compression
Blocking artifact
Visible rectangular grid patterns in compressed images caused by JPEG's 8×8 pixel block compression method.
What is Blocking artifact?
Blocking artifacts are visible rectangular grid patterns that appear in JPEG images when compression quality is too low, caused by JPEG's underlying 8×8 pixel DCT block structure becoming perceptible. Each 8×8 pixel square is compressed independently, and at low quality settings, the boundaries between these blocks become visible as a regular grid pattern. This effect is most noticeable in smooth areas like clear skies, skin tones, and solid backgrounds where the block boundaries create obvious discontinuities in color and tone.
Importance of Blocking artifact
Understanding blocking artifacts helps you choose appropriate JPEG quality settings to maintain professional image quality for web, social media, and email use. When blocking artifacts appear, images look unprofessional and distract viewers from your content. Modern formats like WebP and AVIF avoid this 8×8 block structure entirely, which is why they produce cleaner results at smaller file sizes than JPEG.
Blocking artifact in Practice
A portrait photo compressed at JPEG quality 50 will show obvious 8×8 pixel squares across skin tones and background areas, making the subject look pixelated and unprofessional. The same image at quality 75 maintains smooth gradients without visible block boundaries. Social media platforms like Instagram automatically detect and reject images with severe blocking artifacts, requiring you to upload higher quality versions.
Blocking artifact Best Practices
- → Keep JPEG quality above 70 to prevent blocking artifacts from becoming visible to most viewers.
- → Use WebP or AVIF formats instead of JPEG when possible to avoid DCT block limitations entirely.
- → Compress from the original lossless source rather than re-compressing an already blocked image.
Example of Blocking artifact
Related Terms
Frequently Asked Questions
What are blocking artifacts in JPEG images?
Blocking artifacts are visible rectangular grid patterns that appear in JPEG images when compression quality is too low, caused by JPEG's 8×8 pixel block compression structure becoming perceptible. They appear as a regular grid of squares where each block shows slightly different colors or tones than its neighbors. This effect is most obvious in smooth areas like skies, skin, or solid backgrounds where the block boundaries create noticeable discontinuities.
How do you fix JPEG blocking artifacts?
The only way to fix blocking artifacts is to re-compress from the original uncompressed source image at a higher quality setting, typically above JPEG quality 70. You cannot fix blocking by re-compressing an already artifacted JPEG at higher quality, as this causes double compression and additional quality loss. Prevention is key: always compress from your original RAW or lossless file at an appropriate quality level for your intended use.
Why do some compressed images look like they have a grid of squares?
Images show a grid of squares when JPEG compression quality is too low, making the format's underlying 8×8 pixel block structure visible to viewers. JPEG divides images into these small squares called DCT blocks and compresses each independently, but at low quality settings the boundaries between blocks become obvious as color and tone mismatches. This creates the characteristic grid or quilting effect that makes images look pixelated and unprofessional.