Clever PNG Optimization Techniques

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As a web designer you might be already familiar with the PNG image format which offers a full-featured transparency. It’s a lossless, robust, very good replacement of the elder GIF image format. As a Photoshop (or any other image editor) user you might think that there is not that many options for PNG optimization, especially for truecolor PNG’s (PNG-24 in Photoshop), which doesn’t have any. Some of you may even think that this format is “unoptimizable”. Well, in this post we’ll try to debunk this myth.

This post describes some techniques that may help you optimize your PNG-images. These techniques are derived from laborious hours spent on studying how exactly the PNG encoder saves data. We’ll start with some essentials about the PNG format and will then move to advanced optimization techniques.

You may want to take a look at the following related articles:

The boring part

Before we dive into image optimization techniques, we have to learn some technical details about the PNG format. Each graphic format has its own advantages and weaknesses; knowing them will allow you to modify original image for better visual quality and compression. This is a key concept behind professional image optimization.

PNG was developed as an open-source replacement of the proprietary GIF format. They have some common features (like indexed color palette), but PNG is much better than GIF in every aspect. It introduced some cool features for image packing and compression, but for us – web-designers and developers – the most important one is the scanline filtering (also known as ‘delta filters’).

Scanline filtering

Here is how it works. For example, we have a 5×5 pixels image with horizontal gradient. Here is a schematic view of this image (each number represents a unique color):

Screenshot

As you can see, all identical colors spread vertically, not horizontally. Such images will have a very poor compression ratio in GIF, because it compresses colors that spread horizontally. Let’s see how this image data can be packed by scanline filtering:

Screenshot

Number 2 before each line represents applied filter, which is “Up” in this case. The “Up” filter sends the message to the PNG decoder: “For the current pixel take the value of the above pixel and add it to the current value.” We have 0 value for lines 2—5 because all pixels in each vertical line have the same color. And such data would be compressed better if the image was relatively large. For example, 15 pixels of value 0 can be written as 0(15) and this is much shorter than fifteen 0’s—this is how compression works in common.

I wrote “can be packed” because in this ideal test case the “Sub” filter (number 1 before each line) will give much better result:

Screenshot

The filter “Sub” sends the message to the decoder: “Take the value of the left pixel and add it to the current value.” In this case, it’s 1. As you may already have guessed, such data will be compressed very effectively.

Scanline filtering is important for us because we can use them: in particular, we can do some image manipulation to make filtering better. There are five filters: None (no filtering), Sub (subtract the left pixel value from the current value), Up (subtract the above pixel value), Average (subtract the average of the left and the upper pixels) and Paeth (substitute the upper, left or upper left pixel value, named after Alan Paeth).

And here’s how these filters affect the image size in comparison with the good ol’ GIF:

Screenshot

GIF, 2568 bytes

Screenshot

PNG, 372 bytes

As you can see, the GIF image is 7 times larger than the same PNG-image.

Image type

Another important thing to know about PNG is image type, the meta-data stored inside the file. As a Photoshop user, you are familiar with PNG-8 (indexed image) and PNG-24 (truecolor image). As a Fireworks user, you may know PNG-32 (truecolor with transparency), which is quite confusing, because Photoshop’s PNG-24 may also store truecolor with transparency. Well, it’s worth knowing that these names are not official, and you won’t find them in PNG specs. For your convenience we’ll use Photoshop’s naming convention of PNG image types in this article.

There are 5 available image types in PNG: Grayscale, Truecolor, Indexed-color, Grayscale with alpha and Truecolor with alpha. There are also two subtypes of indexed-color type (non-official, too): bit transparency (each pixel can be fully transparent or fully opaque) and palette transparency (each pixel can be semi-transparent). In second case each color is stored in palette with its alpha value. Thus, opaque red and 50%-transparent red are two different colors and they take 2 cells inside palette.

The worst thing is that Photoshop can save PNG with only 3 of these types: Indexed-color with bit transparency, Truecolor and Truecolor with transparency. That’s why you may find a lot of opinions that Adobe Fireworks is the best tool for PNG optimization. Personally, I don’t agree with them: Fireworks doesn’t have enough tools for image manipulation, it’s only have slightly more options for saving PNG image, but it’s a topic for another discussion.

This is where utilities such as OptiPNG2 or pngcrush3 come in handy. Essentially, these tools do the following:

  1. Pick up the best image type for an image (for example, truecolor can be converted to indexed-color if there aren’t too many colors in the image).
  2. Pick up best delta filters.
  3. Pick up the best compression strategy and, optionally, reduce the color depth.

All these operations do not affect image quality at all, but do reduce the file size of the PNG-images, so I highly recommend you to use such tools every time you save a PNG image.

Now enough with the boring part, let’s do some magic!

1. Posterization

This is a well-known method of the truecolor image optimization. Open up the example image in Photoshop, press the Adjustments layer icon in the Layers palette and choose Posterize:

Screenshot

Pick the smallest possible amount of Levels (usually 40 is enough) and save the image:

Screenshot

Original, 84 KB

Screenshot

Posterized, 53 KB

Here is how it works: the posterization simply reduces the amount of colors, converting similar colors to the single one, thus creating posterized regions. This helps to perform a better scanline filtering and achieve a better compression. The downside of this method is color alternation, which is especially visible if you are trying to stitch image with a HTML background:

Screenshot
Original image

Screenshot
Posterized image

2. Dirty Transparency

Take a look at the following images:

Screenshot
75 KB

Screenshot
30 KB

Both of them were saved in Photoshop without any optimization. Even if you do a per-pixel comparison of these images, you won’t notice any difference. But why is the first image 2.5x larger than the second one?

You need a special plugin for Photoshop to see hidden details. It’s called Remove Transparency and available for free download on the PhotoFreebies plugin suite4. You have to install it first before proceeding with the next step.

Open both images form the example above in Photoshop and choose Filer ? Photo Wiz ? Remove Transparency. Now you can see the actual pixel data that was saved in the image:

Screenshot

Screenshot

What’s happening? How is it possible to reveal the data from the original image from a single-layered PNG image? Well, it’s quite simple. Each pixel in the truecolor image with alpha is described by four bytes: RGBA. The last one is Alpha, which controls pixel transparency: the value of 0 means fully transparent pixel and 255 means fully opaque. And this means that every pixel (with any RGB value) can be hidden with just Alpha byte set to 0. But this RGB data still exists and, moreover, it prevents PNG encoder from effectively packing and encoding the data stream. Thus, we have to remove this hidden data (fill it with solid black, for example) before saving the image. Here is a quick method how to do this:

  1. Open the first image from the example above5 in Photoshop.
  2. Ctrl+click (or ?+click on Mac) on image thumbnail in Layers palette to create a selection, then invert it: Select ? Inverse.

    Screenshot

  3. Switch to Quick Mask mode by pressing Q key:

    Screenshot

  4. We have created a mask for a semi-transparent image, but we need to leave fully transparent pixels only. Choose Image ? Adjustments ? Threshold and move Threshold Level slider to the right, thus leaving fully transparent pixels of the selection:

    Screenshot

  5. Leave Quick Mask mode (press Q key again) and fill the selection with black:

    Screenshot

  6. Invert the seleciton again (Select ? Inverse) and click on the Screenshot icon in the Layers palette to add mask.

That’s it, now you can save this image in PNG-24 and ensure that the 75 KB image is now 30 KB. By the way, all these steps can be easily recorded into Photoshop’s Action (download the Dirty Transparency Photoshop Action6) and reused later with a single keystroke.

You might think about “dirty transparency” as some kind of a bug in image editors: if those image regions can’t be seen and take so much space, why can’t they be removed automatically before saving? Well, this “bug” can be easily turned into a “feature”. Take a look at the following pictures:

Screenshot
5 537 bytes

Screenshot
6 449 bytes

If you remove transparency from those images, you’ll see the following:

Screenshot

Screenshot

Despite the fact that the first image contains more complex image data, it’s 1Kb lighter than the second one, which was optimized as described above. The explanation of this “abnormal” behavior is simple: image data stream in the first example was effectively packed by delta filters, which works better for smooth color transitions (like gradients).

Tech geeks may look at OptiPNG’s output log and ensure that filters were not applied at all for the second image. That’s why I highly recommend you to read The boring part7 of this article first before using these techniques: if you don’t understand what you’re doing, you can make your image even larger.

The ultimate solution to preserve original image data is to create a mask on the image layer in Photoshop (we’ll come back to this later):

Screenshot

As you can see, Dirty transparency is a very powerful yet very delicate technique. You have to know how and why it works before using it. If you are saving PNG-24 images with transparent areas, the first thing you have to do is to check image data in these areas and make the right decision on clearing or leaving them as is.

3. Split by transparency

Sometimes you have to save image in the “heavy” PNG-24 because of few semi-transparent pixels. You can save extra Kbs if you split such images in two parts — one with solid pixels, the second one with semi-transparent — and save them in appropriate graphic formats. For example, you can save semi-transparent pixels in PNG-24, and solid pixels in PNG-8 or even JPEG. Here is a quick (and recordable for Actions) solution to do this. For our experiments we’ll use this elder Russian iPod ancestor:

Screenshot
PNG-24, 62 KB

  1. Ctrl+click/?+click on image thumbnail in Layers palette to create a selection:Screenshot
  2. Go to Channels palette and create new channel from selection:Screenshot
  3. Remove selection (Ctrl+D or ?+D), select the newly created channel and run Threshold (Image ? Adjustments ? Threshold). Move the slider to the very right:Screenshot
  4. We’ve made a mask for selecting dead solid pixels. Now we have to split original layer by this mask. Ctrl+click/?+click on Alpha 1 channel, go to Layers palette, select the original layer and run Layer ? New ? Layer via Cut. As a result, there are two layers with separated solid and semi-transparent pixels.

Now you need to save those two images in separate files: solid pixels in PNG-8, semi-transparent ones in PNG-24. You can apply Posterization8 technique on semi-transparent pixels layer to make image file even smaller.

Screenshot
PNG-8
128 colors + dithering
17 KB

Screenshot
PNG-24
posterization 35
6 KB

And here is the result for comparison:

Screenshot
Before
63 KB

Screenshot
After
23 KB

This method has an obvious drawback: you get two images instead of one, which may be not so convenient to use (for instance, when making a product catalog in the CMS).

4. Influence masks

Actually, is is not a PNG-specific optimization technique, but demonstration of rarely-used Save for Web properties: Color reduction influence mask and Dithering influence mask.

Screenshot

Sadly, these properties were removed in Photoshop CS4, so you can try this optimization approach only in the pre-CS4 versions (I’m using CS3).

To understand how influence masks works, let’s open this demo image9 in PS and save it in PNG-8 with the following settings: Color reduction: Adaptive, Dither: No dithering, Colors: 256.

Screenshot
42 KB

The first thing I’ve noticed about this image is a very fuzzy pendulum. It is a very bright spot on the image and it attracts way too much attention. Let’s try to smooth pendulum’s color transitions by setting dithering to 100%:

Screenshot
46 KB

The pendulum looks better now, but we got another problems: image size increased by 4 KB and solid-color background became very noisy:

Screenshot

We can try to get rid of this noise by lowering dithering value, but the image quality may also be reduced.

Based on these problems, let’s try to do the incredible: increase image quality by lowering the number of colors and image size. Influence masks will help us.

Let’s start with the color. Go to Channels palette, create a new channel and name it color. We’ve already determined that the pendulum is our top priority region to improve image quality, so we need to draw a white circle right on its place (you can enable RGB channel for better precision).

Screenshot

Go to Save for Web dialog and set the following properties: Color reduction: Adaptive, Dither: No, Colors: 128 (as you can see, we reduced number of colors from 256 to 128). Now we have to select an influence mask: click on the Screenshot near Color reduction list and select the color channel from drop-down list: Now our image looks as follows:

Screenshot

You can see the influence mask in action: the pendulum looks perfect, but the other parts of image look really bad. By setting influence mask, we said to Photoshop: “Look, mate, the pendulum is very important part of image so try to preserve as much colors in this area as possible”. Influence mask works exactly the same as regular transparency mask: white color means highest priority in corresponding image region, black color means lowest priority. All intermediate shades of gray affect on image proportionally.

The pendulum now takes the highest color priority, so we have to lower the intensity of white circle to leave more colors for other areas. Close Save for Web dialog, go to Channels palette, select color channel and open Levels dialog (Image ? Adjustments ? Levels). Set the maximum output level to 50 to lower the white color intensity:

Screenshot

Try to save for Web again with the same properties:

Screenshot

Looks better now, but now we’ve got problems in other image areas:

Screenshot

I think you already understand how influence masks works: you provide Photoshop with some clues about important image areas with different shades of gray. With trials and errors I’ve got the following color mask (you can copy it and apply to the image):

Screenshot

Dithering influence mask works exactly the same, but instead of colors, it affects the dithering amount of different image areas. Lighter color means more dithering. This is a very useful feature, because dithering creates irregular pixel patterns which hinders the PNG compressor to use delta filters. You can determine the exact areas where dithering must be applied while leaving other areas intact, thus gaining better compression of image data.

My dithering channel looks like this:

Screenshot

Applying both color and dithering influence channels with the same optimization settings (Adaptive, 128 colors):

Screenshot

Pretty good for 128 colors, isn’t it? Let’s do some finishing touches: set colors to 180 and max dithering to 80%. And here is our final result compared to original, non-optimized version:

Screenshot
256 colors, no dithering, non-optimized
42 KB

Screenshot
180 colors, optimized
34 KB

Please stay tuned (RSS10, Twitter11) for the second part of the article where we’ll cover further techniques; we’ll talk about grayscale images, using less colors, lowering details and discuss further tips for PNG usage and optimization as well as the PNG optimization in practice.

Related posts

You may want to take a look at the following related articles:


13

Footnotes

  1. 1 http://www.smashingmagazine.com/2009/07/01/clever-jpeg-optimization-techniques/
  2. 2 http://optipng.sourceforge.net/
  3. 3 http://pmt.sourceforge.net/pngcrush/
  4. 4 http://www.thepluginsite.com/products/photowiz/photofreebies/index.htm
  5. 5 http://www.smashingmagazine.com/wp-content/uploads/images/png-optimization-guide/dirty-tr-sample1.png
  6. 6 http://www.gracepointafterfive.com/dirty-transparency
  7. 7 #boring
  8. 8 #posterization
  9. 9 http://www.smashingmagazine.com/wp-content/uploads/images/png-optimization-guide/influence/demo.png
  10. 10 http://rss1.smashingmagazine.com/feed/
  11. 11 http://twitter.com/smashingmag
  12. 12 http://www.smashingmagazine.com/2009/07/01/clever-jpeg-optimization-techniques/
  13. 13 http://chikuyonok.ru

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Sergey Chikuyonok is a Russian front-end web-developer and writer with a big passion on optimization: from images and JavaScript effects to working process and time-savings on coding.

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  1. 1

    cancel button,

    I ran some tests using Sergey’s techniques through punypng which uses a variety of open-source compression in its toolbox.

    For 8-bit transparent PNGs, with Sergey’s dirty transparency technique, I saved around 25%. Running that through punypng added another 10% of savings. The punypng part really depends on the type of image you’re working with, but basically, the kind of optimization that Sergey describes are ones that work well in tandem with traditional png compression like in pngcrush or optipng.

    The only time dirty transparency produced larger results and actually worked against punypng was when I was using 1-bit transparent PNGs. In these cases, the better choice would be to output an indexed GIF and then convert it using punypng to an indexed PNG.

    0
  2. 52

    Nice article! You might also want to check my 16 Image Optimization Tips Post: http://codefusionlab.blogspot.com/2009/07/16-impressive-image-optimization-tools.html

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  3. 103

    One other useful technique for transparent PNGs is to create an 8-bit PNG that contains also contains semi-transparent colours.

    Fireworks supports this rare PNG feature as well as pngquant (+ the manfred pngquant GUI).
    http://www.libpng.org/pub/png/apps/pngquant.html
    http://jedisthlm.com/2006/03/16/manfred-a-pngquant-gui/

    This method also has some benefits, as described in this Sitepoint article:
    http://www.sitepoint.com/blogs/2007/09/18/png8-the-clear-winner/

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  4. 154

    One of the best posts that I have read on Smashing: containing both theoretical information and practical techniques that I’d never heard of, and both explained very clearly. Well done.

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  5. 205

    when using the .png file format within design elements don’t forget to remove the gamma: http://morris-photographics.com/photoshop/articles/png-gamma.html

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  6. 256

    That absolutely rocked. Would be very interested in knowing if, of the methods listed in the above article, there are particular techniques that are more or less useful in optimising PNGs (or other image formats) for use in Flash.

    At the moment I tend to save most images as PNG 24 before importing, and Flash looks like it can optionally then do it’s own kind of compression on them (jpeg), while still maintaining transparency/semi-transparency. Any insights as to how Flash is actually interacting with these images, and how to maximise optimisations like those discussed above, would be extremely useful.

    I’ll of course be doing some experimenting with it on my own, in the meanwhile, but don’t think I’ll ever have the level of insight that could be provided by the author of this article!

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  7. 307

    Could it be that I can’t see some images of this article either on my Google Reader or Safari or Firefox? Is it just my problem?

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  8. 358
  9. 409

    very very targeted article

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  10. 460

    Carola, no it’s not just you, half the images are missing for me too, I’m using IE8

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  11. 511

    Some of the images in this article are missing for me as well. Missing in IE and Firefox. Guys, any idea why I can’t see them?

    0
  12. 562

    Some screenshot images missing …

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  13. 613

    very clever technique :D Though some images in comparison are missing, any idea why?

    0
  14. 664

    Very good article!
    But some (comparison) images are missing… :(

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  15. 715

    This is very good.

    Can we use this type of PNG in background repeat-x using with css?

    Thanks in advance

    0
  16. 766

    iPod ancestor :):)

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  17. 817

    Smashing Editorial

    July 15, 2009 10:58 pm

    Sorry guys, we had some media server issues – we are working to solve the problem!

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  18. 868

    Thank you for that article. This information is really appreciated.

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  19. 919

    I used photoshop cs2, and the remove transparency worked same for both the images, the transparent portion was converted to opaque white color and not the entire image as you said.

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  20. 970

    Very helpful, thank you! I’m starting to use .png on the wide range so this article is excellent for me now! Best regards to the Author.

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  21. 1021

    Totally Amazing ! Very good Artikel ! Thanks a Lot !

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  22. 1072

    Informative boring part :)

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  23. 1123

    Thanks again – I can get back to designing again without having to worry about huge PNG file sizes.

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  24. 1174

    amazing post

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  25. 1225

    Sergey Chikuyonok

    July 16, 2009 12:11 am

    I think PNG will work just as well for horizontals, because isn’t it the Paeth-type that does this?

    PNG is better than GIF in every aspect, is saves both vertical- and horizontal-spread colors very good. The real compression is made by Deflate algorithm, scanline filtering is a helper feature that makes compression even better.

    Have you done any side-by-side comparisons of some of these techniques with just running the image through OptiPNG or pngcrush?

    Yes, I did. That’s why I’m using these techniques :) The main idea of these optimization methods is to prepare image for better PNG compression. Savings are very big, sometimes it’s 50–60% off without noticeable quality degradation. Stay tuned for the next article, I made a screencast that shows how these techniques work in real life and affects on OptiPNG compression.

    Would be very interested in knowing if, of the methods listed in the above article, there are particular techniques that are more or less useful in optimising PNGs (or other image formats) for use in Flash.

    You should try “Dirty transparency” method. Also you may try to extract transparency mask from the image, optimize it with Posterization and apply to the original image. This might save you a few bytes.

    I used photoshop cs2, and the remove transparency worked same for both the images, the transparent portion was converted to opaque white color and not the entire image as you said.

    It depends on how these portions were masked. If you delete them, some portions will be filled with solid color (it also depends on mask shape), if you apply a mask to the image layer, the masked areas will be preserved.

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  26. 1276

    wow. those influence masks are incredible. but it does take a lot of time to save 8k

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  27. 1327

    Thanks for a great article, I was always wondering about photoshops lack of PNG compression options, bookmarked for life!

    0
  28. 1378

    cool. i wish PNG could be animated.

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  29. 1429

    Incredible article, thanks!

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  30. 1480

    Thanks smashingmag for very useful article.

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  31. 1531

    Great article – and postet exactly on the day that I needed these infos. :)

    Since you mentionend the tools OptiPNG and pngcrush you might want to take a look at PngOptimizer: http://psydk.org/PngOptimizer. It is IMO much easier to use and gets my PNGs compressed a bit more.

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  32. 1582

    great post !

    is anybody knows the translation for “posterize” in the french photoshop ?
    thx ;-)

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  33. 1633

    Great article! Thank you

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  34. 1684

    Thanks for that detailed tutorial, highly appriciated!

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  35. 1735

    This is an amazingly useful article! Show’s a real understanding of the technical knowledge behind design that so many people lack. Thank you.

    Managed to get a set of 14 images from 900KB down to 300KB with the posterization technique and then pngcrush.

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  36. 1786

    great, great, great!
    Thanks!

    0
  37. 1837

    One of the best articles I’ve seen on Smashing for a LONG time. Well done.

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  38. 1888

    So IE6 is all happy with PNGs then

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  39. 1939

    You don’t have to split the image into two as PNG-8 supports alpha transparency just fine.

    Open the image in Fireworks, change from “No Transparency” to “Alpha Transparency” under the Optimize tab and edit in the semi-transparent pixels you want.

    Save the image through File > Export…

    Semi-transparent pixels will become fully transparent in IE6 and below while in the more capable browsers (IE7+, Firefox, Opera, Safari et al.) it’ll have full-blown alpha transparency.

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  40. 1990

    Great! it must knew every web designer.
    Thanks!

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  41. 2041
  42. 2092

    hans:

    You mean MNG, then: http://en.wikipedia.org/wiki/Mng

    “MNG is closely related to the PNG image format. When PNG development started in early 1995, developers decided not to incorporate support for animation, not least because this feature of GIF was seldom used at the time. However, work soon started on MNG as an animation-supporting version of PNG. Version 1.0 of the MNG specification was released on January 31, 2001.”

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  43. 2143

    Complicated but nice and understandable!!!
    thanks a lot!

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  44. 2194

    crazy…

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  45. 2245

    Amazing Post… Splendid … Hats off to you Sergey!!!!!!!!!!

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  46. 2296

    Two thumbs up. Great work Sergey. Cheers!

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  47. 2347

    Awesome… Very informative

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  48. 2398

    Excellent technique, but its very time consuming. Optimising 200+ PNG files in site means a lot of additional time spent on image editing… My clients wont pay for that.

    Anyway Chapeau Bas Monsieur :-) Great Stuff.

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  49. 2449

    As a png novice – are they safe to use on websites in place of gifs?

    Do all browsers like them?

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  50. 2500

    Sergey Chikuyonok

    July 16, 2009 6:09 am

    Quakeulf, APNG is a Mozilla’s fork of animated PNG: http://en.wikipedia.org/wiki/APNG

    Beacuse of two different forks (APNG and MNG) we still don’t have apropriate standard for everyday use.

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