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PNG Filtering

PNG Filtering reduces data size using coding, prediction, dictionary, or transform structure.

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Quick Facts

Compact metadata for implementation and discovery.

Category: Compression
Difficulty: Intermediate
Family: Compression
Tier: Core
Status: Verified
Review: Source Checked
Sources: 1
Formulas: 1
Relationships: 1
Code example: Ready
Visualization: Ready

Overview

PNG Filtering is a compression method that reduces data size through coding, prediction, transform, or dictionary structure. It works best when the data matches the model, code table, predictor, or transform being used.

Core Concepts

symbol modelbitstreamdictionary or code tableencodingdecoding

Quick facts

Complexity

Runtime and memory behavior at a glance.

Best

O(n)

Average

O(n)

Worst

O(n log n)

Space

O(n)

When to Use

Use PNG Filtering when the data has the repetition, prediction, or symbol distribution it expects.
Use it when encoder and decoder metadata can be versioned and tested together.

When Not to Use

Avoid when the data does not match the assumed distribution, predictor, or dictionary.
Avoid when decompression speed or random access matters more than compression ratio.

What It Does

Models repeated or likely symbols
Encodes data with fewer bits when assumptions hold
Supports decoding with the same model or dictionary

Use Cases

File compression
Columnar storage
Media encoding

Inputs

Input symbols or bytes
Model, dictionary, or coding parameters

Outputs

Compressed bitstream
Code table or dictionary
Decoded output

Key Formulas (1)

Filter residual

residual = pixel − predictor(left, up, upper_left)

residual = pixel - predictor(left, up, upper_left)

Encodes prediction residuals so later compression sees smaller values.

Pseudocode

1.Build or initialize the model, dictionary, transform, or code table
2.Process symbols or blocks in order
3.Emit codes, residuals, dictionary references, or coefficients
4.Store the side information needed for decoding
5.Return the compressed stream

Canonical Example

Input: symbols = 'aaabcc'
Output: encoded stream plus decoding metadata
Explanation: PNG Filtering models repetition, symbol likelihood, or transform coefficients to produce a smaller representation.

Implementation Notes

Write decoder tests before optimizing the encoder.
Store versioned metadata for code tables, dictionaries, predictors, or transforms.
Compression can expand small or already-compressed inputs because metadata has a cost.

Common Pitfalls

Failing to store decoding metadata makes compressed output unusable.
Optimizing only ratio can make decompression too slow or memory-heavy.
Encoder and decoder state must stay synchronized across dictionary updates, blocks, and metadata boundaries.

Interactive Mechanism Walkthrough

Sequence Window & Automaton Alignment

Show symbols being mapped into codes, dictionary entries, residuals, or transform coefficients.

Input:ABABABCABAB

Window:ABAB

Step 1 of 5: Build or initialize the model, dictionary, transform, or code table

compression

Notes

PNG Filtering operates in O(n) time and O(n) space within the Compression family. Key governing relation: residual = pixel - predictor(left, up, upper_left). Often compared with Huffman Coding and LZ77.