PNG compression levels, row filters, and palette quantization

PNG is often treated as an inflexible lossless format, yet libvips pngsave provides two distinct optimization levers:

Truecolour lossless tuning: balancing zlib Compression (0..9) and PNG row prediction filters (PngFilter) for speed vs size;
8-bit indexed palette quantization (Palette: true, similar to pngquant): quantizing 32-bit RGBA graphics into a 256-colour RGBA palette, typically shrinking UI icons, badges, and stickers by 60%–75%.

Translucent ring vector graphic on a transparent background — Input: transparent graphic720 × 480, RGBA

240×160 PNG output with preserved alpha transparency — Output: PNG240 × 160, 4-band RGBA, 35,763 bytes

Cross-binding PNG parameter reference

Setting	govips `PngExportParams`	sharp `.png(options)`	pyvips / vips CLI `pngsave`	Tuning guidance
Compression level	`Compression` (`0..9`, default 6)	`compressionLevel` (`0..9`, default 6)	`compression=6`	`4–6` for real-time HTTP pipelines; `9` for offline static asset builds
Row prediction filter	`Filter` (`PngFilterNone` / `All`)	`adaptiveFiltering` (boolean)	`filter="none"` / `"all"`	`None` is faster and smaller on flat UI/palette graphics; `All` suits continuous-tone photos
Indexed palette mode	`Palette: true`	`palette: true`	`palette=True` / `--palette`	Converts to indexed PNG8 (requires `libimagequant` or `quantizr` in libvips)
Palette colours / bitdepth	`Bitdepth` (`1` / `2` / `4` / `8`)	`colours` (`2..256`) / `bitdepth`	`bitdepth` (or `colours`)	`bitdepth=8` for up to 256 colours; `bitdepth=4` for 16-colour icons
Quantization quality & dither	`Quality` (`0..100`), `Dither` (`0..1`)	`quality`, `dither`, `effort`	`Q=85`, `dither=0.8`, `effort=7`	Lowering `Dither` slightly improves deflate compression on flat colour fills
Adam7 interlacing	`Interlace: true`	`progressive: true`	`interlace=True`	Avoid in production: Adam7 inflates PNG size by 10%–20% and slows decoding

1. Truecolour lossless PNG and row filter selection

Before zlib compression runs, PNG applies a scanline prediction filter (None, Sub, Up, Average, Paeth). On flat UI graphics, QR codes, line art, or indexed images, disabling adaptive filtering (PngFilterNone / adaptiveFiltering: false) encodes several times faster and often yields a smaller file:

params := vips.NewPngExportParams()
params.Compression = 6
params.Filter = vips.PngFilterNone
params.Interlace = false
params.StripMetadata = true

data, _, err := image.ExportPng(params)
if err != nil {
    return err
}
return os.WriteFile("lossless.png", data, 0o644)

import sharp from 'sharp';

await sharp('alpha-rings.svg')
  .png({
    compressionLevel: 6,
    adaptiveFiltering: false,
    progressive: false,
  })
  .toFile('lossless.png');

import pyvips

image = pyvips.Image.new_from_file("alpha-rings.svg")
image.write_to_file(
    "lossless.png",
    compression=6,
    filter="none",
    interlace=False,
    keep="none",
)

vips pngsave alpha-rings.svg lossless.png \
  --compression 6 \
  --filter none \
  --keep none

2. 8-bit indexed palette quantization (PNG8 with full alpha)

When .png output is required (for legacy clients or email templates) and you want to shrink translucent badges, stickers, or UI overlays dramatically, enable Palette: true:

params := vips.NewPngExportParams()
params.Palette = true
params.Bitdepth = 8      // Up to 256 RGBA palette entries
params.Quality = 85      // Quantization quality target (0..100)
params.Dither = 0.8      // Floyd-Steinberg dithering strength (0.0..1.0)
params.Compression = 6
params.Filter = vips.PngFilterNone

data, _, err := image.ExportPng(params)
if err != nil {
    return err
}
return os.WriteFile("palette-8bit.png", data, 0o644)

import sharp from 'sharp';

await sharp('alpha-rings.svg')
  .png({
    palette: true,
    colours: 256,
    quality: 85,
    dither: 0.8,
    effort: 7,
    compressionLevel: 6,
  })
  .toFile('palette-8bit.png');

import pyvips

image = pyvips.Image.new_from_file("alpha-rings.svg")
image.write_to_file(
    "palette-8bit.png",
    palette=True,
    bitdepth=8,
    Q=85,
    dither=0.8,
    effort=7,
    compression=6,
    filter="none",
)

vips pngsave alpha-rings.svg palette-8bit.png \
  --palette \
  --bitdepth 8 \
  --Q 85 \
  --dither 0.8 \
  --effort 7 \
  --compression 6 \
  --filter none

Sources