Editing PDF (section)

=== Raster images ===

Raster images in PDF (called ''Image XObjects'') are represented by dictionaries with an associated stream. The dictionary describes the properties of the image, and the stream contains the image data. (Less commonly, small raster images may be embedded directly in a page description as an ''inline image''.) Images are typically ''filtered'' for compression purposes. Image filters supported in PDF include the following general-purpose filters:

* ''ASCII85Decode'', a filter used to put the stream into 7-bit ASCII,
* ''ASCIIHexDecode'', similar to ASCII85Decode but less compact,
* ''FlateDecode'', a commonly used filter based on the [[deflate]] algorithm defined in {{IETF RFC|1951}} (deflate is also used in the [[gzip]], [[Portable Network Graphics|PNG]], and [[ZIP (file format)|zip]] file formats among others); introduced in PDF 1.2; it can use one of two groups of predictor functions for more compact zlib/deflate compression: ''Predictor 2'' from the [[TIFF]] 6.0 specification and predictors (filters) from the [[Portable Network Graphics|PNG]] specification ({{IETF RFC|2083}}),
* ''LZWDecode'', a filter based on [[LZW]] Compression; it can use one of two groups of predictor functions for more compact LZW compression: ''Predictor 2'' from the TIFF 6.0 specification and predictors (filters) from the PNG specification,
* ''RunLengthDecode'', a simple compression method for streams with repetitive data using the [[run-length encoding]] algorithm and the image-specific filters,
* ''DCTDecode'', a [[lossy]] filter based on the [[JPEG]] standard,
* ''CCITTFaxDecode'', a lossless [[bi-level image|bi-level]] (black/white) filter based on the Group 3 or [[Group 4 compression|Group 4]] [[CCITT]] (ITU-T) [[fax]] compression standard defined in ITU-T [[T.4]] and T.6,
* ''JBIG2Decode'', a lossy or [[lossless]] bi-level (black/white) filter based on the [[JBIG2]] standard, introduced in PDF 1.4, and
* ''JPXDecode'', a lossy or lossless filter based on the [[JPEG 2000]] standard, introduced in PDF 1.5.

Normally all image content in a PDF is embedded in the file. But PDF allows image data to be stored in external files by the use of ''external streams'' or ''Alternate Images''. Standardized subsets of PDF, including [[PDF/A]] and [[PDF/X]], prohibit these features.