for all content types.

Have an image to fix right now?

Compress a copy with Image Compressor and compare it with the original in Compare Images.

Common questions

What quality balances size and clarity?

For screenshots, start at 85, the Temposhot default for JPG, and lower it while checking a zoomed crop.

Can I undo lossy compression?

No. Keep a lossless original if you may need full detail later.

Sources

• Lossy compression (Wikipedia)
• Discrete cosine transform (Wikipedia)

Related resources

• Export formats: JPG quality 1-100 and MP4 quality levels.
• Image quality: what the quality setting changes.