Why do some JPEG images look blocky or blurry after compression?
From NCERTAsked by Anonymous from Frankfurt, Germany23 Oct 2026, 10:53 PM ISTUpdated
In one line
Some JPEG images look blocky or blurry after compression because too much detail is removed to make the file smaller.
JPEG — Lossy compression method for reducing the size of digital images A JPEG image showing blocky compression artifacts — the blocks and blur appear when the file is highly compressed. Image: Wikipedia
The rule
JPEG compression reduces image file size by removing some image details, which can cause visible blocks or blur when the compression is high.
Let's understand it simply
Imagine you are rolling out a big roti, but you want to save time. Instead of making it smooth, you press it quickly with your palm, leaving marks. The roti is still there, but it is not as smooth as before. JPEG compression works like this. To make the image file small, it throws away tiny details. If you compress too much, you start seeing the marks — these are the blocks or blur in the image.
How it works
The JPEG format breaks the image into small squares, usually 8×8 pixels each.
It looks for details in each square and removes the ones it thinks you won't notice.
If you ask for high compression (very small file), it removes more details.
When you open the image, the missing details make the squares stand out — this is called "blockiness".
Fine lines and sharp edges become soft or fuzzy, causing "blurriness".
How JPEG compression causes blockiness and blur
Image is divided into small blocks (8×8 pixels).
Details are removed to shrink file size.
Too much detail removed = blocks and blur appear.
The story behind it
JPEG was created in the late 1980s by a group called the Joint Photographic Experts Group. They wanted a way to store photos using less space, so people could send and save images easily, even with slow internet. They found that our eyes do not notice small changes in colour and detail, so they made a method to remove those bits first.
Where you see it in real life
WhatsApp images: When you send a photo, WhatsApp compresses it. Sometimes, faces or text in the image look blocky.
Online cricket match photos: News websites use compressed images so pages load faster, but players' faces may look blurry.
ID card scans: If you scan your Aadhaar card and save it as a small JPEG, the text might become hard to read.
Old Facebook photos: Older uploads often look fuzzy because they were compressed to save space.
Why it matters
Faster sharing: Compressed images use less data, so they load quickly on slow internet.
Saves storage: You can keep more photos on your phone or computer.
Quality loss: Too much compression can ruin important details, which is bad for documents or art.
Exams and projects: Knowing when to use JPEG or other formats (like PNG) helps you make better presentations.
Common mistakes
Thinking all JPEGs are always blurry — only highly compressed ones are.
Believing the blocks are part of the original photo — they appear only after compression.
Using JPEG for images with text or sharp lines — this makes the text hard to read. Use PNG instead.
Remember it
Too much JPEG compression = smaller file, but more blocks and blur. Balance is key!
Check yourself
Why do JPEG images sometimes look blocky after compression?
Because too much detail is removed, making the blocks visible.
What should you use for images with sharp text: JPEG or PNG? Why?
PNG, because it keeps sharp lines and text clear.
How does JPEG make image files smaller?
By removing small details our eyes may not notice.
Why do websites use compressed JPEG images?
To make pages load faster and save data.
Quick recap
JPEG compression removes details to make files smaller.
Too much compression causes blocky or blurry images.
Blocks appear because the image is divided into squares.
Use less compression for important or sharp images.
