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Scan for outdated or missing drivers - takes under a minuteDriver Scan →Clear out junk files and repair common Windows errorsFree Scan →Run-length encoding (RLE) is a lossless method that represents consecutive repetitions of the same value as a value and its run length. A sequence such as AAAAA can be represented conceptually as (A, 5); decoding repeats the value five times to restore the original sequence. The pair is an illustration, not a universal byte format: each RLE format defines its own counts, literals, boundaries, and limits.
How run-length encoding works
An encoder scans an ordered sequence and groups each maximal adjacent stretch of identical values into a run. For example, AAAABBCCCCC contains three runs: (A, 4), (B, 2), and (C, 5). The decoder uses the values and counts to reconstruct the sequence without losing information.
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Real encodings must also represent values that do not repeat. Many formats handle these as literal segments: they store the values directly, rather than expressing them as repetitions. Microsoft describes its NSCodec stream as segments of runs and literals, while DICOM defines separate replicate-run and literal-run encodings (Microsoft Learn: NSCodec Run-Length Encoding; DICOM PS3.5, 2019a, section G.3).
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1Repair Windows errors before they cause bigger problems2Scan for outdated or missing drivers - takes under a minute3Clear out junk files and repair common Windows errorsA notation such as 4A2B5C can help illustrate the idea, but it is not a safe general-purpose format: if the input itself contains digits, the notation may be ambiguous unless values are escaped or otherwise defined. A real file or protocol needs precise rules for encoding counts and values.
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When RLE saves space—and when it does not
RLE is most useful when identical values occur in long adjacent stretches, such as uniform areas in an image. It only combines neighboring repetitions; values that recur in separate parts of the sequence are not combined unless another transformation or technique is used.
If values alternate or runs are short, the counts and control information can offset any savings or make the result larger. Microsoft’s NSCodec documentation gives examples of encoding that expands the data and says the original stream is sent instead in that case. NVIDIA notes that its value-and-run representation can double data when every run has length one; that example describes that representation, not every RLE format.
Why RLE does not have one universal format
RLE names a family of related encodings, not a single byte-level standard. Implementations differ in what counts as a value, how they encode literal data, the maximum run length, and whether runs may cross boundaries. These differences matter when reading or exchanging compressed data.
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| Implementation | How it handles data | Specific rules |
|---|---|---|
| DICOM RLE | Byte-oriented replicate runs and literal runs. | The DICOM Standards Committee’s 2019a PS3.5 section G.3 specifies replicate runs of 2–128 bytes and literal runs of 1–128 bytes. Each image row is encoded separately; a run must not cross a row boundary. Segments are padded to an even number of bytes when needed. |
| Windows bitmap RLE | BI_RLE8 for 8-bit bitmaps and BI_RLE4 for 4-bit bitmaps; includes encoded and absolute modes. | Escape pairs signal events such as end of line, end of bitmap, and delta movement. See Microsoft’s bitmap compression documentation. |
| Microsoft NSCodec | Processes an image stream as runs and literals. | Its documentation includes cases where encoding expands the data and the original stream is sent instead. See Microsoft Learn: NSCodec Run-Length Encoding. |
R’s rle() function |
Returns vectors of run lengths and corresponding values; inverse.rle() reconstructs the input. |
This is behavior of R’s base-language functions, not a file format. R documents that missing values are treated as unequal to the preceding value, even if it is also missing. See R documentation: Run Length Encoding. |
What to check before using or decoding an RLE stream
The label “RLE” alone does not establish compatibility. Check the specific format or protocol for these details:
Quick Recap
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- What is treated as a value: a bit, byte, pixel, or another unit?
- How are literal segments and run counts represented?
- What are the minimum and maximum run lengths?
- Are there row, block, or segment boundaries that a run cannot cross?
- Do escape codes have special meanings, such as marking the end of a line or image?
- What does the encoder do if the encoded data is larger than the original?
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