Editing PNG (section)

=== File size factors ===
PNG files vary in size due to a number of factors:
;color depth: Color depth can range from 1 to 64 bits per pixel.
;ancillary chunks: PNG supports metadata—this may be useful for editing, but unnecessary for viewing, as on websites.
;interlacing: As each pass of the Adam7 algorithm is separately filtered, this can increase file size.<ref name="pngcf" />
;filter: As a precompression stage, each line is filtered by a predictive filter, which can change from line to line. As the ultimate DEFLATE step operates on the whole image's filtered data, one cannot optimize this row-by-row; the choice of filter for each row is thus potentially very variable, though heuristics exist.<ref group="note">The filtering is used to increase the similarity to the data, hence increasing the compression ratio. However, there is theoretically no formula for similarity, nor absolute relationship between the similarity and compressor, thus unless the compression is done, one can't tell one filter set is better than another.</ref>
;compression: With additional computation, DEFLATE compressors can produce smaller files.
There is thus a filesize trade-off between high color depth, maximal metadata (including color space information, together with information that does not affect display), interlacing, and speed of compression, which all yield large files, with lower color depth, fewer or no ancillary chunks, no interlacing, and tuned but computationally intensive filtering and compression. For different purposes, different trade-offs are chosen: a maximal file may be best for archiving and editing, while a stripped down file may be best for use on a website, and similarly fast but poor compression is preferred when repeatedly editing and saving a file, while slow but high compression is preferred when a file is stable: when archiving or posting.
Interlacing is a trade-off: it dramatically speeds up early rendering of large files (improves latency), but may increase file size (decrease throughput) for little gain, particularly for small files.<ref name="pngcf" />

==== Lossy PNG compression ====
Although PNG is a lossless format, PNG encoders can preprocess image data in a lossy fashion to improve PNG compression. For example, quantizing a truecolor PNG to 256 colors allows the indexed color type to be used for a likely reduction in file size.<ref>{{cite web|url=http://pngmini.com/lossypng.html|title=PNG can be a lossy format|publisher=Pngmini.com|access-date=2014-02-01}}</ref>