science

Group 4 compression

Group 4 compression is a science topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Group 4 compression rather than just read about it. In short: CCITT Group 4 compression, also referred to as G4 or Modified Modified READ (MMR), is a lossless method of image compression used in Group 4 fax machines defined in the ITU-T T.6 fax standard. It is only used for bitonal (black-and-white) images.

Key takeaways

Group 4 compression belongs to science; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Group 4 compression to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Group 4 compression from memory before moving on to harder problems.

Reference excerpt

CCITT Group 4 compression, also referred to as G4 or Modified Modified READ (MMR), is a lossless method of image compression used in Group 4 fax machines defined in the ITU-T T.6 fax standard. It is only used for bitonal (black-and-white) images. Group 4 compression is based on the Group 3 two-dimensional compression scheme (G3-2D), also known as Modified READ, which is in turn based on the Group 3 one-dimensional compression scheme (G3), also known as Modified Huffman coding. Group 4 compression is available in many proprietary image file formats as well as standardized formats such as TIFF, CALS, CIT (Intergraph Raster Type 24) and the PDF document format. G4 offers a small improvement over G3-2D by removing the end-of-line (EOL) codes. G3 and G4 compression both treat an image as a series of horizontal black strips on a white page. Better compression is achieved when there are fewer unique black dots/lines on the page. Both G3-2D and G4 add a two-dimensional feature to achieve greater compression by taking advantage of vertical symmetry. A worst-case image would be an alternating pattern of single-pixel black and white dots offset by one pixel on even/odd lines. G4 compression would actually increase the file size on this type of image. G4 typically achieves a 20:1 (95%) compression ratio. For an 8.5"×11" page scanned at 200 DPI, this equates to a reduction from 467.5 kB to 23.4 kB.

See also JBIG2 Binary image Digital fax compression

External links "Group 4 Compression". GraphicsAcademy.com. Retrieved March 4, 2010. "T.6: Facsimile coding schemes and coding control functions for Group 4 facsimile apparatus". ITU-T. November 1988. Retrieved 2013-12-28.

Worked examples

Example 1 — a first encounter with Group 4 compression

Start with the simplest possible case. Write down what Group 4 compression claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In science, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Group 4 compression before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Group 4 compression ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Group 4 compression

In research: Group 4 compression appears in science research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Group 4 compression in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Group 4 compression is common in secondary-school and first-year university syllabi. It links to neighbouring topics Fax, so understanding it makes those chapters shorter.
In everyday life: Look for Group 4 compression outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

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How to study Group 4 compression in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Group 4 compression means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Group 4 compression out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Group 4 compression in simple terms?

Why does Group 4 compression matter?

Because it connects several science ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Group 4 compression?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Group 4 compression.