Home > PNG Tools Overview
An overview of differents PNG optimization tools, including pure compressors. They are tested with differents PNG files with special charactestics that require a good optimization routine to determine the best output settings. List of tools tested:
See Website - Version tested: 1.15 - Type: Compressor
This project is no longer updated since 2002. AdvPNG is a pure compressor that uses LZMA (7-zip), and it operates some optimizations on the PNG characteristics (it removes unnecessary chunks, compacts iDAT, and tries 8 bits/pixels test when image contains 16 colors or less).
| File Tested | Result | Input Size | Output Size |
|---|---|---|---|
| 16-colors-best-encoded-as-8bits.png | Good | 7223 | 6006 |
| 255-bad-trns-values.png | Bad | 2547 | 2052 |
| 256-entries-to-17.png | Bad | 1243 | 1023 |
| alpha-channel-filtered-grayscale.png | Good | 11349 | 11271 |
| alpha-channel-filtered-rgba.png | Good | 11259 | 11886 |
| bad-transparacy-truecolor-alpha.png | Bad | 85864 | 78376 |
| compact-idat-chunk.png | Good | 17193 | 10056 |
| data-png-header.png | Good | 5129 | 306 |
| dirty-data-rbg.png | Bad | 90027 | 86036 |
| filtering-each-line.png | Bad | 181408 | 181408 |
| grayscale-alpha-filter.png | Bad | 21692 | Not Supported |
| grayscale-alpha-to-grayscale-trns.png | Bad | 7032 | Not Supported |
| grayscale-alpha-to-paletted.png | Bad | 18708 | Not Supported |
| low-compression-huffman-only-is-better.png | Bad | 173207 | 171473 |
| order-entries-in-palette.png | Bad | 24834 | 18403 |
| paletted-as-grayscale.png | Bad | 17304 | Not Supported |
| paletted-to-truecolor.png | Bad | 1362 | 1365 |
| truecolor-alpha-as-filtered-paletted.png | Bad | 72038 | 67144 |
| truecolor-alpha-filter.png | Bad | 46912 | 43414 |
| truecolor-alpha-to-grayscale-alpha.png | Bad | 97074 | 89625 |
| truecolor-alpha-to-paletted-trns.png | Bad | 32473 | 25658 |
| truecolor-alpha-to-truecolor-trns.png | Bad | 16653 | 15515 |
| truecolor-to-paletted-67-colors.png | Bad | 1920 | 1063 |
| Total: 24 Files | 5/24 | 944821 | 894224 |
AdvPNG saved 50597 Bytes (5,36 %). To use to its advantage, you should run AdvDef (not AdvPNG) after another PNG optimizer utility, capable of performing tests on ColorType, filters, etc. Used alone, it is less efficient because it allows only a few images reductions, and it does not support GrayScale formats.
See Website - Version tested: 0.6.4 - Type: RGB+Alpha optimizer
This project is credited OptiPNG, but it is actually a modification of OptiPNG, called CryoPNG. This utility can be used on TrueColor + Alpha PNG, and lets you change the RGB space of a PNG file in order to optimize the filtering. It can convert black pixels using existing filters.
| File Tested | Result | Input Size | Output Size |
|---|---|---|---|
| 16-colors-best-encoded-as-8bits.png | Bad | 7223 | Not Supported |
| 255-bad-trns-values.png | Bad | 2547 | Not Supported |
| 256-entries-to-17.png | Bad | 1243 | Not Supported |
| alpha-channel-filtered-grayscale.png | Bad | 11349 | 9563 |
| alpha-channel-filtered-rgba.png | Correct | 11259 | 9420 |
| bad-transparacy-truecolor-alpha.png | Correct | 85864 | 995 |
| compact-idat-chunk.png | Good | 17193 | 10563 |
| data-png-header.png | Bad | 5129 | Not Supported |
| dirty-data-rbg.png | Good | 90027 | 44923 |
| filtering-each-line.png | Bad | 181408 | Not Supported |
| grayscale-alpha-filter.png | Bad | 21692 | Not Supported |
| grayscale-alpha-to-grayscale-trns.png | Bad | 7032 | Not Supported |
| grayscale-alpha-to-paletted.png | Bad | 18708 | Not Supported |
| low-compression-huffman-only-is-better.png | Bad | 173207 | Not Supported |
| order-entries-in-palette.png | Bad | 24834 | Not Supported |
| paletted-as-grayscale.png | Bad | 17304 | Not Supported |
| paletted-to-truecolor.png | Bad | 1362 | Not Supported |
| truecolor-alpha-as-filtered-paletted.png | Bad | 72038 | 28869 |
| truecolor-alpha-filter.png | Bad | 46912 | 39791 |
| truecolor-alpha-to-grayscale-alpha.png | Bad | 97074 | 54359 |
| truecolor-alpha-to-paletted-trns.png | Bad | 32473 | 27655 |
| truecolor-alpha-to-truecolor-trns.png | Bad | 16653 | 10203 |
| truecolor-to-paletted-67-colors.png | Bad | 1920 | Not Supported |
| Total: 24 Files | 3/24 | 944821 | 700120 |
CryoPNG saved 244701 Bytes (25,90 %). Although it can not process all files, CryoPNG can significantly reduce the weight, because it destroys (or changes) RGB values that will not be returned in a Web browser. You can use it only on TrueColor + Alpha PNG, and can be supplemented by a compressor.
See Website - Version tested: 0.6.5 - Type: Optimizer
This project is based on PNGCrush. According to the author, it's running faster because all trials are performed in memory. It also includes parts of PNGRewrite code for some images reductions. OptiPNG is used by a lot of optimizers, including onlines services as SmushIt.
| File Tested | Result | Input Size | Output Size |
|---|---|---|---|
| 16-colors-best-encoded-as-8bits.png | Bad | 7223 | 6700 |
| 255-bad-trns-values.png | Bad | 2547 | 2051 |
| 256-entries-to-17.png | Good | 1243 | 303 |
| alpha-channel-filtered-grayscale.png | Good | 11349 | 7827 |
| alpha-channel-filtered-rgba.png | Good | 11259 | 9463 |
| bad-transparacy-truecolor-alpha.png | Bad | 85864 | 68796 |
| compact-idat-chunk.png | Good | 17193 | 10563 |
| data-png-header.png | Bad | 5129 | 4905 |
| dirty-data-rbg.png | Bad | 90027 | 63955 |
| filtering-each-line.png | Good | 181408 | 757 |
| grayscale-alpha-filter.png | Bad | 21692 | 18885 |
| grayscale-alpha-to-grayscale-trns.png | Good | 7032 | 4308 |
| grayscale-alpha-to-paletted.png | Good | 18708 | 13832 |
| low-compression-huffman-only-is-better.png | Good | 173207 | 100444 |
| order-entries-in-palette.png | Bad | 24834 | 19296 |
| paletted-as-grayscale.png | Bad | 17304 | 15571 |
| paletted-to-truecolor.png | Bad | 1362 | 1058 |
| truecolor-alpha-as-filtered-paletted.png | Bad | 72038 | 60638 |
| truecolor-alpha-filter.png | Bad | 46912 | 41937 |
| truecolor-alpha-to-grayscale-alpha.png | Bad | 97074 | 76654 |
| truecolor-alpha-to-paletted-trns.png | Bad | 32473 | 20012 |
| truecolor-alpha-to-truecolor-trns.png | Bad | 16653 | 10227 |
| truecolor-to-paletted-67-colors.png | Bad | 1920 | 1104 |
| Total: 24 Files | 8/24 | 944821 | 559486 |
OptiPNG saved 385335 Bytes (40,78 %). OptiPNG is a moderate optimizer, and run a good compression routine, but it should be more efficient if it runs others optimization tricks. It does not support RGB filtering, or other images reductions which can be very useful and efficient for Web.
See Website - Version tested: 1.7.15 - Type: Optimizer
Pngcrush is one of the first optimization utilities for PNG, published in 1998. The author also wrote the specifications of PNG. PNGCrush has a lot of options, and most of tools are based on it (OptiPNG, TruePNG, and indirectly all optimizers based on OptiPNG as SmushIt).
| File Tested | Option | Result | Input Size | Output Size |
|---|---|---|---|---|
| 16-colors-best-encoded-as-8bits.png | -bit_depth 8 | Good | 7223 | 6630 |
| 255-bad-trns-values.png | Bug! | Bad | 2547 | 2051 |
| 256-entries-to-17.png | -plte_len 17 | Good | 1243 | 303 |
| alpha-channel-filtered-grayscale.png | -c 4 -brute | Good | 11349 | 7827 |
| alpha-channel-filtered-rgba.png | -brute | Good | 11259 | 9463 |
| bad-transparacy-truecolor-alpha.png | -brute | Bad | 85864 | 69307 |
| compact-idat-chunk.png | -brute | Good | 17193 | 10610 |
| data-png-header.png | -rem alla | Good | 5129 | 303 |
| dirty-data-rbg.png | -brute | Bad | 90027 | 62479 |
| filtering-each-line.png | -brute | Good | 181408 | 757 |
| grayscale-alpha-filter.png | -c 4 -brute | Bad | 21692 | 18885 |
| grayscale-alpha-to-grayscale-trns.png | -c 0 -brute | Good | 7032 | 3543 |
| grayscale-alpha-to-paletted.png | -reduce -brute | Bad | 18708 | 16074 |
| low-compression-huffman-only-is-better.png | -huffman | Good | 173207 | 100344 |
| order-entries-in-palette.png | -brute | Bad | 24834 | 19319 |
| paletted-as-grayscale.png | -reduce -brute | Good | 17304 | 15771 |
| paletted-to-truecolor.png | -c 2 -rem PLTE -brute | Good | 1362 | 459 |
| truecolor-alpha-as-filtered-paletted.png | -reduce -brute | Bad | 72038 | 60605 |
| truecolor-alpha-filter.png | -brute | Bad | 46912 | 41937 |
| truecolor-alpha-to-grayscale-alpha.png | -c 4 -brute | Bad | 97074 | 49886 |
| truecolor-alpha-to-paletted-trns.png | -reduce -brute | Bad | 32473 | 27687 |
| truecolor-alpha-to-truecolor-trns.png | -c 2 -brute | Good | 32473 | 9162 |
| truecolor-to-paletted-67-colors.png | -reduce -brute | Bad | 1920 | 1187 |
| Total: 24 Files | - | 12/24 | 944821 | 536169 |
PNGCrush saved 408652 Bytes (43,25 %). To correctly use pngcrush, you must know the PNG specification, and control options of the utility, because without them, it is unlikely to get a properly optimized PNG. Others optimizers have the advantage to do that automatically.
See Website - Version tested: 2.0 - Type: Optimizer
PngOptimizer is a very simple tool to use: there is no option for images reductions, it does it itself automatically (clean RGB space, palette optimizer, bitdepth reduction, etc.), and also does an average compression. Another good point, it's generally fast.
| File Tested | Result | Input Size | Output Size |
|---|---|---|---|
| 16-colors-best-encoded-as-8bits.png | Bad | 7223 | 6697 |
| 255-bad-trns-values.png | Good | 2547 | 1825 |
| 256-entries-to-17.png | Good | 1243 | 307 |
| alpha-channel-filtered-grayscale.png | Bad | 11349 | 10712 |
| alpha-channel-filtered-rgba.png | Bad | 11259 | 10973 |
| bad-transparacy-truecolor-alpha.png | Correct | 85864 | 761 |
| compact-idat-chunk.png | Good | 17193 | 10564 |
| data-png-header.png | Good | 5129 | 307 |
| dirty-data-rbg.png | Correct | 90027 | 46768 |
| filtering-each-line.png | Good | 181408 | 790 |
| grayscale-alpha-filter.png | Bad | 21692 | 18925 |
| grayscale-alpha-to-grayscale-trns.png | Good | 7032 | 4354 |
| grayscale-alpha-to-paletted.png | Bad | 18708 | 18708 |
| low-compression-huffman-only-is-better.png | Correct | 173207 | 102327 |
| order-entries-in-palette.png | Good | 24834 | 15978 |
| paletted-as-grayscale.png | Bad | 17304 | 15835 |
| paletted-to-truecolor.png | Bad | 1362 | 1059 |
| truecolor-alpha-as-filtered-paletted.png | Bad | 72038 | 28952 |
| truecolor-alpha-filter.png | Correct | 46912 | 39374 |
| truecolor-alpha-to-grayscale-alpha.png | Bad | 97074 | 54451 |
| truecolor-alpha-to-paletted-trns.png | Bad | 32473 | 27657 |
| truecolor-alpha-to-truecolor-trns.png | Good | 16653 | 9168 |
| truecolor-to-paletted-67-colors.png | Bad | 1920 | 1100 |
| Total: 24 Files | 10/24 | 944821 | 427592 |
PNGOptimizer saved 517229 Bytes (54,74 %). It reduces all PNG Test Corpus files by more than 50%. This tool includes most of the critical images reductions, but there is still more improvement to do to get even better results. It can be completed by a good compressor.
See Website - Version tested: 07/02/2011 - Type: Compressor/ Manual Optimizer
PNGOUT is undoubtedly the best PNG compressor, based on a modified algorithm of Deflate, called KFlate
. It can also achieve images reductions, mainly manually. That's mean you've to know about PNG specifications to use it efficiently.
| File Tested | Option | Result | Input Size | Output Size |
|---|---|---|---|---|
| 16-colors-best-encoded-as-8bits.png | /c3 /f0 /d8 /n2 | Good | 7223 | 5900 |
| 255-bad-trns-values.png | /c3 /f0 /n2 | Good | 2547 | 1766 |
| 256-entries-to-17.png | /c3 /f0 /n1 | Good | 1243 | 305 |
| alpha-channel-filtered-grayscale.png | /c4 /f5 /n2 | Good | 11349 | 6974 |
| alpha-channel-filtered-rgba.png | /c6 /f5 /n5 | Good | 11259 | 8841 |
| bad-transparacy-truecolor-alpha.png | /c6 /f5 /n8 | Bad | 85864 | 66320 |
| compact-idat-chunk.png | /c6 /f0 /n1 | Good | 17193 | 9957 |
| data-png-header.png | /c3 /f0 /n1 | Good | 5129 | 305 |
| dirty-data-rbg.png | /c6 /f5 /n8 | Bad | 90027 | 62479 |
| filtering-each-line.png | /c2 /f5 /n1 | Good | 181408 | 751 |
| grayscale-alpha-filter.png | /c4 /f5 /n1 | Bad | 21692 | 17874 |
| grayscale-alpha-to-grayscale-trns.png | /c0 /f4 /n1 | Good | 7032 | 3389 |
| grayscale-alpha-to-paletted.png | /c3 /f0 /n5 | Good | 18708 | 13161 |
| low-compression-huffman-only-is-better.png | /s3 /c2 /f5 /n5 | Good | 173207 | 100168 |
| order-entries-in-palette.png | /c3 /f1 /n3 | Bad | 24834 | 20604 |
| paletted-as-grayscale.png | /c0 /f5 /d8 /n2 | Bad | 17304 | 15439 |
| paletted-to-truecolor.png | /c2 /f5 /n2 | Good | 1362 | 445 |
| truecolor-alpha-as-filtered-paletted.png | /c6 /f5 /n8 | Bad | 72038 | 57849 |
| truecolor-alpha-filter.png | /c6 /f5 /n3 | Bad | 46912 | 39812 |
| truecolor-alpha-to-grayscale-alpha.png | /c6 /f5 /n2 | Bad | 97074 | 72681 |
| truecolor-alpha-to-paletted-trns.png | /c3 /f0 /n4 | Good | 32473 | 19171 |
| truecolor-alpha-to-truecolor-trns.png | /c2 /f5 /n2 | Good | 16653 | 8745 |
| truecolor-to-paletted-67-colors.png | /c3 /f0 /n1 | Good | 1920 | 1039 |
| Total: 24 Files | - | 16/24 | 944821 | 533990 |
PNGOUT saved 410831 Bytes (43,48 %). The best usage of PNGOUT is to run it after a good PNG optimizer, as TruePNG. You can use it as a pure compressor, because PNGOUT does not operate some images reductions for Web, as the cleaning of RGB space.
See Website - Version tested: 1.4.0 - Type: Paletted/GrayScale Optimizer
PNGRewrite is command-line utility that reduces the unnecessarily large palettes that some programs write into PNG files. It also optimizes transparency data, and reduces the bits-per-pixel if possible. It only runs on Paletted or GrayScale images.
| File Tested | Result | Input Size | Output Size |
|---|---|---|---|
| 16-colors-best-encoded-as-8bits.png | Bad | 7223 | 6683 |
| 255-bad-trns-values.png | Good | 2547 | 1829 |
| 256-entries-to-17.png | Good | 1243 | 307 |
| alpha-channel-filtered-grayscale.png | Bad | 11349 | Not Supported |
| alpha-channel-filtered-rgba.png | Bad | 11259 | Not Supported |
| bad-transparacy-truecolor-alpha.png | Bad | 85864 | Not Supported |
| compact-idat-chunk.png | Bad | 17193 | Not Supported |
| data-png-header.png | Good | 5129 | 389 |
| dirty-data-rbg.png | Bad | 90027 | Not Supported |
| filtering-each-line.png | Bad | 181408 | Not Supported |
| grayscale-alpha-filter.png | Bad | 21692 | Not Supported |
| grayscale-alpha-to-grayscale-trns.png | Good | 7032 | 3548 |
| grayscale-alpha-to-paletted.png | Bad | 18708 | 13783 |
| low-compression-huffman-only-is-better.png | Bad | 173207 | Not Supported |
| order-entries-in-palette.png | Bad | 24834 | 19319 |
| paletted-as-grayscale.png | Good | 17304 | 15835 |
| paletted-to-truecolor.png | Bad | 1362 | 1365 |
| truecolor-alpha-as-filtered-paletted.png | Bad | 72038 | Not Supported |
| truecolor-alpha-filter.png | Bad | 46912 | Not Supported |
| truecolor-alpha-to-grayscale-alpha.png | Bad | 97074 | Not Supported |
| truecolor-alpha-to-paletted-trns.png | Good | 32473 | 20049 |
| truecolor-alpha-to-truecolor-trns.png | Bad | 16653 | Not Supported |
| truecolor-to-paletted-67-colors.png | Good | 1920 | 1100 |
| Total: 24 Files | 6/24 | 944821 | 908983 |
PNGRewrite saved 35838 Bytes (3,79 %), without any comparison to others utility: it only works on Paletted images. Most of PNGRewrite tricks are included into others tools, as OptiPNG, which is using its code. This tool is today partially obsolete.
See Website - Version tested: March 2011 - Type: Optimizer
pngwolf is a great PNG optimizer, which have the ability to test differents filtering methods. According to its author, it uses a genetic algorithm for finding the best scanline filter for each scanline in the image. It's very efficient on RGB (24-bit) or RGB+Alpha (32-bit).
| File Tested | Result | Input Size | Output Size |
|---|---|---|---|
| 16-colors-best-encoded-as-8bits.png | Bad | 7223 | 6263 |
| 255-bad-trns-values.png | Bad | 2547 | 1993 |
| 256-entries-to-17.png | Good | 1243 | 1024 |
| alpha-channel-filtered-grayscale.png | Bad | 11349 | 8828 |
| alpha-channel-filtered-rgba.png | Good | 11259 | 8848 |
| bad-transparacy-truecolor-alpha.png | Bad | 85864 | 984 |
| compact-idat-chunk.png | Good | 17193 | 10030 |
| data-png-header.png | Bad | 5129 | 4909 |
| dirty-data-rbg.png | Correct | 90027 | 45632 |
| filtering-each-line.png | Good | 181408 | 757 |
| grayscale-alpha-filter.png | Bad | 21692 | 18004 |
| grayscale-alpha-to-grayscale-trns.png | Good | 7032 | 4102 |
| grayscale-alpha-to-paletted.png | Bad | 18708 | 14965 |
| low-compression-huffman-only-is-better.png | Correct | 173207 | 106835 |
| order-entries-in-palette.png | Bad | 24834 | 18251 |
| paletted-as-grayscale.png | Bad | 17304 | 15480 |
| paletted-to-truecolor.png | Bad | 1362 | 1058 |
| truecolor-alpha-as-filtered-paletted.png | Bad | 72038 | 26266 |
| truecolor-alpha-filter.png | Correct | 46912 | 37339 |
| truecolor-alpha-to-grayscale-alpha.png | Bad | 97074 | 51910 |
| truecolor-alpha-to-paletted-trns.png | Bad | 32473 | 25640 |
| truecolor-alpha-to-truecolor-trns.png | Good | 16653 | 9683 |
| truecolor-to-paletted-67-colors.png | Bad | 1920 | 1118 |
| Total: 24 Files | 7,5/24 | 944821 | 559486 |
PNGWolf saved 524907 Bytes (55,56 %). It is an excellent tool which can be useful for all filtered images (including Filtered Paletted images). It does not support yet any image reduction (except RGB Filtering with None Filter). Combined with PNGOUT and -ks -f6 option, you can get good results.
See Website - Version tested: 0.4.0.3 - Type: Optimizer
If you have to choose one, TruePNG is the must have tool, because it operates the most of images reductions (i've returned a lot of suggestions to its author, based on PNG Test Corpus files). Completed with PNGOUT, it gives excellent results — this combination is used by ScriptPNG.
| File Tested | Result | Input Size | Output Size |
|---|---|---|---|
| 16-colors-best-encoded-as-8bits.png | Good | 7223 | 6647 |
| 255-bad-trns-values.png | Good | 2547 | 1766 |
| 256-entries-to-17.png | Good | 1243 | 303 |
| alpha-channel-filtered-grayscale.png | Good | 11349 | 7512 |
| alpha-channel-filtered-rgba.png | Good | 11259 | 9151 |
| bad-transparacy-truecolor-alpha.png | Good | 85864 | 568 |
| compact-idat-chunk.png | Good | 17193 | 10549 |
| data-png-header.png | Good | 5129 | 319 |
| dirty-data-rbg.png | Good | 90027 | 45173 |
| filtering-each-line.png | Good | 181408 | 754 |
| grayscale-alpha-filter.png | Good | 21692 | 17468 |
| grayscale-alpha-to-grayscale-trns.png | Good | 7032 | 3537 |
| grayscale-alpha-to-paletted.png | Good | 18708 | 13426 |
| low-compression-huffman-only-is-better.png | Good | 173207 | 100360 |
| order-entries-in-palette.png | Good | 24834 | 15361 |
| paletted-as-grayscale.png | Good | 17304 | 14657 |
| paletted-to-truecolor.png | Good | 1362 | 459 |
| truecolor-alpha-as-filtered-paletted.png | Good | 72038 | 17241 |
| truecolor-alpha-filter.png | Good | 46912 | 38996 |
| truecolor-alpha-to-grayscale-alpha.png | Good | 97074 | 41296 |
| truecolor-alpha-to-paletted-trns.png | Good | 32473 | 19894 |
| truecolor-alpha-to-truecolor-trns.png | Good | 16653 | 9150 |
| truecolor-to-paletted-67-colors.png | Good | 1920 | 1101 |
| Total: 24 Files | 24/24 | 944821 | 376628 |
TruePNG saved 568193 Bytes (60,14 %). It does RGB space cleaning, palette optimization, ColorType best choice, BitDepth reduction, etc. Thanks to that, it also achieves the best compression results. You can use PNGOUT to compress files further, using -ks -f6options.
See Website - Type: Deflate Structure Optimizer
It removes garbage
from PNG files, cleans huffman symbols and does others tricks to optimize the Deflate structure. It can save some extra bytes after any PNG compression process. You should use it after a PNG optimizer, and a PNG compressor.
See Website - Type: Deflate Structure Optimizer
This tool is very closed to DeflOpt, and does the same things with different strategies. To get bests results, you can use multiple combination of them, using that way:
deflopt -k "myfile.png" defluff < "myfile.png" > "myfile-o.png" deflopt -k "myfile-o.png"
See Website - Type: Compressor
PNGZopfli is a tool that uses the powerful Zopfli Deflation. It works like Advdef: you have to optimize your PNG before using PNGZopfli because it does not any reductions before compression.
See Website or Mirror- Type: Block code Optimizer for PNGOUT
Huffmix gets the best blocks results from two trials files created by PNGOUT, and creates a new output file with best results. It can let you to optimize trials results of PNGOUT.
See Website - Type: PNG Reset Tool
Prepng rewrite input PNG without compression, but with filtering step (delta, average, and Paeth). Setting no compression to a file can help to improve new trial compression.
See Website - Type: Quantizer, converts to Paletted
pngquant — new version — uses modified version of Median Cut quantization algorithm. It converts PNG files to Paletted PNG (256 colors or less). It converts alpha channel to alpha values in palette. New version reloaded by Kornel Lesiński seems to be more efficient thant pngnq.
See Website - Type: Quantizer, converts to Paletted
pngnq is based on pngquant, but it uses a different quantization algorithm that is more efficient than the old version of pngquant, and gives better quality / size ratio. Only produces PNG-8 (Palette images) with alpha values if necessary.
See Website - Type: Quantizer, Optimizer
Color Quantizer is a GUI which contains other programs, like PNGOUT, TruePNG, DeflOpt, jpegtran and Intel library for JPG. It uses a close quantizer than pngquant — new version —, but you can control lot of parameters in quantization.
See Website - Type: RGB Space Cleaner
PNGKT scans RGB values in the RGB space that are not displayed cause to the alpha channel. Corresponding RGB values to 0 with alpha channel are converted to 0,0,0. This can improve compression of PNG Truecolor+alpha files.
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